In humans, the gene for polydactyly (having extra fingers or toes) is dominant over the gene for the normal number of digits. If parents who are both
heterozygous for polydactyly have four children, how many of these children
would most likely have extra fingers or toes?

multiple allele inheritance is traits controlled by more than 2 alleles (ex: pigeons having 3 alleles of one gene govern their feather color).   so the answer is a.

the answer to the blank provided is “an autosomal gene that is required for the expression of genes on the x chromosome”.

testis-determining factor, abbreviated as td, which is more popularly known as sex-determining region y or sry protein, is a dna-binding protein or  gene-regulatory protein/transcription factor, encoded by the  sry gene. it is the gene responsible for the initiation of male gender determination in humans.