NEET Biology Notes Genetics concept of Genetics
concept of Genetics
Gregor Johann Mendel is called the Father of Genetics. Archibald Garrod is considered as the Father of Experimental Genetics. The term ‘Genetics’ was coined by W Bateson in 1905. It is the stucly of heredity and variations. Genetics is generally of two types:
- Forward genetics It involves initial study on the basis of phenotype, leading ultimately to the study off DNA sequencing comprising the gene for this phenotype.
- Reverse genetics It involves initial study with a DNA segment, the phenotypic effect of
which we do not know and then introduce this segment in some plant/animal to study its phenotypic effects.
Heredity Heredity (e.g. like begets like) is the transmission of characteristics from parent to offsprings. Such transmissible characters are called hereditary characters.
Variations These are differences in morphological, physiological, cytological and behavioural characters shown by the individuals of the same species, race and family. The main sources of variations are crossing over, chance combination of chromosomes during meiosis, fertilisation and mutations. Mutation is the ultimate source of variations.
Terms used in Inheritance Studies
The following terms are used in inheritance studies. Different genetic terminology are as follows :
- Allele One of two or more alternative forms of a gene.
- Diploid Having two sets of chromosomes, which are referred to as homologoues. Animals and plants are diploid in the dominant phase of their life cycles as are some protists.
- Dominant allele An allele that dictates the appearance of heterozygotes. One allele is said to be dominant over another if a heterozygous individuals with one copy of that allele has the same appearance as a homozygous individual with two copies of it.
- Gene The basic unit of heredity; a sequence of DNA nucleotides on a chromosome that encodes a polypeptide or RNA molecule and so determines the nature of an individual’s inherited traits.
- Genotype The total set of genes present in the cells of an organism. This term is often used to refer the set of alleles at a single gene.
- Haploid Having only one set of chromosomes. Gametes, certain animals, protists and fungi and certain stages in the life cycle of plants are haploid.
- Heterozygote A diploid individual carrying two different alleles of a gene on two homologous chromosomes. Most human beings are heterozygous for many genes.
- Homozygote A diploid individual carrying identical alleles of a gene on both homologous chromosomes.
- Locus The location of a gene on a chromosome.
- Phenotype The realized expression of the genotype; the observable manifestation of a trait (affecting an individual’s structure, physiology, or behaviour) that results from the biological activity of the DNA molecules.
- Recessive allele An allele whose phenotypic effect is masked in heterozygotes by the presence of a dominant allele.
- Checker board Punnett square ibis a square (Punnett; 1927) divided into smaller squares^ which shows the mathematical (probable) result of a cross, both phenotypic and genotypic. It is of three types, i.e. gametic, phenotypic and genotypic. Forked line or branching system is also used to know phenotypic and genotypic probabilities.
- Wild and mutant alleles It is the character, which was , originally present in the population ?ppd is usually most common and dominant, whereas recessive allele is less common and forms from wild allele through mutation. It is also called mutant allele. /
- Hybrid It is an organism produced after crossing two genetically different individuals, such process of obtaining hybrids is known as hybridisation.
- Reciprocal cross It is a cross involving two types of individuals, where the male of one type is crossed with female of the second type and vice-versa.
- Back cross It is performed between Ft hybrid and any one of its parents.
- Test cross When Ft hybrid is crossed with its recessive parent, it is called as test cross. The test cross ratio in monohybrid cross is 1 :1 and in dihybrid cross, ratio will be 1 : 1: 1 : 1.
- Monohybrid cross It is made to study the inheritance of a single pair of allele.
- Dihybrid cross It is made to study the inheritance of two pairs of factors or alleles of two genes. .
- Trihybrid ratio It is the ratio obtained in F2 -generation raised from a trihybrid cross followed by selfing or inbreeding of Fj individuals.
- Eight phenotypes are formed. The phenotypic ratio is 27 ; 9 : 9 : 9 : 3 : 3 : 3 :1.
- Pure Line It is a strain of genetically pure, true breeding individuals, which have been derived from a single self-fertilised homozygous ancestor.
- Genome It is the complete set of chromosome where every gene chromosome is represented singly as in gamete. A single genome is present in haploid cells, two in diploid cells and many in polyploid cells.
Mendel and his Experiments
Gregor Johann Mendel, an Austrian monk performed breeding experiments on the garden pea (Pisum sativum) and is credited with being the Father of Genetics.
Mendel used garden pea (Pisum sativum) for his experiments due to its well defined characters, hermaphroditism predominance of self-fertilisation and easy hybridisation and emasculation (i.e. removal of anthers before maturity).
Mendel’s success was mainly based on the fact that he considered a single character at one time.
Genes controlling seven traits (characters) in pea studied by Mendel are now known to be located on only four chromosomes out of seven.
Mendel’s work was rediscovered by Hugo de Vries (Dutch biologist) Carl Correns (German botanist) and Erich von Tschermak (Austrian botanist) in 1900.
Mendel proposed three laws of heredity which are as follows :
- Law of Dominance
It states the expression of only one of the forms of the parental traits in the Fj hybrid, i.e. the Fa-generation always display only one of the parental traits. He described that trait which was always seen in Fx-generation as dominant and the trait that disappeared as recessive.
- Law of Segregation
It states that members of eaqji pair of alleles of a gene separate when gametes are produced in meiosis. .
Mendel’s law of segregation is universal and without any exception.
It is also called law of purity of gametes or law of splitting of hybrids because gametes always remain pure and may carry either the dominant or the recessive factor but never both.
- Law of Independent Assortment
It states that pairs of alleles separate independently of each other during gamete formation.
When a dihybrid cross is made and the offsprings of Fj-generation are self-bred, dihybrid ratio is 9 : 3 : 3 : 1 (phenotypic ratio), where 9/16 individuals carry both the dominant traits, 3/16 first dominant and second recessive, 3/16 first recessive and second dominant, 1/16 carry both the recessive traits.
Deviation from Mendelism
There are many exceptions and deviations of Mendel’s laws such as:
- Mendel observed that one allele dominated over the other but incomplete dominance in 4 O’clock plant (Mirabilis jalapa) and snapdragon are some exceptions.
- Mendel observed only two alleles for a character but body colour of rabbit shows multiple allelism (i.e. the presence of more than two alleles for a gene).
- As per law of independent assortment, any two or more than two pairs of characters assort independently but linkage is exception for law of independent assortment.
- Incomplete Dominance
It is the exception of Law of dominance, where none of the two contrasting alleles is dominant. Thes expression of character in Fj-generation is intermediate or mixture of expression of two characters. This phenomenon can be observed in Mirabilis jalapa (4 O’clock plant) in which, as a result of hybridisation between white (rr) and red (RR) flowered plants, pink flowers are formed in Fa-generation and this event is known as incomplete dominance.
Incomplete dominance is also known as blending inheritance. First case of incomplete dominance or blending inheritance was reported in Mirabilis jalapa (4 O’clock plant) by Carl Correns (1903).
In some cases, both alleles are expressed at the same time, this results in a different phenotype in the heterozygous individual, e.g. the roan coat of horses (i.e. patches of two different colours on the skin) illustrate this. This phenomenon is known as codominance.
Multiple Allelism and Inheritance of Blood Group
Some traits have genes with more than two alleles. These are called multiple alleles. The human blood group gene provides an example of multiple allele as well as an interesting dominance relationship. There are three A, B, O blood group alleles, usually given the symbolsandandare codominant to each other, but are both dominant to
Inheritance of Blood Groups
Blood group A, B and O were discovered by Karl Landsteiner, whereas blood group AB was discovered by Von Decastello and Sturli.
A set of three multiple alleles present on the chromosome are responsible for four types of blood groups.
Some other examples of multiple allelism are:
- Colour loci in com.
- Skin colour in rodents.
- Eye colour in Drosophila.
- Self-incompatibility genes in some plants.
It was discovered by Landsteiner and Wiener (1940) in RBCs of rhesus monkey (Macaca rhesus).
Person with Rh factor is called Rh+ (Rh positive), Rh+ blood group is found in about 85% people and person without Rh factor is called Rh” (Rh negative), such type of blood , group is found in about 15% people. 0 Erythroblastosis foetalis or Haemolytic Disease T of the Newborn (HDN) occurs when the mother is Rh” and father as well as foetus is Rh+.
When a gene affects many aspects of a phenotype or controls several phenotypic characters, is called pleiotropic gene and this phenomenon is called pleiotropy.
Some examples of pleiotropy are:
- ‘Vestigial wing’ mutation of Drosophila not only controls the size and shape of the wings but also affects several other features including reduced fecundity.
- Gene for phenylketonuria in human beings also interferes with synthesis of melanin pigment.
- Sickle-cell anaemia.
Pleiotropic genes along with their main effect may also act as modifers for different genes and thus, have more than one effect.
Genes when acting individually have a small effect but that collectively produce a significant phenotypic expression are called polygenes, e.g. genes for height or weight.
Polygenes show polygenic inheritance or quantitative inheritance. As genes are present on different chromosomes, they will segregate independently during meiosis. That’s why polygenic inheritance becomes a genetic problem for dihybrid cross (i.e. for two genes) and possibly much more, if more than two genes are involved for a particular trait.