Linkage & Crossing OVER

Sem. 4 – paper 2– unit 2

Prof. Swatantrata M. Sharma

LINKAGE -

Definitions -

The tendency of two or more genes to stay together

during inheritence is known as linkage.

The tendency for certain genes to be inherited together

due to their presence on the same chromosomes is

known as Linkage.

Tendency for two or more non –allelic gene to be

inherited together, because they are located more or

less closely on the same chromosome known as

Linkage.

Linked genes do not show independent segregation.

Crossing over -

The phenomenon of recombinant of linked genes, which is

the result of exchange of homologous segment between

non –sister chromatins of homologous chromosomal pairs

known as Crossing over.

The exchange of genes between homologous

chromosomes, resulting in a mixture of parental

characteristics in offspring known as crossing over.

MECHANISM -

Theory – (Breakage & Reunion)

C.D. Darlington (1932) proposed this theory

It is an established model of crossing over by the

physical breakage & crosswise reunion of broken

chromatids during the pairing of homologous

chromosomes

Occurs at Prophase stage of meiosis.

Also known as Darlington’s Theory

Breaks take place in two homologous chromatids, followed by

reunion of parts which results in new combinations of genes.

The breakage & reunion among non sister chromatids must

essentially be based on breakage & reunion of DNA double helices.

In this process Endonuclease & DNA ligase enzyme plays

important role.

The hybrid DNA undergoes reorientation to form “X – Shaped ”

figure.

One end of X Rotates 180 degree and reunite.