The Biology Topics of biotechnology involve using living organisms to develop new products or solve problems.
Polygenic Inheritance of Traits Like Eye Color and Skin Color
Meaning of Polygene:
A group of non-epistatic genes that collectively influence a phenotypic trait is known as polygene. Polygene is also known as quantitative gene or multiple factor and inheritance of the polygene is called multiple gene inheritance. Traits with polygenic determination are often called a quantitative character and such character is sharply different from monogenic or oligogenic character.
Characteristics of Polygenic Trait:
- Each polygenic character is controlled by several non-allelic genes each contributing to the expression of a phenotype.
- Polygenic characters in the population exhibit a continuous distribution.
- The effect of individual genes in the case of a polygenic character is not easily detectable.
- Polygenic characters may only be analyzed by statistical methods such as means, variances, covariance, etc. whereas the discontinuous characters may be analyzed by ratio.
- The environment may exert great influence on the expression of a polygenic character but the environment can influence very little a monogenic or oligogenic character.
- Polygenic characters may hardly be classified into clear-cut groups.
- Polygenic characters are expressed based on additive effects.
- Polygenic characters are contiguous and may be measured by the metric system.
- Considerable modification in the expression of a polygenic character may occur by environmental influence.
- The genotypic and phenotypic difference is very prominent in the case of polygenic character and therefore, the transmission of low grade.
Distribution of Polygenic Trait:
Phenotypic variations of the polygenic traits show a normal continuous distribution. Hence, the frequency of the phenotypes with relation to variables of the phenotypes if are plotted on a graph paper it gives a bell-shaped curve which the property of normal distribution. In a standard normal distribution mean+/- one standard deviation includes 68% of the whole distribution. Extreme values higher or lower in a normal distribution are rare distributions, while the medium values of the distribution are maximum. Therefore, normal distribution gives a bell shapes curve. The curve will be more smooth if the number of genes involved is increased.
Types of Polygenic Traits
Polygenic traits may be three types, namely metric or continuous trait, meristic trait, and threshold trait.
1. Metric or Continuous Trait:
The measures of a trait when showing a continuous distribution can be obtained in metric scale, it is called a metric or continuous trait. For example, human height, weight, intelligence, milk produced by a cow, etc. are examples of continuous traits.
2. Meristic Trait:
The measures of the trait when are obtained by whole numbers and if the values in the distribution of measures of the trait never come in fractional numbers the trait is called a metric trait. For example, grains produced in the ears of rice, litter produced by a dog, progeny produced by the couples in the population, etc., come under the meristic trait.
3. Threshold Trait:
The threshold trait represents the character that has two or few phenotypic expressions. Because these characters also are controlled by polygene, they are regarded as polygenic traits. For example, the twinning of castles, parthenogenesis in Turkey, incidence of a pathogenic disease, etc., are considered threshold traits.
Polygenic Inheritance in Living Organisms:
Mendel’s discovery clearly explained how the candidate genes and their alleles control the expression of many phenotypic characters. But the problem arises in explaining the inheritance of quantitative characters in living organisms. George Udny Yule (1905), a mathematician first proposed that continuous characters could be produced by the conjoined action of several genes. This idea was later authenticated by Herman Nilsson-Ehle in 1909 from his work on wheat and tobacco. Ronald Fisher (1918) demonstrated the inheritance of quantitative characters and he also argued that quantitative characters were also inherited following Mendelian pattern. In this regard, the experiments carried out by Nilsson-Ehle may be cited. He obtained several homozygous varieties of wheat that differed by their grain colour. In a cross when Nilsson Ehle used plants with white grains and plants with purple grains, he obtained a result in F2 generation that may be shown in the following manner.
The cross between white and purple kernelled wheat and its results:
Cross P generation – Plant with purple kernel × Plant with white kernel
F1 generation – Plant with Red Kernel on Selfing
F2 generation – 1/16 purple, 4/16 Dark Red, 6/16 Red, 4/16 Light Red, 1/16 White
Ratio – 1 : 4 : 6 : 4 : 1
Based on this observation he proposed that the phenotypic variations were the cumulative effect of two non-allelic genes when each locus contains two alleles. Suppose the genes are a+ and b+ and their alleles are a- and b-. In this system a+ and b+ are contributors to the red pigment and a- and b- cannot produce any pigment. Hence, the genotype of the purple kernelled plant was a+a+b+b+ and that of the white kernelled plant was a-a-b-b-. From a cross of plants with these genotypes progeny production may be shown in the following checkerboard. Purple kernelled plants with genotype a+a+b+b+ may reproduce gametes as a+b+ and white kernelled plants with genotype a-a-b-b- may produce a-b-. In the F1 progeny with the genotype a+b+/a-b- may be formed and the kernel colour appeared as red. Selfing of the F1 plants then produced F2 progeny, when purple, dark red, red, light red, and white coloured plants were produced as under.
Production of Various Types of Kernel in Wheat:
Let the a+ and b+ be the genes contributing pigment to the plants for colour generation and contribution from both genes may generate more colour. On the other hand when the genes are a- or b- no contribution for colour generation may come. Therefore, the presence of a- or b- means there is no contribution colour to the kernel. Further, colour production exhibits a distinct dose effect, because when the homozygous condition i.e., a+b+/a+b+ containing 4 doses of color-producing genes in the plant could develop purple colour and the plants having only a single dose of pigment-producing gene as a+b-/a-b-, a-b+/a-b-, a-b-/a+b- and a-b-/a-b+ could produce light red colour in the kernel in the plants.
The presence of three doses of the pigment-producing genes and the presence of two doses of pigment-producing genes produced dark red and red coloured kernels respectively. Hence, the cumulative effects of the genes appeared quite prominent in the experiment. This experiment of Nilsson-Ehle also showed a polygenic inheritance in kernel colour inheritance in wheat with a qualitative effect. Along with this a meticulous analysis of the inheritance of two pairs of non-allelic genes in this case clearly show that cumulative effects do not come in contradiction to the Mendelian pattern of inheritance of the genes in this case.
Cumulative Action of the Genes Involved in Developing Kernel Colour in Wheat:
Skin Colour in Human
In humans, skin colour may vary depending on the presence of melanin in the skin. The negros have very dark skin colour because of excessive melanism in the skin. On the other hand, the Europeans contain very little melanin in the skin and therefore, they are very white in appearance. Davenport working on black negros and white Europeans predicted that there are several genes that controlled the skin colour in humans. Three genes are considered as A, B, and C, then they also possess allelic forms as a, b, and c. The extremely black Negro contains homozygous conditions for genes A, B, and C. Therefore, a black Negro has the genotype ABC/ABC.
On the other hand, the white European has the genotype abc/abc with white skin color. Davenport found that when Negro male married a white European girl they produced children of intermediate skin colour which was called mulatto. But when two mulattoes marry their children appeared to be skin colour of different shades. Each of the hybrid mulattoes may produce gametes of eight different types and the gametic union may produce 64 different combinations. Among these progeny, seven different skin colours could be recognized and the skin colour of the progeny depended on the number of genes they inherited from their parents. The progeny skin colour varies from extremely black with phenotype ABC/ABC to pure white with genotype abc/abc. However, the progeny phenotypes appeared in the proportion indicated below.
- Pure black (AABBCC): 1/64
- Very dark brown: 6/64
- Dark brown: 15/64
- Mulatto (AaBbCc): 20/64
- Light brown: 15/64
- Very light brown: 6/64
- Pure white (aabbcc): 1/64
Cross between a Negro man and a white woman, the production of their progeny and expected progeny types in the F2 generation may be shown as under.
Different body colours are developed depending on the presence of dominant genes. Hence, the genotypes developing different skin colours may be indicated in the following table.
Showing additive effects of the genes:
|Number of Dominant Genes||Skin Colour||The ratio in F2 Generation|
|5||Very dark brown||6|
|1||Very light brown||6|
Dominant Genes Contributing to Skin Colour:
Note how the effects of the genes become additive in developing skin colour. It is evident that each dominant gene directs the synthesis of the same amount of melanin and contributes to the development of skin colour. The more dominant genes the more dark becomes the skin colour. While the recessive genes do not have a role in developing pigment and therefore, all the dominant genes show an additive effect. It is to be kept in mind that the genes denoted by capital letters are not truly dominant and so the genes denoted by small letters are not truly recessive in nature. The genes mentioned in capitals may produce pigment in the maximum amount and those denoted by small cases can produce a very minimum amount of pigment. Therefore, genotypes with small cases appear white in complexion. If the capital letters are considered as a complex ‘a’ and the small cases as another unit as ‘b’, the combinations formed by these genes may be shown by the binomial expansion (a + b)6.
The distribution of the various phenotypes also shows clear continuous nature in the population. In this pattern of distribution maximum number of individuals exhibit the intermediate skin colour (Mulatto), while the extreme of the features is represented by sufficiently less in number. Therefore, polygenic inheritance is based on six important assumptions as indicated below.
- Each of the genes involved in skin polygenic inheritance is involved in the expression of the character under concern.
- The contributing genes usually have a cumulative or additive effect.
- The genes associated with the expression of a character lack dominance and the alleles of a gene show intermediate expression.
- The genes involved are non-allelic and occupy different loci. There is no epistatic effect between the genes.
- The genes do not show linkage.
- Polygenic inheritance may cause a trait to have continuous variation which means the characteristic does not have discrete forms and instead varies gradually between extremes. The continuous variation shown by human traits includes height, weight, and skin colour.
In humans, there are atleast 3 genes coding for skin colour. Each skin colour gene has two forms. One form codes for high levels of melanin production and the other form codes for low levels of melanin production. Skin colour is the most visible of all human traits. In fact, it is so obvious, and the variation between people so striking, that it is not surprising that it has caused much interest and controversy. The colour of human skin is influenced by both internal and external factors but is primarily due to pigments. Like all characteristics of humans, the colour of our skin is controlled by genes and inheritance.
In conclusion, human skin colour is controlled by the complex interplay of many different genetic variants, most of which control the production and cellular arrangement of melanin and has also been molded by the environment in different regions of the world over 1000 years.
Differences between Monogenic and Polygenic Character:
|Monogenic Character||Polygenic Character|
|1. The character is determined by a single gene.||1. The character is determined by more than one gene.|
|2. Phenotypic features are usually two, but sometimes three.||2. Phenotypic features are many.|
|3. Variable characters are discrete.||3. Variable characters show continuity.|
|4. F2 phenotypic ratio becomes 3 : 1 or 1 : 2 : 1||4. F2 phenotypic distribution is difficult to detect.|
|5. Heterozygotes show one genotype.||5. Heterozygotes become variable and of many genotypes.|
Analysis of Mendel’s Laws of Heredity
Mendel’s laws of inheritance developed the base of the science of heredity. After the rediscovery of Mendelism in 1900, the laws of inheritance given by Mendel got a special emphasis. However, these could not influence the elites of the period between 1866 and 1900. Besides this, Mendel himself did not give much importance to his discovery for which his concepts did not get much scope to be evaluated in the Mendelian time period. Now we can realize that if Mendel’s discovery would not come, there would have been no improvement in the science of heredity.
Diploid animals and plants transmit genes from generation to generation following this principle of segregation, but the law of independent assortment is not universal in its application. However, the genes or alleles that do not come under the purview of linkage, may be assorted at random showing independent segregation. The genes that are present over a particular chromosome are the linked genes and such genes cannot exhibit independent assortment. However, the genes residing on different chromosomes may exhibit independent assortment. If Mendel would have encountered linkage, the law of independent assortment could probably not be discovered. Because of this reason, Mendel is often said to be lucky. The experimental material and the characters considered in hybridization kept him away from any sort of problem. The genes for the characters considered by Mendel were present on 7 chromosomes of the pea plant in such a way that they could be assorted at random with independent segregation.
In 1975 Blixt showed that the genes determining the colour of the flower and the colour of the cotyledon are present on 1st chromosome. On the other hand, genes determining the shape of the pod, the position of the flower, and the height of the vine are located on the 4th chromosome. Therefore, the genes of chromosomes I and IV are linked, but the genes are located so distantly over the chromosomes that during gamete formation they can be assorted independently. Hence, several genes for Mendelian features, in spite of being linked, were not influenced by linkage to prevent an independent assortment of the genes.
Reasons Behind Mendel’s Success
Mendel discovered the principles of inheritance were his success. The reasons behind his success are
- The experimental material, the pea plant taken by Mendel is easy to culture and its generation time is reasonably short.
- Characters with their phenotypic expressions considered in Mendelian experiments are very prominent and visibly detectable.
- The experimental protocols of Mendel were well planned and the experiments were carried out with great care.
- Mendel did a careful recording of the experimental results and he could always analyze those mathematically.
- Above all the characters considered by Mendel were not influenced by linkage.