complete linkage
Complete linkage is a linkage or grouping of genes on a chromosome
Fig.:Cross showing complete linkagewhich is not altered and is inherited as such from generation to
generation without any cross-over. In such cases, linked genes do not
separate to form new or non-parental combinations.
It is rare but has been reported in certain cases. for example a pure
breeding red eyed and normal winged female Drosophila (PV/PV) is
crossed with pure breeding purple eyed and vestigial winged male
fly (pv/pv). f 1 generation is red eyed and normal winged showing
that both the traits are dominant. f 1 hybrid males are test crossed
with purple eyed and vestigial winged females. The offspring were
only of two types, red eyed normal winged and purple eyed vestigial
winged in the ratio of 1 : 1. There was no crossing over (non-parental
combination) indicating that the linkage in male Drosophila was
complete.
Incomplete linkage
Incomplete linkage is the phenomenon of an occasional crossing over
between two homologous chromosomes so that one or more alleles
present in a linkage group are replaced by other alleles.
It produces both parental and recombinant individuals. The
percentage of each parental type is more than 25% while that of each
recombinant type is less than 25%, i.e., parental types are more than
50% of population while recombinant types are less than 50%.
For example, homologous pure breeding red eyed normal winged
female Drosophila (PV/PV) is crossed to homozygous pure breeding
purple eyed and vestigial winged (double recessive) male Drosophila
(pv/pv). f 1 flies are hybrid red eyed and normal winged.
a female hybrid or f 1 fly (PV/pv) is crossed with double recessive
Fig.:Cross showing incomplete linkagepurple eyed and vestigial winged male fly. 90.7% offspring were
found to be parental types (red eyed normal winged, purple eyed
vestigial winged) while rest were recombinant types (red eyed
vestigial winged, purple eyed normal winged).
The ratio comes out to be 9 : 1 : 1 : 8 while it should have been
1 : 1 : 1 : 1 in case of independent assortment and 1 : 1 in case of
complete linkage. This shows that the two genes did not segregate
independently of each other.
Linkage group
a linkage group is a linearly arranged group of linked genes which
are normally inherited together except for crossing over during cell
division. They act and move as a unit rather than independently.
because the two homologous chromosomes possess either similar or
allelic genes on the same loci, they constitute the same linkage group.
Therefore, the number of linkage groups present in an individual
corresponds to number of chromosomes in its one genome (all the
chromosomes if haploid or homologous pairs if diploid).
It is known as principle of limitation of linkage groups. fruitfly Drosophila melanogaster has four linkage groups (4 pairs
of chromosomes), human beings have 23 linkage groups (23 pairs of chromosomes), pea has seven linkage groups (7 pairs of
chromosomes).