Question

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

1) Aa Bb Cc dd X aa BB Cc Dd

let`s divide this into four different crosses

Aa * aa

	A	a
a	Aa	aa

probability of getting recessive phenotype for A= probability of getting aa= number of aa/total number= 1/2

Bb * BB

	B	b
B	BB	Bb

probability of getting dominant phenotype for B= probability of getting progeny with B allele= number of progeny with B allele/total number=2/2=1

Cc* Cc

	C	c
C	CC	Cc
c	Cc	cc

probability of getting dominant phenotype for C= probability of getting progeny with C allele= number of progeny with C allele/total number=2/3

Dd * dd

	D	d
d	Dd	dd

probability of getting recessive phenotype for D= probability of getting dd=number of dd/total number=1/2

probability of getting recessive phenotype for A, dominant phenotype for B, dominant phenotype for C, and recessive phenotype for D = probability of getting recessive phenotype for A probability of getting dominant phenotype for B probability of getting dominant phenotype for C probability of getting recessive phenotype for D=1/212/31/2=1/6

2) in incomplete dominance the heterozygotes shows intermediate phenotype compared to homozygotes, so heterozygotes do not show dominant phenotype.

Aa * aa

	A	a
a	Aa	aa

probability of getting recessive phenotype for A= probability of getting aa= number of aa/total number= 1/2

Bb * BB

	B	b
B	BB	Bb

probability of getting dominant phenotype for B= number of progeny with BB genotype/total number=1/2

Cc* Cc

	C	c
C	CC	Cc
c	Cc	cc

probability of getting dominant phenotype for C= probability of getting progeny with CC = number of progeny with CC genotype/total number=1/4

Dd * dd

	D	d
d	Dd	dd

probability of getting recessive phenotype for D= probability of getting dd=number of dd/total number=1/2

probability of getting recessive phenotype for A, dominant phenotype for B, dominant phenotype for C, and recessive phenotype for D = probability of getting recessive phenotype for A probability of getting dominant phenotype for B probability of getting dominant phenotype for C probability of getting recessive phenotype for D=1/21/21/41/2=1/32

3) dominant, One allele is dominant to the other so homozygous dominant and heterozygotes shows dominant phenotype.

incomplete dominant:one allele is incompletely dominant to another so heterozygotes show incomplete phenotype compared to homozygotes.

co-dominance: both alleles are expressed in the heterozygous condition.