In a particular human family, John and his mother both have brachydactyly (a rare autosomal dominant allele causing short fingers). John’s father has Huntington disease (another rare autosomal dominant allele). John’s wife is phenotypically normal and is pregnant. Two-thirds of people who inherit the Huntington (HD) allele show symptoms by age 50, and John is 50 and has no symptoms. Brachydactyly is 90% penetrant.

What are the genotypes of John's parents?

Brachydactyly and Huntington are phenotypic diseases that develop when a person has one or two dominant alleles for these traits.

John's father has the Huntington disease, but not brachydactyly so his genotype is Hh/bb.

John's mother is a healthy women with brachydactyly, so her's genotype is probably hh/BB or hh/Bb.

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genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).