Question

Assume that a bi-allelic locus controls a (a) codominant trait, and (b) homozygous recessive lethal trait.

Show a Mendelian monohybrid intercross, and the expected phenotypic ratios in the F2 in each case.

Answer 1

a) A codominant trait means both alleles get to express when present. Let's say the locus controls the fur patterns, A is for strips and B for spots. We have to make a parental cross first, this will use pure breeds, that is AA x BB:

	A
B	AB

Every individual in the F1 is AB genotype, that is both stripped and spotted. Now, let's see the F2:

	A	B
A	AA	AB
B	AB	BB

In the F2 we expected the following phenotype ratios:

Stripped: 0.25 (AA)

Spotted: 0.25 (BB)

Stripped and spotted: 0.5 (AB)

b) In this new case the homozygous recessive individuals die before reproducing, this tends to occur before birth or around birth in real cases. We have alleles A and a, let's propose AA genotypes express a strong legged animals, heterozygous Aa present short weak legs, and the homozygous aa do not survive.

We cannot make a pure breed cross in the parentals as the recessive homozygous is never available for breeding, we are going to use an heterozygous instead: AA x Aa

	A	a
A	AA	Aa

In the F1 we obtained 2 different genotypes and phenotypes, 0.5 and 0.5 in proportion. Now, the next cross:

	A	a
A	AA	Aa
a	Aa	aa

The phenotypic ratios in this case are going to be affected by the lethal trait, we are going to see offspring either strong legged or short legged, in a ratio of 1:2, that is 0.33 strong legged and 0.66 short legged. We excluded the aa occurrence, since we won't be able to see them in the offspring.