Linkage

Mendel’s law of independent assortment is applicable to the gene which are situated in separate chromosome but when two gene for different character are located in same chromosomes, they are tied to one another and they are said to be linked .They are inheritate together by the offspring and they will not be assorted independently .Thus the tendency of two or more genes of same chromosomes to remain together in the process of inheritance is called Linkage.

History of linkage

i.                  Coupling  and Repulsion Hypothesis

W.Batson and R.C Punnet(1906) for the first time reported 2 pairs of allele which do not follow the law of independent assortment. They crossed two Sweat Pea Plants having 2 pairs of contrasting character i.e. color of flower and shape of pollen grain. They crossed two pure varieties having blue flower and long pollen grain with red flower and round pollen grain.

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In F1 they found almost all the plant with blue flower with long pollen grain (BbLl).In the next experiment they test crossed the F1 with double recessive parent. Here the test cross was expected to give 4 F2 Phenotypes in the ratio 1:1:1:1 (Original dihybrid test cross ratio).But in their practical field the actual result showed the blue long ,blue round, red long and red round in the ratio of 7:1:1:7 .

This result of the test cross indicates that the parental combinations are 7 times more than the new combination. Here Bateson and Punnet concluded that the allele BL, bl which come from the same parent enter to the same gamete to be inherited together and the allele which come from different parent enter to different gametes. In this way according to them the allele coming from same parent which inherit together is due to a process called Genetic Coupling. But the other one where gene comes from different parent is due to a process called Genetic Repulsion.

ii. Morgan’s view of linkage

T.H Morgan (1910) obtained similar result in Drosophila melanogaster. According to his experiment the two Drosophila- one having grey body with long wings and the other with black body with vestigial wings. He found all the individuals in F1 with grey body with long wings. When he crossed the F1 male hybrid (Grey and Long) with recessive female parent  (Black and Vestigial) ,Only two types of offspring having  grey long and black vestigial variety were obtained in F2 in equal proportion. But the recombinant i.e. the new combination does not appear at all in F2. Here Morgan concluded that the grey body character is always inheritate is long wing character and similarly the black body character is always associated with vestigial wing.

Since only parental character and no new non parental character appears in F2 this show complete linkage and such type of complete linkage is found in Drosophila male.

In his next experiment Morgan has crossed the F1 female hybrid (grey long) with recessive male parent (black vestigial),the expected new combination i.e. grey vestigial and black long appeared along with the parental combination and the ratio become 7:1:1:7.The parental combination become 7 times more than the new combination. Here Morgan concluded that this type of appearance of new combination is due to incomplete linkage and such incomplete linkage leads to the process of crossing over. Thus, it appears that the situation of linkage for Drosophila is different in male and females.

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iii. Chromosomal Theory of Linkage

Morgan and Castle formulated the theory. According to this theory-

*The genes are arranged in a linear fashion in the chromosomes.

*Genes that show linkage are situated in same chromosome.

*The distance between the two linked genes indicates the strength of linkage. The genes which are closely located show strong linkage where as those are widely separated shows weak linkage.

*The linked genes always remain in their original combination during the course of inheritance.

*The Linked genes show 2 types of arrangement on the chromosome. If the dominant alleles are present on chromosome, the arrangement is also known as Cis-Arrangement of linked gene the dominant allele of one pair and recessive allele of second pair are present on one chromosome and vice-versa the arrangement is called Trans Arrangement of Linked gene.

Types of Linkage

There are 4 types of linkages.

1.  Complete linkage

2.  Incomplete linkage

3.  Inter chromosomal linkage

4.  Sex linkage

(1)Complete linkage

As per Morgan’s test cross experiment with F1 male hybrid and female recessive parental character in F2 only parental combination are formed without the appearance of any new combination. This indicates that the linked gene of main parent inherited together and the chromosome do not undergo any breakage. This is called complete linkage. But this phenomenon is very rare and only found in some insects. There is no independent Assortment between such linked genes.

(2)Incomplete linkage

In majority of the cases the homologous chromosomes undergo breakage and again reunite during gametogenesis. This interferes with the linkage of genes which could not remain together. So, that there is the exchange of chromosomal segment and it results in recombination of gene. Such a linkage is called Incomplete Linkage. It was observed by Morgan by test crossing the F1 female hybrid with parental recessive male character. Here in F1 parental combination appears in about 96% where as the new combination appears only in 4%.

(3)Inter Chromosomal linkage

This type of linkage occurs between the gene coming from different linkage group present an separate chromosome.

(4)Sex linkage

Genes present on Sex chromosome (either X or Y or both X and Y) are called Sex linked gene. The inheritance of such gene is called Sex linkage.

Linkage Group

All these gene which are linking together located in a single chromosome constitute a linkage groups. The total number of linkage group in an organism is equal to the number of chromosome pairs 4 linkage group in drosophila.

23 Linkage group in Human being

7 linkage group in sweat Pea (Lathyrus odoratus)

Factor effecting linkage

Different physiological as well as environmental factor effect the strength of linkage like-

i.                  Age of the animal- With increasing age the chances of crossing over is reduced and therefore strength of linkage increases.

ii.               Temperature - The increase of temperature the frequency of chiasma formation and therefore it also decrease the strength of linkage.

iii.            X ray- Exposure to X ray reduces the strength of linkage.

Significance of Linkage

The phenomenon of Linkage has great significance for living organisms because it is responsible for holding the parental characters together and therefore it reduces the possibility of variability of gametes. So it check the appearance of new characters.

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