Question

In the following biochemical pathway, the D allele codes for a functional form of the enzyme D and the recessive allele d does not. The F gene alleles function in a similar manner with an active product produced by the dominant allele F and not by the recessive allele f. If two individuals, both heterozygous for the 2 genes, are crossed, what would be the expected phenotypic ratio of the resulting progeny?

1) 1:1:1:1
2) 3:1:1:3
3) 1:2:1:2
4) 9:3:3:1

Answer 1

The expected phenotypic ratio of the progeny resulting from the cross of two individuals, both heterozygous for two genes (DdFf x DdFf), is 9 dominant:3 dominant for one trait and recessive for the other:3 recessive for one trait and dominant for the other:1 recessive for both. This 9:3:3:1 ratio is derived from the Mendelian principles of independent assortment and dominance in dihybrid crosses.

Step-by-step explanation:

To solve for the expected phenotypic ratio of the progeny when two individuals heterozygous for two genes (D and F) are crossed, we must perform a dihybrid cross. Since each parent is heterozygous for both genes (DdFf), we need to set up a Punnett square to predict the offspring's genotypes. The gametes can be DF, Df, dF, and df. Crossing these will give us the following genotypic outcomes: DDFF, DDFf, DdFF, DdFf (these four are phenotypically similar), DdFf again, DdfF, ddFF, ddfF (again phenotypically similar), DdFf repeating from above, DdfF repeating, ddFf, ddfF (similar phenotypes), DdFf and DdfF repeating, dDFf, and finally ddFf and ddfF both repeating.

When we look at these outcomes and categorize them based on phenotypes, we see that only ddff will show a recessive phenotype for both genes, while Ddff, ddFf, and DdFf each show dominance for one gene and recessiveness for the other in any combination. Hence the phenotypes in the F2 generation will be:

• 9 showing dominance for both traits (DD or Dd for D, and FF or Ff for F),
• 3 showing dominance for D and recessive for f (ddF_),
• 3 showing recessive d and dominance for F (D_ff), and
• 1 showing recessive for both traits (ddff).

Therefore, the final answer in two line explanation in 300 words is that the expected phenotypic ratio for the progeny of two heterozygous individuals (DdFf x DdFf) is 9:3:3:1. This is due to the independent assortment of the alleles for each gene and the combinations of dominant and recessive alleles that express the corresponding phenotypes.