Question

Albinism is determined by a recessive gene in man. The presence of albinism in 50% children born to a couple proves that

• A. Both parents are heterozygous for albinism.
• B. Father is homozygous normal and mother is heterozygous.
• C. Father is homozygous for albinism but mother heterozygous.
• D. Both are homozygous.

Answer 1

The correct option is C Father is homozygous for albinism but mother heterozygous.

Albinism is determined by a recessive gene in man. Only homozygous recessive (aa) individuals will be albino, both homozygous dominant (AA) and heterozygous dominant (Aa) individuals will have normal phenotype.
Cross 1 : Both parents are heterozygous for albinism:
Parent generation : Aa X Aa

Gametes : Aa Aa	A	a
A	AA normal	Aa normal
a	Aa normal	aa albino

Offspring phenotypic ratio = 3 normal : 1 albino

Cross 2 : Father is homozygous normal and mother is heterozygous.
Parent generation : AA (father) X Aa (mother)

Gametes : AA Aa	A	A
A	AA normal	AA normal
a	Aa normal	Aa normal

Offspring phenotype ratio = All normal

Cross 3 : Father is homozygous for albinism but mother is heterozygous.
Parent generation : aa (father) X Aa (mother)

Gametes : aa Aa	a	a
A	Aa normal	Aa normal
a	aa albino	aa albino

Offspring phenotype ratio = 1 normal : 1 albino; Option C is the correct answer.