Huntington's Disease Inheritance: What If One or Both Parents Are Affected?

A Simple Guide to Understanding Genetic Risk with Punnett Squares

1. If only one parent is affected

A. Inheritance When One Parent Is Affected with Heterozygous Huntington’s Disease (Hh)

Mother: Heterozygous for the dominant HD gene → genotype Hh

(H = dominant mutant allele causing Huntington's disease, h = normal allele)

Father: Homozygous recessive (normal) → genotype hh

Punnett Square:         

	h (father)	h (father)
H (mother)	Hh	Hh
h (mother)	hh	hh

Offspring outcomes:

50% chance (Hh) → Inherits Huntington's disease

50% chance (hh) → Does not inherit the disease

B. Inheritance When One Parent Is Affected with Homozygous Huntington’s Disease (HH)

Mother: Homozygous for the dominant HD gene → genotype HH

(H = dominant mutant allele causing Huntington's disease, h = normal allele)

Father: Homozygous recessive (normal) → genotype hh

Punnett Square:         

	h (father)	h (father)
H (mother)	Hh	Hh
H (mother)	Hh	Hh

Offspring outcomes:

100% chance (Hh) → Inherits Huntington's disease

2. If both parents are affected

A. Inheritance When Both Parents are Affected with Heterozygous Huntington’s Disease (Hh)

Mother: Heterozygous for the dominant HD gene → genotype Hh

(H = dominant mutant allele causing Huntington's disease, h = normal allele)

Father: Heterozygous recessive (normal) → genotype Hh

Punnett Square:         

	H (father)	h (father)
H (mother)	HH	Hh
h (mother)	Hh	hh

Offspring outcomes:

50% chance (Hh) → Inherits Huntington's disease

25% chance (HH) → Inherits Huntington's disease

25% chance (hh) → Does not inherit the disease

B. Inheritance with one parent with Heterozygous Huntington’s Disease (Hh) and the other with Homozygous Huntington’s Disease (HH)

Mother: Homozygous for the dominant HD gene → genotype HH

(H = dominant mutant allele causing Huntington's disease, h = normal allele)

Father: Heterozygous recessive (normal) → genotype Hh

Punnett Square:         

	H (father)	h (father)
H (mother)	HH	Hh
H (mother)	HH	Hh

Offspring outcomes:

50% chance (Hh) → Inherits Huntington's disease

50% chance (HH) → Inherits Huntington's disease

C. Inheritance When Both Parents are Affected with Homozygous Huntington’s Disease (HH)

Mother: Homozygous for the dominant HD gene → genotype HH

(H = dominant mutant allele causing Huntington's disease, h = normal allele)

Father: Homozygous recessive (normal) → genotype HH

Punnett Square:         

	H (father)	H (father)
H (mother)	HH	HH
H (mother)	HH	HH

Offspring outcomes:

100% chance (HH) → Inherits Huntington's disease (likely earlier onset and more severe)