This document discusses linkage, which is the tendency of genes located physically close to each other on a chromosome to be inherited together. The closer the genes are, the greater the chance they will be inherited together. Linkage was first reported in 1906 and describes two phenomena: coupling, where dominant alleles are inherited together, and repulsion, where they are inherited separately. The document provides an example of linkage in maize, describing two experimental crosses and their progeny.

1. ATHIRA RAJAN
Athirarajanmt@gmail.com
LINKAGE AND ITS ESTIMATION

2. LINKAGE
✓Linkage is the close association of genes or other DNA sequences on the same
chromosome.
✓The closer two genes are to each other on the chromosome, the greater the
probability that they will be inherited together
✓The degree of strength of linkage depends upon the distance between the linked
genes
✓The phenomenon of linkage was first reported by Bateson and Punnet in 1906
✓T.H Morgan put forth the theory of linkage and concluded that coupling and
repulsion were two phases of single phenomenon,linkage

3. ✓Exceptions to Mendel’s law of independent assortment were discovered
in 1906 by Bateson and Punnet
✓They found that two pairs of alleles in sweet peas did not assort
independently
✓When the two alleles A and B came from the same parent (AABB x aabb),
they tend to enter the same gamete and were transmitted together- coupling
✓when the same allele came from different parents (AAbb x aaBB) they tend
to enter different gametes and remain apart - Repulsion

4. COUPLING
✓The two alleles of two linked genes are said to be in coupling phase if the
two dominant alleles are located in one chromosome and recessive alleles in
its homologous chromosome (AB/ab)
REPULSION
✓A double heterozygote is said to be in repulsion phase when the dominant
alleles are located one in each of the homologous chromosomes (Ab/aB)

6. EXAMPLE
✓In maize, coloured pericarp and full endosperm are dominant, Two
experimental results are given below:
a) Pure breeding parents: Coloured, full endosperm x Colourless shrunken
F1 Coloured, full x test cross
b) Pure breed parents: Coloured, shrunken x Colourless, full
F1 Coloured, full x test cross

7. EXPERIMENT (A)
COLOURED
:COLOURLESS
PROGENIES
CHARACTERS
EXPERIMENT (A) EXPERIMENT (B)
Coloured, full 4032 639
Coloured, shrunken 149 21379
Colourless, full 152 21906
Colourless, shrunken 4035 672