Question

3.) Why are genes more than 50cM away from each other considered "unlinked?” 4.) If two genes are 75CM away from each other, they are not genetically linked, so how do we even know that they are 75CM away from each other?

Answer 1

3) suppose the A and B are two unlinked genes.

the alleles of A are A and a and alleles of B are B and b.

the first cross is

AABB * aabb

AB/ab (F1)

F1 is test crossed

AB/ab * aabb

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

if the genes are not linked all the phenotypes are expected in equal proportion, that is 25% each.

the parental type gametes have the same allele configuration as in the parent, the parent is AB/ab A and B alleles are on one chromosome and a and b alleles are on another chromosome, so parental gametes are AB and ab.

the parental progenies are AB/ab and ab/ab and recombinant progenies are Ab/ab and aB/ab.

if there are 100 progenies

AB/ab=25, Ab/ab=25, aB/ab=25, ab/ab=25

the recombination frequency= (number of recombinant progenies/total number of progenies )$\times$100

=((25+25)/100)$\times$100

= 50%

so if the genes are not linked recombination frequency is 50%,

recombination frequency in percentage is the distance between genes in map units.

so genes more than 50 cM apart are considered unlinked.