In humans, polydactyly (an extra finger on each hand or toe on each foot) is due to a dominant gene. When
one parent is polydactylous, but heterozygous, and the other parent is normal, what are the genotypic and
phenotypic ratios of their children?

The genotype ratio of their children is 2Ff : 2 ff

The phenotype ratio is 2 polydactyl individuals : 2 normal individuals.

Explanation: Let F represent the gene for extra finger while f represent the gene for normal finger. From the information provided, F is dominant while f is recessive. A heterozygous parent will have a genotype of Ff while a normal parent will have a genotype of ff. A cross between them will produce 2 Ff and 2 ff. Ff will manifest phenotypically as polydactyl because F is a dominant allele while ff will manifest phenotypically as normal fingers because f is recessive. For a recessive allele to express itself outwardly, it must occur in a homozygous state. Ff is heterozygous polydactyl.

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