A recessive phenotype such as smooth chin is expressed only when the individual is:

1. A dominant allele produces its phenotype whether the individual has one copy or two, written Bb or BB. 2. A recessive phenotype appears only if the individual is homozygous for that recessive allele, written bb. 3. A heterozygous individual carries the recessive allele but expresses the dominant phenotype, which is what makes them a carrier. 4. In the worked cross, a heterozygous mother and a homozygous recessive father produce a fifty per cent chance of each chin type. _Source: OpenStax Psychology (1st ed., CC BY 4.0), Ch 3 "Biopsychology", §3.1 Human Genetics_