Question

HUNTINGTON’S DISEASE is an autosomal dominantly inherited, degenerative human disease of the nervous system. Individuals homozygous for this dominant gene almost always die as a fetus. The disease has no obvious phenotypic effects in a heterozygous individual, until a person is about 35-40 years old, well into child-rearing years. There is no known cure for this genetic disease.

E. Determine the results of a mating between two parents, both of whom will get Huntington’s disease.

1. Provide the Punnett square and the potential offspring phenotypes and genotypes.
2. What percentage of children from this mating will develop Huntington’s?
3. What percentage of children from this mating will carry at least one Huntington’s allele?

Answer 1

Since Huntington's Disease is a dominant disorder, individuals that have the dominant allele develop this disorder. Additionally, since individuals homozygous for this disorder die as fetuses, all individuals that are affected by this disorder will be heterozygous, that is they will have one dominant allele that causes Huntington's disease and one recessive allele. If the allele that causes Huntington's disease is noted as 'H' and the allele that is recessive is noted as 'h', then the genotype of both parents is Hh.

i) Punnett square for mating between these parents.

		Maternal Gametes
		H	h
Paternal Gametes	H	HH	Hh
h	Hh	hh

Potential Genotypes: HH, Hh, hh
Potential Phenotypes: Embryonic lethal(HH), Have Huntington's (Hh), don't have Huntington's (hh)

ii)

Individuals of the genotype Hh will develop Huntington's Disease. Out of all possible progeny, 50% will develop Huntington's, however, since HH individuals die as fetuses, 66% of children born will develop Huntington's.

iii)

Again, individuals that arry this allele in the heterozygous condition will develop the disorder, and therefore 66% of individuals born from this mating will carry the dominant, H allele.