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.