Question

Explain how a dihybrid test cross can be used to determine if the autosomal linkage exists

Answer 1

autosomal linkage means two genes on an autosome is linked to each other.

to test whether the genes are linked or not, we can use the dihybrid test cross, test cross is the cross with a homozygous recessive individual.

let the genes be A and B.

alleles of gene A are A and a

alleles of gene B are B and b.

AABB * aabb

AaBb

cross this dihybrid with aabb ( homozygous recessive individual, a and b are recessive alleles)

AaBb * aabb

	AB	Ab	aB	ab
ab	AB/ab	Ab/ab	aB/ab	ab/ab

if the genes are not linked all the genotypes are expected in equal proportion

AB/ab: Ab/ab: aB/ab: ab/ab=1:1:1:1

recombination frequency in percentage is the distance between genes in map units if the distance between genes is less the genes are closely linked to each other.

recombination frequency= ( number of recombinants/total number of progenies)100

here AB and ab gametes produced by the dihybrid are parental type gametes, these gametes have the same allelic combination in the chromosomes of the dihybrid, Ab and aB are recombinant gametes, formed by the recombination between the homologus chromosomes in the dihybrid, the progenies formed from the recombinant gametes are recombinant progenies.

Recombination frequency if the genes are not linked

If genes are not linked, the ratio is

AB/ab: Ab/ab: aB/ab: ab/ab=1:1:1:1

recombination frequency= ((number of Ab/ab + number of aB/ab)/total number ) 100

= ((1+1)/4)100

= 50%

if the recombination frequency is 50% then the genes are not linked, if the recombination frequency is very less than 50%, then the genes are unlinked, if the recombination frequency is closer to 50% but less than 50%, then we have to do a chi-square test to determine whether the genes are linked or not.