Question

Assume that a cross is made between two organisms that are both heterozygous for a gene that shows incomplete dominance. What phenotypic and genotypic ratios are expected in the offspring?

1) 1:2:1
2) 9:3:4
3) 12:3:1
4) 9:6:1
5) 10:3:3

Answer 1

The expected genotypic and phenotypic ratios for offspring from a cross between two heterozygous organisms showing incomplete dominance are both 1:2:1. In the case of a recessive lethal allele expressed in utero, the genotypic ratio observed would be 2:1, as homozygous recessive individuals would not survive.The correct option is 1).

Step-by-step explanation:

When two organisms that are heterozygous for a gene that shows incomplete dominance are crossed, the expected genotypic ratio in the offspring is 1:2:1. This means that there is a 1 in 4 chance for homozygous dominant, a 2 in 4 (or 1 in 2) chance for heterozygous, and a 1 in 4 chance for homozygous recessive genotypes. Meanwhile, the phenotypic ratio will also be 1:2:1 because incomplete dominance results in the heterozygous phenotype being a blend of the dominant and recessive traits, creating a distinct third phenotype.

Using a Punnett square, the grid would show that for two heterozygous parents (Aa x Aa), the offspring possibilities are AA, Aa, aA, and aa. The heterozygous outcomes (Aa and aA) are phenotypically identical, thereby providing the 1:2:1 phenotype as well. This same reasoning applies for a recessive lethal allele expressed in utero, with the exception that homozygous recessive individuals will not survive, thus the genotypic ratio would be 2:1 because the expected 1:2:1 ratio would lose the 1 part representing the lethal homozygous recessive.