Question

A 1 : 1 phenotypic ratio in a test cross indicated that ______.

Options

• alleles are codominant

• dominant phenotype parent was heterozygous

• dominant phenotype parent was homozygous

• both parents were heterozygotes

Answer 1

A 1 : 1 phenotypic ratio in a test cross indicated that dominant phenotype parent was heterozygous.

Explanation:

A 1 : 1 phenotypic ratio in a test cross indicates that the parent showing the dominant phenotype was heterozygous. In a test cross, the individual with the dominant phenotype (but unknown genotype) is crossed with a homozygous recessive individual. If the dominant phenotype parent is heterozygous, the offspring will segregate into two phenotypes (dominant and recessive) in a 1:1 ratio. If the dominant parent were homozygous, all offspring would show the dominant phenotype, resulting in a 100% dominant phenotype ratio, not 1:1.