Question

What are linked genes? Which of Mendel’s laws does this violate? If you perform a test cross with a dihdybrid (hint: aabb X AaBb), what are the possible genotypes of the offspring and indicate which genotypes are parental and which ones are recombinant? If there are 1000 offspring, how many of each type of offspring would you expect if the genes are unlinked and how many of each type of offspring would you expect if there are 20 map units between the two genes. Explain your answer (again, cannot just give the numbers)

Answer 1

Linked gene are pair of genes which are present on same chromosome and inherited from parents to next generation together.

They violate the law of independent assortment of genes which states that  the allele of one gene separates independently of an allele of another gene.but this is not the case in linked genes where linked genes inherited together.

Case1: When genes are not linked both genes are assort independent of each other

Test cross parent genotype :aabb XAaBb

Gametes:    ab    AB, Ab, aB, ab

Progeny:

AaBb

Aabb

aaBb

aabb

parental genotype in offspring =AaBb and aabb

Recombinant genotype in offspring =Aabb and aaBb

Ratio of parental genotype offsprig to recombinant genotype offspring is 1:1

genotype ratio in offspring-

AaBb:Aabb:aaBb:aabb

1 : 1 : 1: 1

If total no of offspring is 1000 then  no. Of offspring of each genotype Is1000/4=250

Total parental genotype offspring=500

Total recombinant genotype offspring=500

Case2: When genes are with map distance20cM

Distance between genes=20cM

Recombinant frequency between two genes=20/100=0.2

RF=Total recombinant/total population

Total recombinant(Aabb+aaBb)=0.2x1000=200

Total parental(AaBb+aabb)=1000-200=800