Please explain in detail I dont understand A geneticist disc

Please explain in detail.. I dont understand.

A geneticist discovers two genes in the zebrafish genome that are 42 cM apart. A homozygous dominant fish is crossed with a homozygous recessive fish, and one of the F1 offspring is testcrossed. Which of the following is true of the F2 fish?

A. 42% have phenotypes like the parental fish.

B. 58% have phenotypes like the parental fish. Correct Answer

C. 21% came from gametes that had undergone crossing over between the two genes.

D. 58% represent recombinant offspring.

Solution

Two genes are 42 cM or map units (m.u) apart.

1 map unit or 1 cM = 1 % recombination

42 cM or m.u = 42 % recombination

So 42 % of the phenotypes will be recombinant

Remaining 58 % of the phenotypes will be parental

Example:

Two genes A and B

Homozygous dominant : AABB

Homozygous recessive : aabb

Cross between AABB x aabb produces the following genotype,

AABB - Parental genotype - parental phenotype

aabb - Parental genotype - parental phenotype

And the following are recombinants,

AaBb, AABb, aaBB, AAbb, AaBB