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.

D.    Determine the results of a mating between a person who will get Huntington’s disease and one who will not develop this 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

Here there is mating between a heterozygous individual having allele for Huntington's disease and homozygous individual with no alleles for Huntington's disease.

h,h - alleles for homozygous recessive parent . ( presence of normal alleles)

H,h - alleles for heterozygous parent ( presence of Huntington's allele)

Given below is the punnet square for the above mentioned information:

	h	h
H	Hh	Hh
h	hh	hh

Genotypic variations: Hh, hh

Phenotypic variations: Neurodegenerative changes, no change

Percentage of children that can develop the disease = number of cases that can develop the disease divided by the total number of possibilities = 2/4 = 50%

Percentage of children that will carry at least one Huntington’s allele = number of cases having allele for Huntington's disease (H) divided by total number of possibilities = 2/4 = 50%