How are dominant traits inherited?
• For one thing, if we use Huntington disease as a typical example, they are almost always of the P1 genotypes: P1 Hh x hh where H is the allele for Huntington disease and h is the normal allele. • That’s because it is very rare for a person with some medical disorder to marry a person with the same medical disorder. More often a person with a rare dominant disorder marries a person who does not have that disorder. • And that in turn is because most dominant disorders are very rare in the population (about 1 in 25,000 people). • If we look at the P1 cross, the heterozygous individual expresses the dominant trait. Thus this person produces two types of gametes, half with the dominant allele H and half without it and thus of h genotype in the haploid condition. • The other parent is homozygous normal, so all those gametes are h. • This gives us .50 fertilizations are Hh and .50 fertilizations are hh. We say the odds are 50% that a person with a dominant trait passes on the defect to a ch
