Linked genes are linearly located on same chromosome they get separated if exchange (crossing over) takes place between them.
Strength of linkage α 1/distance between the genes it means if the distance between two genes is increased then strength of linkage is reduced and it proves that greater is the distance between genes the greater is the probability of their crossing over. Crossing over obviously disturbs or degenerates linkage. Linked genes can be separated by crossing over.
Factors effecting crossing over (c. o) –
(1) Distance increase = crossing over increase
(2)Temperature increase = crossing over increase
(3)X-ray increase = crossing over increase
(4)Age increase then crossing over decrease
(5)Sex, male crossing over decrease (crossing over totally absent in male drosophila)
Arrangement of linked genes on chromosome
The arrangement of linked genes in any dihybrid plant is two types-
(a) Cis –Arrangement :- When two dominant genes located on one chromosome and both recessive genes located on another chromosome such type of arrangement is termed as cis arrangement Cis arrangement is an original arrangement.
Two types of gamete can be produced in cis arrangement –(AB) and (ab)
(b) Trans arrangement :- When a chromosome bears on dominant and one excessive gene and another chromosome also possess one dominant and one recessive gene such type of arrangement is called trans arrangement. Trans arrangement is not an original form It is due to crossing over. Two types of gamete also formed in trans arrangement but it is different from Cis arrangement (Ab) and (aB)
Types of linkage:-
There are two types of linkage-
- COMPLETE LINKAGE:- linkage in which genes always show parental combination. It never forms new combination.
Crossing over is absent in it, such genes are located very close on the chromosomes. such type of linkage occurs very rare in nature e.g male drosophila, female silk moth.
- INCOMPLETE LINKAGE:- When new combination also appear along with parental combination in off springs this type of linkage is called incomplete linkage, the new combinations form due to crossing over.
Linkage group:- All the genes which are located on one pair of homologous chromosome form one linkage group. Genes which are located on homologous chromosomes are allelic so we consider one linkage group.
Linkage group = haploid no of homologous chromosomes
2n N Pair Linkage group
Pea 14 7 7 7
Maize 20 10 10 10
Drosophila 8 4 4 4
Barley 14 7 7 7
Mouse 42 21 21 21
Application of linkage
Distance can be identified by the incomplete linkage. Its unit is Centi morgan.
Genetic map/linkage map /chromosome map
In genetic map different genes are linearly arranged according to % crossing over (µDistance) between them. With the help of genetic map we can find out the position of a particular gene on chromosome. Genetic map is helpful in the study of genome.