Law of independent assortment suggests that when two or more than two factors are considered together, each member of a pair of homologous chromosomes segregates during meiosis independently of the members of other pairs, so that alleles carried on different chromosomes are distributed randomly to the gametes. For example Plants having round and yellow seeds (RRYY) crossed with plants having wrinkled green(rryy) seeds, all the F1 plants have the genotype RrYy. When these are selfed, , the factors for the two characters assort or separated independently of each other. R may enter the gamete either withYor y and r also may ener the gamete with Y ory. Thus four types of gametes are formed :- RY, Ry, rY and ry are formed,which can recombine in 16 ways to produce 9 genotypes and four phenotypes in the F2 generation.
Dihybrid F2 ratio of 9 : 3 :3 : 1 and testcross ratio of 1 : 1: 1: 1 in diploid plants are expected on this general principle of independent assortment.
Some genetic terms:-
- Allele or allelomorph :- One of two or more alternative forms of a gene at a given position (locus) on a chromosome, caused by a difference in the sequence of DNA.Example In a plant that occurs in tall and short forms, there may be an allele that tends to produce tall plants and an alternative allele that produces short plants.
- Homozygous :-An plants with two identical alleles for a character is homozygous for that character.
- Heterozygous:- Plants with two different alleles for a character is heterozygous for that character.
- Phenotype :- Physical or chemical expression of a characteristic.
- Genotype :- Composition of characteristics in terms of alleles. Or a description of its genetic makeup is its genotype.
Linkage:- In every plants, there are present thousands of genes. The number of genes is more as compared to the number of chromosomes e.g., pea. It is clear that every chromosome possesses more than one gene. In 1903, William Sutton had expressed the possibility that every chromosome must have more than one unit factors.
Linkage may be complete, incomplete, or absent (not detectable), depending upon the distance between linked genes in a chromosome.
W. Bateson, Saunders and Punnett (1909) during experiments, found that the result obtained from a cross in sweet pea show a deviation from law of independent assortment
They were crossed the plant Lathyrus odoratus (sweet pea). In this plant, purple colour of flower is dominant over red colour and long pollen are dominant over round pollen. The purple flowers (B) and long pollen (L) were crossed with red flowers (b) and round pollen (l), in F1 generation the plants (BbLl) produced purple flower and long pollen, as expected.These plants were crossed with plants having red flowers and round pollen (bbll)
The resulting genotypes, and their actual and expected numbers under independent assortment, were as follows:
In F2 generation 1 : 1 : 1: 1 ratio was expected after testcross but 7 : 1 : 1 : 7 ratio was actually obtained. This indicates that there is a tendency in dominant alleles to remain together. And similar is the case with recessive alleles.This deviation was, termed as coupling by Bateson. It was also observed that when two such dominant alleles or two recessive alleles come from different parents, they tend to remain separate and was termed as repulsion.When the plants with purple flowers and round pollen (Bbll) were crossed with red flowers and long pollen (bbLL). The results of testcross were similar to that coupling phase 1 : 7 : 7 : 1 ratio instead of expected 1 : 1 : 1 : 1. Therefore Bateson and co-workers coined two words as coupling and repulsion. Bateson and Punnett could not explain the exact reasons of coupling and repulsion, and it was T.H. Morgan who while performing experiments with Drosophila, in 1910, found that coupling or repulsion was not complete.
Linked genesBy studying the inheritance of characters in the fruitfly Drosophila, TH Morgan and colleagues (1910) determined that genes are not completely independent as Mendel had thought, but that they tend to be inherited in groups. Since independent assortment does not occur, a dihybrid cross following two linked genes will not produce and F2 phenotypic ratio of 9:3:3:1. They observed that genes in the same chromosome are often transmitted together as a group, but that this was not always so and that ‘crossing-over’ between chromosomes could occur to disrupt these linkage groups. Genes that are present on the same chromosome, and that tend to be inherited (transmitted to the gametes) together, are termed linked genes because the DNA sequence containing the genes is passed along as a unit during meiosis. The closer that genes reside on a particular chromosome, the higher the probability that they will be inherited as a unit, since crossing over between two linked genes is not as common.The genes present on same chromosome, thus, would not assort (separated) independently. Such type of genes are called linked genes and this phenomenon is called linkage.
Morgan defined linkage as follows: "that the pairs of genes of homozygous parents tend to enter in the same gametes and to remain together, whereas same genes from heterozygous parent tend to enter in the different gametes and remain apart from each other. He further stated that the tendency of linked genes remaining together in original combination is due to their location in the same chromosome ".According to him the degree or strength of linkage depends upon the distance between the linked genes in the chromosome.
Chromosomes Theory of Linkage:- Morgan along with castle formulated the chromosome theory of linkage which is as follows :-
- The genes which show the phenomenon of linkage are situated in the same chromosomes and separated during the process of inheritance.
- The distance between the linked genes determines the strength of linkage. The closely located genes show strong linkage than the widely located genes which show the weak linkage.
- The genes are arranged in linear fashion in the chromosomes.
Types of linkage :- Linkage may be complete, incomplete, or absent (not detectable)linkage, depending upon the distance between linked genes in a chromosome.
Linkage groups :- The group of linked genes that are located on the same chromosome, called linkage groups.Because, all the genes of a chromosome have their identical genes (alleomorphs) on the homologous chromosome, therefore linkage groups of a homologous pair of chromosome is considered as one. In any species, the number of linkage groups is equal to the number of pairs of chromosomes. e.g. Corn (Zea mays) has 10 pairs of chromosomes and 10 linkage groups.
Arrangement of genes