Question

35. The following is a cross between two individuals with the genotypes as indicated for four unlinked autosomal genes.

Aa Bb Cc dd X aa BB Cc Dd

1. For each gene, assume the capital letter indicates a dominant allele and the lower case letter (small) indicates a recessive allele. What proportion of the progeny would have the following overall phenotype: recessive phenotype for A, dominant phenotype for B, dominant phenotype for C, and recessive phenotype for D?

2. Now assume that, unlike part 1, the two alleles of gene B show incomplete dominance and that the two alleles of gene C also show incomplete dominance. Like in part 1, for the alleles of genes A and D the capital letter is dominant and the lower case (small) letter is recessive. In this situation, what would be the proportion of the progeny with the following overall phenotype: recessive phenotype for A, dominant phenotype for B, dominant phenotype for C, and recessive phenotype for D?

3. Explain the differences between the terms dominant, incompletely dominant, and co-dominant.

Answer 1

Parental genotypes- Aa Bb Cc dd X aa BB Cc Dd

1. Desired pheotype- recessive phenotype for A i.e. aa, dominant phenotype for B i.e. BB or Bb, dominant phenotype for C i.e. CC or Cc, and recessive phenotype for D i.e. dd.

Now, from the cross between parental A gene i.e.

Aa X aa

Constructing the Punnett square-

Gametes	A	a
a	Aa	aa
a	Aa	aa

From the cross the chance of getting aa genotype i.e. the recessive phenotype is= 2/4=1/2

Similarly, from the cross between parental B gene i.e.

Bb X BB

Constructing the Punnett square-

From the cross the chance of getting BB & Bb genotype i.e. the dominant phenotype is= 4/4=1

Similarly, from the cross between parental C gene i.e.

Cc X Cc

Constructing the Punnett square-

From the cross the chance of getting CC & Cc genotype i.e. the dominant phenotype is= 3/4

Similarly, from the cross between parental D gene i.e.

dd X Dd

Constructing the Punnett square-

From the cross the chance of getting dd genotype i.e. the recessive phenotype is= 2/4=1/2

So, the proportion of the progeny with the desired phenotype=chance of recessive phenotype for A*chance for dominant phenotype for B*chance for dominant phenotype for C*chance for recessive phenotype for D= 1/2*1*3/4*1/2=3/16 (Ans)

2. Now two alleles of both gene B and gene C show incomplete dominance. So, the heterozygous genotype i.e. Bb and Cc will show a different phenotype from the dominant phenotype.

Desired pheotype- recessive phenotype for A i.e. aa, dominant phenotype for B i.e. BB, dominant phenotype for C i.e. CC, and recessive phenotype for D i.e. dd.

Now, from the cross between parental A gene i.e.

Aa X aa

Constructing the Punnett square-

Gametes	A	a
a	Aa	aa
a	Aa	aa

From the cross the chance of getting aa genotype i.e. the recessive phenotype is= 2/4=1/2

Similarly, from the cross between parental B gene i.e.

Bb X BB

Constructing the Punnett square-

From the cross the chance of getting BB genotype i.e. the dominant phenotype is= 2/4=1/2

Similarly, from the cross between parental C gene i.e.

Cc X Cc

Constructing the Punnett square-

From the cross the chance of getting CC genotype i.e. the dominant phenotype is= 1/4

Similarly, from the cross between parental D gene i.e.

dd X Dd

Constructing the Punnett square-

From the cross the chance of getting dd genotype i.e. the recessive phenotype is= 2/4=1/2

So, the proportion of the progeny with the desired phenotype=chance of recessive phenotype for A*chance for dominant phenotype for B*chance for dominant phenotype for C*chance for recessive phenotype for D= 1/2*1/2*1/4*1/2= 1/32 (Ans)

3. In case of the dominant, one allele i.e. the dominant allele is completely dominant to the other allele i.e. the recessive allele. The dominant allele masks the effect of the recessive allele. So, in heterozygous condition the dominant allele expresses masking the recessive allele.

In case of incompletely dominant, one allele i.e. the dominant allele is incompletely dominant to the other allele i.e. the recessive allele. The dominant allele doesn't completely mask the effect of the recessive allele and both alleles are partially expressed resulting in an intermediate phenotype. As a result in heterozygous condition, an inbetween phenotype is expressed which different from both the dominant and recessive phenotype.

In case of codominant, both the alleles are completely expressed and a third phenotype is produced different from both allelic phenotypes. In heterozygous condition both alleles are expressed.