- 1 Rules for the Inheritance of Traits: Mendel’s Contribution
- 1.1 Initial Discovery of Heredity of Gregor Johann Mendel
- 1.2 Life of Mendel
- 1.3 Mendelian Concepts of Heredity or Mendelism
- 1.4 Experiment of Mendel
- 1.5 1. Monohybrid Inheritance and the Law of Segregation
- 1.6 Explanation of Results of Monohybrid Inheritance
- 1.7 Monohybrid Experiments
- 1.8 Monohybrid Experiments in Plants:
- 1.9 Monohybrid Cross in Animal:
- 1.10 Mendel’s Laws of Heredity derived from the Monohybrid Experiment:
- 1.11 2. Dihybrid Inheritance and the Law of Independent Assortment
- 1.12 Explanation of Results of Dihybrid Inheritance
- 1.13 Dihybrid Experiments
- 1.14 Dihybrid Experiment and Law of Independent Assortment:
- 1.15 Test Cross and Determination of the Genotype:
- 1.16 Law of Heredity Derived from Dihybrid Experiments:
- 1.17 How are Characteristics (or Traits) Transmitted to Progeny
- 1.18 How do Genes Control the Characteristics (or Traits)
- 1.19 How Blood Groups Are Inherited
- 1.20 Sex Determination in Humans
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Rules for the Inheritance of Traits: Mendel’s Contribution
Inheritance is the transmission of genetically controlled characteristics (or traits) from one generation to the next. We will now describe how Mendel studied the inheritance of characters or traits in various generations of pea plants cultivated by him. First, we will discuss ‘monohybrid inheritance’ which concerns the inheritance of a single characteristic (or single trait) such as plant height. After that, we will describe the dihybrid inheritance which involves the inheritance of two characteristics (or two traits) such as seed shape and seed colour.
Initial Discovery of Heredity of Gregor Johann Mendel
The transmission of characters from parents to progeny was known to common people from the prehistoric period. In agriculture, the raising of quality plants as well as a selection of high-yielding life was a common practice in civilized man. Only through selection attractive breeds of plants and animals could be produced by people and they would feel that good quality plants and animals could only be produced from their selected breeds. But during that period nothing was known to people about the basis of the quality lives. There was no scientific investigation to elucidate the mystery of inheritance. A monk named Gregor Johann Mendel first carried out experiments on garden pea plants to elucidate the mystery behind the inheritance of characters. The experiments carried out by Mendel were so designed that he could be able to derive the principles of inheritance.
Life of Mendel
Gregor Johann Mendel was born in 1822 in a peasant family in the village of Czechoslovakia. Because of his poor family background, he could not continue his education up to university level. Only at the age of 21, he entered an Augustinian monastery at Brunn so as to get a scope to continue his education and he was ordained as a priest in 1847. From this monastery, he was sent to Vienna University in 1851 for further studies. He took courses in Mathematics, Physics, Chemistry, and Natural Sciences over there. In 1854 he returned to Brunn and started teaching in a local school.
His training at Vienna University provoked him to think about the phenomenon of heredity and as such on return to Brunn, he planned to perform research on garden pea to reveal the mystery behind the phenomenon of heredity. He collected a number of pea plants and cultivated them in the monastery garden in a controlled environment to carry out their hybridization. From 1856 he continued his experiments for about eight years and then he disclosed his observations before the Brunn Natural History Society in 1865. The paper prepared by Mendel in this regard was published in the proceedings of the Brunn Natural History Society in 1866 with the title, “On plant Hybridization”.
Though the discovery of Mendel was the footstone in the history of the science of heredity yet it did not receive any recognition by the scientific society at that period and as such in great despair Mendel gave up further experimentation for the cause of science. He spent the latter part of his life in a monastery and devoted himself fully to the religious cause. He died in 1884. Mendel’s work was reassessed in 1900 by Hugo de Vries of Holland, Karl Correns of Germany, and Eric von Tschermack of Austria independently and they established Mendel as the father of Genetics. Their work thus is regarded as the rediscovery of Mendel.
Mendelian Concepts of Heredity or Mendelism
On the basis of his hybridization experiments on garden peas, Mendel could be able to realize some principles related to heredity and he also formulated two laws that are responsible for governing the process of inheritance. All these Mendelian ideas and principles relating to heredity are considered Mendeiism. Analysis of the inheritance of features from one generation to the other in pea plants led him to predict the existence of some particulate material or factor responsible for the expression of character. Every such particulate factor also maintains its individuality through succeeding generations. This is known as the law of unit character. That the inheritance is controlled by particulate material is known as the ‘Particulate theory of inheritance’. Besides these, he also realized that factors for contrasting features may differ by their power of expression and hence some factors may dominate the other.
Contrasting factors of character thus may be marked as dominant and recessive by nature. One character expressed by the involvement of a dominant factor may be called a dominant character. On the contrary, a feature expressed by the involvement of a recessive factor is designated as a recessive character. Mendel also realized that in the presence of a dominant factor, the recessive one cannot express itself. This phenomenon is known as the Principle of dominance and recessiveness. Mendelian factors were subsequently named as genes by W.L. Johanssen in 1909 and a gene is considered as the structural and functional unit of heredity.
Besides these ideas, Mendel also realized the basic principles of heredity. He expressed the basic principles in the form of two laws namely the Law of Segregation and the Law of Independent Assortment. Mendel advocated that for a character there is one pair of factors (Genes) which may be identical or non-identical in nature and expression of a character in an organism may occur due to the presence of this pair of factors. The law of segregation states that the pair of factors or genes contrasting or identical in nature present in an individual for any specific character are segregated during gamete formation and each factor goes into one gamete. On the other hand, the Law of Independent Assortment states that two or more pairs of contrasting factors or genes for two or more characters of an organism segregate independently during gamete formation appearing in all possible combinations in the gametes.
Experiment of Mendel
Experimental Material of Mendel: Mendel selected a common pea plant (Pisum sativum) as material in his experiments. Initially, he collected 34 varieties of pea plants in consideration of their phenotypic features but later on he used only 22 varieties in different experiments.
Reasons behind selecting pea plants for the experiment by Mendel:
- Pea plants are easy to develop in the garden.
- Pea plants with contrasting characters are available in nature.
- The pea plants are able to propagate generations with such contrasting characters.
- The flowers of pea plants are bisexual and show self-pollination.
- The floral structure in the pea plant is such that female and male parts are protected within the petal and therefore, no external agent can transmit pollen from one flower to the stigma of another flower. So cross-pollination in pea plants hardly occurs.
- Mechanically cross-pollination may be carried out in pea plants.
- The cross-pollinated flowers produce seeds and from them, new plants may be grown.
Protocol of Mendelian Experiments:
Mendel carried out hybridization on pea plants considering various contrasting features of characters. Following such hybridization, F1 and F2 generations were raised and results obtained in this way were analyzed statistically to ascertain the mode of inheritance of features. In a generalized way, the Mendelian protocol of experiments may be stated in the following points:
- Pea plants having contrasting features for one or more characters were grown in the green house.
- Hybridization was carried out between plants having contrasting features in controlled conditions.
- Mechanically petals of a flower were separated to expose the reproductive parts like stamens, anthers, and pistils. The anthers of the flower were excised with a pair of scissors.
- Reciprocal transfer of pollens from the anthers of flowers from plants with contrasting features of a character to the stigma was carried out with the help of a brush.
- After such transfer of pollens, petals were oriented in position and the treated flower was covered with polythene paper to prevent any further pollination by some other way.
- Pods developed from the treated flowers were collected after ripening.
- The seeds from such pods were then sown to raise next-generation plants (F1 generation). Phenotypic features of the F1 plants were noted for analysis.
- The F1 plants were then allowed to self-pollinate and the seeds of these plants were collected to grow F2 generation plants.
- Expected results from each type of hybridization test were correlated with the observed results statistically with the help of a significance test (χ2 tests) to find out any meaningful discrepancy. This testing helped Mendel to accept or reject the hypothesis made upon some prediction by Mendel regarding the transmission of characters from parents to offspring.
Mendel carried out monohybrid, dihybrid, and even trihybrid experiments considering several characters of pea plants. Altogether he considered seven pairs of contrasting features of seven characters in his hybridization experiments. Besides normal hybridization, he also carried out some form of crosses between pea plants which were different test crosses to ascertain the genotype of the progeny plants. In primary hybridization experiments, Mendel always used pure breeding plants for the purpose of cross-pollination. The F1 plants, therefore, were hybrids for one or more characters.
Seven Characters and their Contrasting Features in Pea Plants:
1. Monohybrid Inheritance and the Law of Segregation
In order to trace the inheritance of a single pair of contrasting characteristics among the pea plants (like tall stem and short stem), Mendel crossed (cross-bred) the pure-bred pea plants differing in these traits and noted their occurrence in the progeny of succeeding generations.
Mendel first crossed pure-bred tall pea plants with pure-bred dwarf pea plants and found that only tall pea plants were produced in the first generation or F1 generation (see Figure). No dwarf pea plants (or short pea plants) were obtained in the first generation of progeny. From this Mendel concluded that the first generation (or F1 cross) showed the traits of only one of the parent plants: tallness. The trait of the other parent plant, dwarfness, did not show up in the progeny of the first generation.
A cross of purebred tall and dwarf pea plants.
Mendel then crossed the tall pea plants of the first generation (F1 generation) and found that tall plants and dwarf plants were obtained in the second generation (or F2 generation) in the ratio of 3 : 1. In other words in the F2 generation, three-fourth plants were tall and one-fourth were dwarf (see Figure). Mendel noted that the dwarf trait of the parent pea plant which had seemingly disappeared in the first-generation progeny, reappeared in the second generation. Mendel said that the trait of dwarfness of one of the parent pea plants had not been lost, it was merely concealed or suppressed in the first generation to re-emerge in the second generation. Mendel called the repressed trait of ‘dwarfness’ a ‘recessive trait’ and the expressed trait of ‘tallness’ the ‘dominant trait’. In this way, Mendel’s experiments with tall and dwarf pea plants showed that the traits may be dominant or recessive.
A cross of tall plants of the F1 generation produces tall and dwarf plants in a ratio of 3 : 1.
Mendel also noted that all the pea plants produced from the hybrid tall parents of the F1 generation were either tall or dwarf. There were no plants with intermediate height (or medium height) in between the tall and dwarf plants. In this way, Mendel’s experiment showed that the traits (like tallness and dwarfness) are inherited independently. This is because if the traits of tallness or dwarfness had blended (or mixed up), then medium-sized pea plants would have been produced.
Out of a total of 1064 pea plants of F2 generation, Mendel found that there were 787 tall pea plants and 277 dwarf pea plants. The ratio of tall plants to dwarf plants comes to be 787 : 277 = 2.84 : 1, which is approximately equal to 3 : 1. Thus, yet another result obtained from Mendel’s monohybrid inheritance experiment is that the ratio of tall plants to dwarf plants in the F2 generation is 3 : 1. Since tallness is a dominant trait and dwarfness is a recessive trait, so we can also say that the contrasting progeny in the F2 generation occurs in the ratio of 3 dominant to 1 recessive. The ratio 3 : 1 is known as the monohybrid ratio.
The results of the monohybrid cross enabled Mendel to formulate his first law of inheritance which is called the law of segregation. According to Mendel’s first law of inheritance: The characteristics (or traits) of an organism are determined by internal ‘factors’ which occur in pairs. Only one of a pair of such factors can be present in a single gamete. We will now explain the results of the monohybrid cross of tall and dwarf pea plants theoretically by using Mendel’s first law of inheritance.
Explanation of Results of Monohybrid Inheritance
Mendel said that each trait is determined by a pair of ‘factors’. This means that the pure-bred tall pea plant has two factors TT for the trait of tallness, and the pure-bred dwarf pea plant also has two factors tt for the trait of dwarfness. The factors of inheritance of tallness TT separate into two gametes T and T, and the factors for the inheritance of dwarfness tt separate into two other gametes t and t (The traits are transmitted to progeny through these gametes).
The gametes of the tall pea plant then cross with the gametes of the dwarf pea plant by the process of fertilization to form zygotes which then produce various progeny in the F1 generation (or first generation) which consists of all tall plants. Thus, the F1 generation possesses one factor of inheritance from each parent plant which was carried in gametes. The parental cross is shown clearly in the following chart:
In the F1 generation shown above, all the progeny plants have factors Tt in which T is the factor for tallness which is a dominant trait. Since all the plants in the F1 generation have the factors Tt, so all of them are tall. The small letter t represents the recessive trait of dwarfness, which does not show up in the first generation in the presence of the dominant trait T.
When two hybrids, tall pea plants (Tt) produced in the first generation (F1) are now cross-bred with each other, then they will produce second-generation (F2) pea plants. This again happens by the separation of factors of inheritance of these tall plants into individual gametes and then crossing of the gametes during fertilisation as shown below:
We can see from the above chart that in the F2 generation (or second generation), the pea plants produced have genotype or inheritance factors TT, Tt, Tt, and tt. Now, the plants having genotypes TT, Tt, and Tt all contain the factor T for the dominant trait ‘tallness’, so all three plants (TT, Tt, and Tt) are tall. The plant having the genotype tt has both factors t for the recessive trait ‘dwarfness’, so it is a dwarf plant. Please note that though a single copy of factor T is enough to make a plant tall but both copies of factor t (that is tt) are necessary to make a plant dwarf (or short).
In the F2 generation, we get 1 plant having genotype TT, 2 plants having genotype Tt, and 1 plant having genotype tt. So, the genotypic ratio in a monohybrid cross will be TT : Tt : tt = 1 : 2 : 1
Again, in the F2 generation, we get 3 tall plants and 1 dwarf plant, so the phenotypic ratio in monohybrid cross will be Tall plants : Dwarf plants = 3 : 1
This result is the same as that obtained by Mendel through experiments.
A cross involving two animals or plants of the same species concerning a pair of contrasting features of a character is termed a monohybrid cross. From the monohybrid cross the progeny produced are hybrids or heterozygous organisms for one character.
Monohybrid Experiments in Plants:
At first, Mendel carried out monohybrid experiments in pea plants considering seven pairs of contrasting features of seven characters. Results of all the monohybrid experiments have been included in the next table. An example of one such experiment by Mendel and its explanation may be given for the sake of elucidation.
Mendel collected pure breeding pea plants of contrasting categories such as tall and dwarf, yellow and green, purple and white, etc. In one experiment he carried out cross-pollination of a tall plant with a dwarf plant and the F1 plants all appeared to be tall. The F1 plants on self-pollination produced 3/4 tall and 1/4 dwarf in F2 generation, the F1 plants were hybrids or heterozygous in nature and the results of different monohybrid experiments as obtained by Mendel have been shown in the next table. Mendel carried out monohybrid experiments considering seven pairs of contrasting features of seven characters and in all the cases he obtained identical results. The results of all his experiments have been shown in the following next table.
Gametes from these two plants are united to form the combination of Tt in the Fx progeny plants. Because of the dominance of T, the F1 progeny plants become tall by appearance, but they are hybrid. The hybrid tall plants (Tt) during gamete formation show segregation of the contrasting factors producing (T) and (t) gamates. Due to segregation, two types of gamates may be formed from the F1 plant and they by their union at random may produce tall and dwarf plants in 3 : 1 ratio.
The results of seven monohybrid experiments with seven different characters:
It is noteworthy that the phenotypic ratio of the plants produced in the F2 generation did not fit exactly with 3 : 1 ratio though Mendel expected a 3 : 1 ratio of the phenotypes to appear in the F2 generation. The discrepancy as appeared in the F2 phenotypes from the expected ratio was due to sampling error or chance error. This error may appear by chance only and the discrepancy was insignificant which could be calculated by a statistical test called the chi-square test. Mendel carried out a chi-square test for the results of F2 phenotypes that appeared in his experiments and observed that the deviation was insignificant. Therefore, F2 phenotypic ratio in his monohybrid experiments was equivalent to 3 : 1.
Further, on analysis of the results of the monohybrid cross of Mendel, it could be realized that the F1 plants were hybrids having alternative factors for a character. Because one of the factors in this combination was dominant the F, plant showed only the dominant feature of a character. These alternative factors for a character were the alleles and during gamete formation in the hybrids, the factors were segregated into different types of gametes. Therefore, both the male and female reproductive parts of the flower in the pea plant could produce two types of gametes T and t. Random union of these male and female gametes produced three types of progeny with three genotypes TT, Tt, and tt, Both TT and Tt gene combi¬nations produced tall plants as T was dominant over t. Such an analysis helped Mendel to formulate the law of segregation and the law may be stated as under.
Monohybrid Cross in Animal:
A cross between a black guinea pig and a white guinea pig may be one monohybrid cross in animals when body colour is taken into consideration. The black body colour, in this case, is contrasting with the white body colour. It is to be mentioned that the black guineapig and white guineapig in the cross are true breeding i.e. they are homozygous in genotype.
Black coat colour in guineapig is determined by a dominant gene, say B and its contrasting feature white coat colour is due to its recessive allele b, B is dominant over b and hence black bodied guinea pigs may have the genotype BB or Bb, while white guineapig possesses genotype bb. One pure-breeding black guineapig (BB) when was crossed with a white guineapig, the cross produced only black guineapig in the F1 generation. When the F1 black guineapigs were allowed to cross each other, in the F2 generation they, produced black and white guineapigs in 3 : 1 ratio similar to the Mendelian monohybrid cross. The cross and results may be shown as under.
The black and white guineapigs of the parental generation produce all black guinea pigs in the F1 generation. The F1 black guineapigs are hybrids and they show black body colour because black body colour is dominant over white body colour. Hence, the genotypes of the parental black and white guineapigs may be indicated as BB (black) and bb (white). These two types of guineapigs produce (B) and (b) gametes. These gametes during reproduction unit to produce Bb (Black) progeny. These black F, progeny when intercrossed produce black and white F2 progeny in 3 : 1 ratio. The cross of the black and white guineapigs and their progeny production may be indicated in the following manner (Diagram).
As black is pure homozygous and dominant, its geno¬type will be BB and as white is pure homozygous but recessive, its genotype will be bb. BB will produce one type of gamete B. In F1 the genotype will be Bb. The black hybrid of F, when mated among themselves produces black and white offspring in 3 : 1 ratio i.e., 15% black and 25% white. Out of 75% black Guineapig 25% are pure black (BB), 50% are hybrid (heterozygous) black (Bb), and 25% are pure white (bb). So, the genotypic ratio becomes 1 : 2 : 1.
Phenotypic and Genotypic Ratio of F2 generation shown in Checkerboard:
|Phenotype||Genotype||Genotypic Ratio||Phenotypic Ratio|
Phenotypic Ratio = 3 : 1, Genotypic Ratio = 1 : 2 : 1
From this experiment, it is concluded that the inheritance of colour character in Guineapig follows Mendel’s law of segregation. This cross was made to study the inheritance of one character or two contrasting forms or two different alleles. Here the crossing of black and white guineapigs is the monohybrid cross.
Mendel’s Laws of Heredity derived from the Monohybrid Experiment:
1. Law of Segregation:
Pair of contrasting factors or genes for a given character, never blend blit segregate during gametogenesis. Besides the law of segregation, Mendel could also be able to propose the law of unit character and the law of dominance from his monohybrid experiment. These laws may be explained in the following way.
2. Law of Unit Character:
For the expression of any feature in living organisms, there is a unit particulate factor or element within the body and this element is very stable in nature and never intermingles with any other such element. This is known as the law of unit character.
Explanation: Mendel observed in the pea plants of the garden alternative features in different characters. On the basis of this, he felt that each and every characteristic feature is determined by the existence of a stable internal unit factor and this factor may be transmitted from one generation to the other in an unaltered form.
3. Law of Dominance and Recessiveness:
When individuals having a pair of contrasting features are crossed, the feature that is expressed is said to be dominant and the feature that remains suppressed is called recessive. Therefore, the property for which a feature becomes dominant or recessive is known as dominance or recessiveness.
Explanation: The pairs of contrasting features which Mendel selected for his hybridization experiments are dominant and recessive in nature. He also found that in each case of hybridization between pure breeding pea plants with dominant and recessive features, the progeny plants bore the dominant feature in the F1 generation. This becomes possible due to the dominant potentiality of the factor determining the character of an organism. The factor for dominance and for recessiveness were called by Mendel as a dominant factor and recessive factor respectively.
Test Cross and Determination of the Genotypes of the Progeny:
Mendel performed a cross where a progeny plant was crossed with a recessive variety plant. In the case of monohybrid cross the F1 progeny was a hybrid with the heterozygous gene combination. Such a cross resulted in the production of two types of progeny; one type expressing the dominant feature and the other type showing the recessive feature. The two types of progeny were also developed in equal proportion i.e., 1 : 1. This indicated that the F1 progeny contained the inner heterozygotic gene combination. With relation to the previous cross between tall and dwarf plants, the test cross may be indicated in the following manner.
Analysis: A result from the Test cross as indicated above suggested that the F, tall plant was hybrid having two Mendelian factors in its inner make up producing tall features as tall was dominant over dwarf features. If the factor or gene for tallness is taken as T and that for dwarfness is t, the hybrid tall plant had the genotype Tt. The dwarf plant had the genotype tt. Therefore, the hybrid tall plant could produce two types of gametes as T and t in equal proportion, while the dwarf plant produced only one type of gamete as t. The gametic union of the hybrid tall plant and the dwarf plant could produce tall and dwarf plants in equal proportion. This supported the segregation of factors in the hybrid tall plant during its gametogenesis as illustrated below.
Test cross ratio thus helps to know the genotype of the progeny. Mendel performed a Test cross for all the F2 plants developed in the F2 generation and he found that out of all the progeny plants third were pure Tall, two-thirds were hybrid Tall and one-third were pure Dwarf. Hence when in the monohybrid cross the F2 phenotypic ratio is 3 : 1, then F2 genotypic ratio was 1 : 2 : 1.
|Hybrid Tall Plant (Tt)|
|Dwarf Plant (tt)||Gametes||T||T|
Deviation from Mendelian Ratio in Monohybrid Crosses:
A typical monohybrid cross as per Mendelian observation should show a phenotypic ratio of 3 : 1 in F2 generation. But there are certain exceptions where one may find a deviation from the Mendelian ratio.
1. Incomplete Dominance:
The situation where one allele of a gene cannot fully suppress the recessive allele is known as incomplete dominance. In the 4 O’clock plant (Mirabilis jalapa) two contrasting flower colours are red and white. The red flower feature in this plant is a normal feature supposed to be dominant by nature. But the gene for the red coloration of the flower, in this case, cannot fully suppress the gene for the white flower feature. Because of this fact when a pure breeding red variety plant is crossed with a white variety plant, all progeny in the next generation becomes pink in nature.
When such pink variety plant is allowed to self-pollinate, the plants produced in the next generation show red, pink, and white variety plants in 1 : 2 : 1 proportion. This suggests that the gene for the red flower colour is not fully dominant over the white feature. Hence, the hybrids contain contrasting alleles of the gene and produce some intermediate expression, i.e., the flower not being fully red becomes pink in colour. It also appears that in case of incomplete dominance genotypic and phenotypic ratio of F2 generation becomes the same i.e., 1 : 2 : 1.
In the case of codominance, the alleles of a gene are equal potential for expression. Neither of the alleles of the gene in this case can be designated as dominant or recessive in nature. When pure breeding varieties having contrasting features are crossed, the resultant hybrids of the F1 generation would exhibit both the phenotypes their parents possess. The F1 hybrids in this situation may produce three types of offspring in the ratio 1 : 2 : 1.
In men, the MN blood group is controlled by codominant alleles, LM and LN. Individuals with the M phenotype have the genotype LMLM and that in N-type is LNLN. Mating of M and N individuals produces only MN-type individuals which means that both LM and LN alleles become expressed in the hybrids. The MN individuals in the next generation may produce M, N, and MN progeny in the F2 generation. Besides this in human ABO blood group system also exhibits the codominant expression of genes. There are three alleles namely IA, IB, and I, which are associated with the expression of four phenotypes of the blood group such as A, B, AB, and O types. Out of the three alleles gene IA and IB are co-dominant and therefore, individuals with genotype IAIB show the expression of both IA and IB genes and the phenotype of the individual becomes AB.
Another example of codominance in animals may be cited from lady bird beetle, Harmonia aziridines in consideration of a character in the elytra of this insect. Elytra in this, insect characteristically may contain either dark round spots on the surface or dark broad bands on its surface. These characters have been found to be controlled by a pair of alleles. But the hybrids of these two varieties of the beetle contain both dark spots and dark bands on the elytra surface indicating codominant expression of two allelic genes. The hybrids in mating may produce three types of offspring in the ratio 1 : 2 : 1.
The effect of lethal mutation may also modify the F2 phenotypic ratio in organisms and the effect of such genes may vary depending upon conditions. In Drosophila melanogaster curly wing feature is due to a dominant mutation (Cy) and the mutant gene in the homozygous condition is lethal for the organism. Hence, a curly-winged fly always is heterozygous (Cy+/Cy) for the gene. This type of lethality is called a balanced lethal system. When two curly-winged heterozygous male and female flies mate, two types of progeny as curly and normal individuals develop in a proportion 2 : 1.
A similar effect may be observed in mice in relation to their body colour. Normal body colour in mice is gray or agouti, but a mutation results in yellow body colour which is having a lethal effect. The gene for agouti coat colour (a) is present on the second chromosome of the mouse, when its allele Ay is having a dominant effect over a. In homozygous condition as Ay Ay the mouse cannot survive. Therefore, when two yellow mice crossed yellow and agouti mice develop in 2 : 1 ratio. The homozygous yellow condition does not survive due to lethality.
2. Dihybrid Inheritance and the Law of Independent Assortment
Dihybrid inheritance involves the inheritance of two pairs of contrasting characteristics (or contrasting traits) at the same time. The two pairs of contrasting characteristics chosen by Mendel were the shape and colour of seeds: round-yellow seeds, and wrinkled-green seeds (see Figures).
In order to trace the inheritance of two pairs of contrasting traits, Mendel crossed pea plants having round-yellow seeds with pea plants having wrinkled-green seeds and noted their occurrence in the succeeding generations of pea plants.
Mendel made the following observations:
Mendel first crossed pure-bred pea plants having round-yellow seeds with pure-bred pea plants having wrinkled-green seeds and found that only round-yellow seeds were produced in the first generation. No wrinkled-green seeds were obtained in the F1 generation. From this, it was concluded that the round shape and yellow colour of the seeds were dominant traits over the wrinkled shape and green colour of the seeds.
When the F1 generation pea plants having round-yellow seeds were cross-bred by self-pollination, then four types of seeds having different combinations of shape and colour were obtained in the second generation or F2 generation. These were round-yellow, round-green, wrinkled-yellow, and wrinkled-green seeds. Mendel collected a total of 556 F2 seeds and counted them shape-wise and color-wise. He got the following result:
Round-yellow seeds = 315
Round-green seeds = 108
Wrinkled-yellow seeds = 101
Wrinkled-green seeds = 32
The phenotypic ratio of different types of seeds can be written as:
Round yellow seeds : Round green seeds : Wrinkled yellow seeds : Wrinkled green seeds = 315 : 108 : 101 : 32 = 9 : 3 : 3 : 1
Thus, the ratio of each phenotype (or appearance) of the seeds in the F2 generation is 9 : 3 : 3 : 1. This is known as the dihybrid ratio.
Mendel observed that he had started with two combinations of characteristics in seeds: round-yellow and wrinkled-green, and two new combinations of characteristics had appeared in the F2 generation: round-green and wrinkled-yellow (see Figure). On the basis of this observation, Mendel concluded that though the two pairs of original characteristics (seed shape and colour) combine in the F2 generation they separate and behave independently in subsequent generations.
The results of the dihybrid cross enabled Mendel to formulate his second law of inheritance which is called the law of independent assortment. According to Mendel’s second law of inheritance: In the inheritance of more than one pair of traits in a cross simultaneously, the factors responsible for each pair of traits are distributed independently to the gametes.
Mendel started with round-yellow and wrinkled green pea seeds and found that two new combinations of characteristics, round-green and wrinkled yellow seeds, appeared in the F2 generation.
Explanation of Results of Dihybrid Inheritance
In the dihybrid cross, the parent plants having the phenotype round-yellow seeds have the factors of inheritance or gene combination RRYY (in which RR are the dominant genes for round shape whereas YY is the dominant gene for yellow colour). On the other hand, the parent plants having the phenotype wrinkled-green seeds have the factors of inheritance or gene combination rryy (in which rr are the recessive genes for wrinkled shape and yy are the recessive genes for green colour).
RRYY are the factors of inheritance or genes for the round-yellow seeds (these are dominant genes). On the other hand, rryy are the factors of inheritance or genes for the wrinkled-green seeds (which are recessive genes).
Keeping these points in mind, we can now show the dihybrid cross by drawing a chart as we did in the case of a monohybrid cross. The chart showing the dihybrid cross between pea plants having round-yellow seeds and wrinkled-green seeds is given below.
Round-yellow = 9; Round-green = 3; Wrinkled-yellow = 3; Wrinkled-green = 1
This result is the same as that obtained by Mendel through experiments.
An amazing thing about Mendel’s work is that he worked out the underlying rules of inheritance before any knowledge of DNA, chromosomes or genes became available. Let us answer one question now.
(a) What do the progeny of a tall plant with round seeds and a short plant with wrinkled seeds look like? Why?
(b) What happens when the F1 progeny obtained above are used to produce F2 progeny by self-pollination?
(a) The progeny of a tall plant having round seeds crossed with short plants having wrinkled seeds are all tall plants having round seeds. This is because the ‘tallness’ and ’round shape’ of seeds are dominant traits. On the other hand, the ‘shortness’ and ‘wrinkled shape’ of seeds are recessive traits.
(b) When F1 progeny are cross-bred by self-pollination, then we will get four types of progeny in the F2 generation. Of these four types of progeny, two types will have traits like parents and the other two will have new combinations of traits. Thus,
- Some F2 progeny will be tall plants with round seeds (9).
- Some F2 progeny will be tall plants with wrinkled seeds (3).
- Some F2 progeny will be short plants with round seeds (3).
- Some F2 progeny will be short plants with wrinkled seeds (1).
Please note that though Mendel studied the inheritance of characteristics by using plants (or rather pea plants) the rules for the inheritance of traits given by Mendel are also applicable to the inheritance of traits in animals (including human beings). Thus, human genetics follows Mendelian principles.
Across between two individuals of the same species containing two pairs of contrasting features of two characters may be called as dihybrid experiment. Individuals formed such cross are also hybrids for two characters or heterozygous for tivo characters.
Dihybrid Experiment and Law of Independent Assortment:
With the help of a monohybrid experiment, Mendel could be able to reveal the relationship between the alleles and formulate the principle relating to the transmission of alleles from one generation to the other. After this, to reveal the relationship among two or more pairs of alleles and their mode of inheritance Mendel carried out dihybrid experiments. In the dihybrid experiments plants having contrasting features for two characters are crossed with each other.
Dihybrid Experiments on Plants:
Mendel performed his dihybrid cross by taking two pairs of allelomorphic characters. He crossed a garden pea plant having yellow cotyledon (YY) and round seed (RR) with another variety having green cotyledon (yy) and wrinkled seed (rr).
In one of his experiments he crossed two pea plants showing contrasting features for two characters viz. colour of the cotyledon and shape of the seed. A plant homozygous for yellow cotyledon and round seed shape was crossed with plant having green cotyledon colour and wrinkled seed shape. From the cross between two plants as stated, progeny plants were produced in F1 generation when all plants showed the expression of yellow cotyledon colour and round seed shape. When the F1 plants were allowed to self-pollinate, in the F2 generation four types of progeny namely yellow round, yellow wrinkled, green round and green wrinkled plants were produced in the proportion 9 : 3 : 3 : 1.
According to the phenotypic ratio i.e., 9 : 3 : 3 : 1, where the 9/16 represents the proportion of the individuals, displaying both the dominant traits, secondly the first 3/16 represents the individuals displaying the first dominant trait and the second recessive trait; thirdly the second 3/16 represents those displaying the first recessive trait and second dominant trait and finally represents the homozygote displaying both recessive traits.
Determination of Phenotypic Ratio by Checker Board Method:
Phenotype and Genotype Ratio of F2 Generation:
Phenotypic Ratio = 9 : 3 : 3 : 1, Genotypic ratio = 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1
After analysis of the results of his dihybrid experiments on pea plants Mendel proposed the law of independent assortment. The law states that two or more pairs of alleles in an individual during formation of gametes are segregated independently and assort at random appearing in all possible combinations into the gametes.
Determination of phenotypic ratio with the help of branching method:
One dihybrid experiment may be considered a combination of two monohybrid experiments. The branching method may help us to combine the results of two monohybrid experiments as indicated below.
Test Cross and Determination of the Genotype:
In the case of monohybrid experiments as test cross is used for determining the genotypes of the progeny, so also this cross may be used in the case of dihybrid experiments to determine the genotype of the F1 or F2 progeny. As usual in test crosses, the F1 or F2 progeny is mated with plants showing only recessive features, and analysis of the phenotypes resulting from such mating may indicate the genotype of the F1 or F2 progeny. The F1 progeny plants showing yellow round features were crossed by Mendel both with parental yellow round and green wrinkled plants. The cross between F1 yellow round progeny and parental yellow round parent produced only yellow round progeny, but the cross between F1 yellow round and parental green wrinkled plant produced four types of plants yellow round, yellow wrinkled, green round, and green wrinkled plants in equal proportions as shown in the checker board.
The test cross carried out by Mendel in this case produced 55 yellow round, 49 yellow wrinkled, 51 green round, and 52 green wrinkled plants which with minor deviation indicated a phenotypic ratio of 1 : 1 : 1 : 1. Such a result indicated that the F1 yellow round plants were hybrids and that the F1 yellow round plants could produce four types of gametes as YR, Yr, yR and yr in equal proportion. These gametes being united with yr gamete from green wrinkled (yyrr) plant could give four types of progeny in equal proportion.
Dihybrid Experiment on Animals:
Experiment on Guineapig: Dihybrid experiment taking a guinea pig as a model may be shown with respect to black rough and white smooth features. The two features are related to hair colour and texture when black colour is dominant over white colour and rough is dominant with the smooth feature. Dihybrid cross in Mendelian Pattern may produce four phenotypes in F2 in 9 : 3 : 3 : 1 ratio as indicated below.
Therefore, F2 phenotypes and their ratio become 9 Black Rough : 3 Black smooth : 3 White Rough : 1 White smooth.
Analysis of Genotypes gives the following ratio:
1BBRR : 2BBRr : 2BbRR : 4BbRr : 1BBrr : 2Bbrr : 1bbRR : 2bbRr : 1bbrr
Law of Heredity Derived from Dihybrid Experiments:
Two or more pairs of contrasting factors or genes during gamete formation not only segregate independently but also assort at random appearing in all possible combinations in the gametes.
Principles and Laws Derived from Mendelian Experiments and their Critical Analysis:
The discoveries of Mendel are milestones in the field of inheritance of characters and an assessment of these discoveries are needed to be discussed. The Mendelian discoveries were based on the observation of his monohybrid and dihybrid experiments on pea plants and these are the Law of unit character, the Principle of dominance and recessiveness, the Law of segregation, and the Law of independent assortment. Out of these first three were derived from Mendel’s monohybrid experiment and the fourth was derived from his dihybrid experiment.
1. Law of Unit Character:
It states that each character of the organism is determined by a particulate factor or gene. It is not absolutely true because there are some characters that are polygenic meaning a character may be determined by more than one gene. For example, human body colour (skin colour) is polygenic by nature. Another example is pleiotropy which implies that one gene may influence more than one character. Further, there is also an example that the character of an individual may be solely dependant upon the environment, and the gene has nothing to do with this.
2. Principle of Dominance and Recessiveness:
Mendel would feel that if there are two contrasting features of a character, then one of the genes would be dominant and the other would be recessive. The factors or genes of these two characters would behave in a dominant or recessive fashion. For the expression of the dominant character presence of one gene would be sufficient, while for the expression of the recessive feature homozygous condition would be necessary. Therefore, pea plants could be produced by the gene combination TT or Tt but dwarf pea plants appeared only in the tt homozygous gene combination. This concept is partially true because sometimes a gene can be incompletely dominant and an intermediate expression comes in a heterozygous condition. Such a condition is found in evening primrose with respect to its flower colour.
3. Law of Segregation:
According to Mendel behind any character presence of a pair of factors (identical or contrasting) is necessary in an organism. During gametogenesis, these pair of genes are separated and each of them goes into a gamete. This means that pair of genes for a character are segregated during gamete formation. This law is true in all cases and because of this gametes become diploid in nature. During reproduction when male and female gamete unite, the diploid condition is achieved when the genes for a character come in pair promoting the expression of the character in the progeny.
4. Law of Independent Assortment:
This law could be derived from the dihybrid experiments of Mendel. The results of the experiment showed that a pair of contrasting features of two characters assorted independently forming four combinations of features in a proportion 9 : 3 : 3 : 1. Such a combination of characters may appear if the pairs of alternate factors or genes segregate independently so that they can appear in possible combinations in the gametes. The male and female gametes being united give the progeny types in 9 : 3 : 3 : 1 proportion. This law may be true for Mendelian characters in pea plants. But an independent assortment of the genes is impossible when the genes are. Genes are present over chromosomes in linear order and the genes present over a chromosome are linked together. The linked genes move together with chromosomes. Therefore, the homologous pairs of chromosomes of male or female organisms during gamete formation segregate independently but not the genes present over them.
How are Characteristics (or Traits) Transmitted to Progeny
Genes are responsible for the characteristic features (or traits) of an organism: plant or animal. The characteristics or traits of parents are transmitted to their progeny (offsprings) through genes present on their chromosomes during the process of sexual reproduction. This happens as follows.
Genes work in pairs. There is a pair of genes for each characteristic of an organism (one is the dominant gene and the other is the recessive gene). Each parent possesses a pair of genes for each characteristic on a pair of chromosomes. However, each parent passes only one of the two genes of the pair for each characteristic to its progeny through gametes. Thus, the male gamete and female gamete carry one gene for each characteristic from the gene pairs of parents (which are located on the pair of chromosomes). But when a male gamete fuses with a female gamete during fertilization, they make a new cell called a zygote with a full set of genes (on a full set of chromosomes). This zygote grows and develops to form a new organism having characteristics (or traits) from both the parents which it has inherited through genes.
The two genes (or pair of genes) responsible for a particular characteristic are always present on the corresponding positions of the pair of chromosomes. For example, in Figure, the two genes for the same characteristic (length of plant stem), are present on the corresponding positions of the pair of chromosomes. One gene of the pair is for ‘tallness’ and the other is for ‘dwarfness’. Please note that though the progeny inherits two genes (or a pair of genes) for each trait from its parents the trait shown by the progeny depends on which inherited gene is dominant of the two.
For example, if a pea plant progeny (or hybrid) inherits the gene for tallness (T) from one parent and the gene for dwarfness (t) from the other parent, then it will show the trait of ‘tallness’ and become a tall plant because the gene for tallness is dominant over the gene for dwarfness. So, although the gene for dwarfness (t) is present in all the cells of the hybrid plants, it does not show its effect (because it is a recessive gene). If, however, both the parent plants pass on one copy of each of the recessive genes for dwarfness (t) making the genotype (tt), then the traits of dwarfness will appear in the progeny plant.
Please note that the genes for ‘tallness’ and ‘dwarfness’ are not to be considered two different genes. They are just the two forms of the same gene that controls only one characteristic feature of a plant: the length of its stem. But there can be an increase in the length of the stem making the plant tall or a decrease in the length of the stem, making the plant dwarf.
How do Genes Control the Characteristics (or Traits)
A gene is the section of DNA on a chromosome that codes for the formation of a protein controlling a specific characteristic (or trait) of the organism. Suppose a plant progeny has the gene for the characteristic called ‘tallness’. Now, the gene for tallness will give instructions to the plant cells to make a lot of plant growth hormones. And due to the formation of excess plant growth hormones, the plant will grow too much and hence become tall. On the other hand, if the plant has the gene set for dwarfness, then fewer plant growth hormones will be produced due to which the plant will grow less, remain short and hence become a dwarf plant.
Just like plants, the characteristics (or traits) in animals (including human beings) are also transmitted from the parents through genes by the process of sexual reproduction. We will now give an example of the transmission of colour of hair from the parents (father and mother) to the child. Before we do that please keep in mind that black hair is a phenotype produced by the genotype HH or Hh. On the other hand, blonde hair (pale yellow hair) is a phenotype produced by the genotype hh. Let us give an example now.
A mother has black hair, the father has blonde hair (pale yellow hair), and the child has black hair (see Figures). This can be explained on the basis of the transmission of genes for ‘hair colour’ from the mother and father to the child as follows: Mother’s cell contains two genes HH for black hair. Both the genes HH are dominant genes, so the mother has black hair (see Figure). Father’s cells contain two genes (hh) for blonde hair. The two genes hh are recessive genes, so the father has blonde hair (or pale yellow hair) (see Figure). Now, during the process of reproduction, the mother transmits one of the dominant genes H for black hair to the child, and the father transmits one of his recessive genes h for blonde hair to the child. Due to this, the child has the genes Hh for her hair.
Mother’s cells contain two dominant genes HH for black hair, so she has black hair.
Father’s cells contain two recessive genes hh for blonde hair, so he has blonde hair.
The child has one dominant gene F1 for black hair (from mother) and one recessive gene h for blonde hair (from father), so her genotype is Hh and her phenotype is black hair.
Now, the gene H for black hair is the dominant gene but the gene h for blonde hair is the recessive gene. The dominant gene H for black hair shows its effect due to which the child has black hair (see Figure). The recessive gene h for blonde hair cannot show its effect in the presence of dominant gene H for black hair. Please note that the genes which dominate other genes are called dominant genes, and the genes which get dominated are called recessive genes.
We will now describe the inheritance of blood groups by the children from their parents. Please note that the gene which controls the blood groups is represented by the letter I. This gene has three different forms (called alleles) which are represented as IA, IB, and IO.
How Blood Groups Are Inherited
A person has one of the four blood groups: A, B, AB, or O. This blood group system is controlled by a gene that has three different forms denoted by the symbols IA, IB, and IO. The genes IA and IB show no dominance over each other, that is, they are codominant. However, genes IA and IB both are dominant over the gene IO. In other words, the blood gene IO is recessive in relation to genes IA and IB.
Although there are three gene forms (called alleles) for blood, any one person can have only two of them. So, the blood group of a person depends on which two forms of the genes he possesses.
- If the genotype (gene combination) is IAIA, then the blood group of the person is A. And if the genotype is IAIO even then the blood group is A (because IO is a recessive gene).
- If the genotype is IBIB, then the blood group of the person is B. And if the genotype is IBIO even then the blood group is B (because IO is a recessive gene).
- If the genotype is IAIB, then the blood group of the person is AB.
- If the genotype is IOIO, then the blood group of the person is O.
Let us solve one problem now.
A man having blood group A marries a woman having blood group O and they have a child. What will be the blood group of the child?
The answer to this question depends on whether the blood group A of the man has a gene combination IAIA or IAIO.
(i) When the blood group A has genotype IAIA: In this case the genotype of the man’s blood is IAIA and that of the woman’s blood is IOIO. So, the child will have blood group A (because the gene IA is dominant over gene IO).
(ii) When the blood group A has genotype IAIO: Here the genotype of man’s blood is IAIO and that of woman’s blood is IOIO. So, in this case, there is an equal chance that the genotype of the child’s blood can be either IAIO or IOIO. Due to this, there is an equal chance of the child acquiring blood group A or blood group O.
Just as the blood group is inherited by a child from its parents, in the same way, the sex of a child (boy or girl) is also inherited from the parents: mother and father. We will now describe the inheritance of sex by a child from the parents. Inheritance of sex is also known as sex determination. Please note that while discussing the determination of the sex of a child, we use letter symbols to describe whole sex chromosomes rather than individual genes. The sex chromosomes are XX for a female (girl) and XY for a male (boy)
Sex Determination in Humans
A person can have a male sex or a female sex. The process by which the sex of a person is determined is called sex determination. Genetics is involved in the determination of the sex of a person. This can be explained as follows.
The chromosomes which determine the sex of a person are called sex chromosomes. There are two types of sex chromosomes, one is called the X chromosome and the other is called the Y chromosome.
- A male (man or father) has one X chromosome and one Y chromosome [see Figure (a)]. This means that half the male gametes or half the sperms will have X chromosomes and the other half will have Y chromosomes.
- A female (woman or mother) has two X chromosomes (but no Y chromosomes) [see Figure (b)], This means that all the female gametes called ova (or eggs) will have only X chromosomes.
The sex chromosomes.
The sex of a child depends on what happens at fertilization: (a) If a sperm carrying an X chromosome fertilizes an ovum (or egg) that carries an X chromosome, then the child born will be a girl (or female). This is because the child will have XX combinations of sex chromosomes (see Figure).
Inheritance of sex in humans.
If a sperm carrying a Y chromosome fertilizes an ovum (or egg) that carries an X chromosome, then the child born will be a boy (or male). This is because the child will have an XY combination of sex chromosomes (see Figure).
Please note that it is the sperm that determines the sex of the child. This is because half of the sperms have X chromosomes and the other half have Y chromosomes. Thus, there is a 50 percent chance of a boy and a 50 percent chance of a girl being born to the parents. This is why the human population is roughly half male and half female.
From the above discussion, we conclude that if the father (man or husband) contributes an X sex chromosome at fertilization through his sperm, the baby born will be a girl. On the other hand, if the father (man or husband) contributes a Y sex chromosome at fertilization through his sperm, then the baby born will be a boy. This means that it is the sex chromosome contributed by the father (man or husband) which decides the sex of the baby to which the mother (woman or wife) will give birth. Thus, the father (man or husband) is responsible for the sex of the baby (boy or girl) which is born.
The belief that a mother (woman or wife) is responsible for the sex of her baby is absolutely wrong. In many ignorant Indian families, the mother (woman or wife) is held responsible for the birth of a girl child and unnecessarily harassed by her in-laws (sasural). Such people should understand that it is the husband who is responsible for the birth of a girl child (and not his wife). Moreover, a girl is no less than a boy.
In some of the animals, sex determination is also controlled by environmental factors. For example, in some reptiles, the temperature at which the fertilized egg is incubated before hatching plays a role in determining the sex of the offspring. It has been found that in a turtle (Chrysema picta), high incubation temperature leads to the development of female offspring (or female progeny). On the other hand, in the case of a lizard (Agama agama), high incubation temperature results in male offspring (or male progeny). In some animals, such as snails, individuals can change sex, indicating that sex is not determined genetically in such animals.