Question

A man with a specific unusual genetic trait (leading to heterozygosity) is
house with an unaffected and non-carrying woman, and they have four children. The pedigrees of
this family is shown in graphs a-e. For each inheritance mode indicate
how many children of each sex will express the trait (color those affected in each tree
genealogical according to the type of inheritance).

a) Autosomal recessive trait:
b) Autosomal dominant trait:
c) X-linked recessive trait:
d) X-linked dominant trait:
e) Trait linked to Y:

오모 모 오

Answer 1

Answer a) The man is heterozygous for this unusual genetic trait hence it can not be an autosomal recessive trait, as autosomal recessive trait can be expressed only in homozygous condition.

Answer b) If the trait is autosomal dominant then there are chances that 50% of the children will be affected out of 4

If the genotype of the man is considered as “Aa”(A is disorder causing allele)

And genotype of the woman is “aa”

Then the outcome of mating will be

gamates	A	a
a	Aa affected	aa Not affected
a	Aa affected	aa Not affected

The chances of a male or a female being affected are 50%

Answer c) If the trait is X-linked recessive

then let the genotype of man be XaY (Xa is disorder causing allele)

And the genotype of woman be XX

Then the outcome of mating will be

gamates	Xa	Y
X	XXa Not affected(carrier)	XY Not affected
X	XXa Not affected(carrier)	XY Not affected

In this case the females will be carriers(as the disorder is recessive) and the males will not be affected as the disorder is X linked and males get only Y chromosome from the father.

Answer d) If the trait is X-linked dominant

then let the genotype of man be XAY(XAis disorder causing allele)

And the genotype of woman be XX

Then the outcome of mating will be

gamates	XA	Y
X	XXA affected	XY Not affected
X	XXA affected	XY Not affected

In this case all the females will be affected(as the trait is dominant) and the males will be not affected as the disorder is X linked and males get only Y chromosome from the father.

Answer e) If the trait is Y linked

Then then let the genotype of man be XYA(YA is disorder causing allele)

And the genotype of woman be XX

gamates	X	YA
X	XX Not affected	X YA affected
X	XX Not affected	X YA affected

In this case all the females will not be affected and all the males will be affected as the disorder is Y linked and males get only Y chromosome from the father.

Note in answer a the condition is not possible as the recessive character is not expressed in the heterozygous condition, if the father would have been homozygous for the trait and the trait was recessive then none of the children would have been affected.