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A recessive phenotype such as smooth chin is expressed only when the individual is:
AHomozygous for the recessive allele
BHeterozygous for the two alleles
CHomozygous for the dominant allele
DCarrying a new spontaneous mutation
Answer & Solution
Correct answer: A. Homozygous for the recessive allele
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_
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