The disease galactosemia is an autosomal recessive trait. A man whose sister had galactosemia intends to start a family with a woman whose great-grandfather was galactosemic. They are worried about having a galactosemic child.
(assume no other relatives brought galactosemia-causing variants into the family)
What is the probability of their first child being affected with the condition?
The disease galactosemia is an autosomal recessive trait. Let GG denotes non galactosomic normal, Gg denotes non galactosomic but carrier and gg denotes galactosomic.
Since the man's sister is galactosomic, thus it can be said that the man is carrier of galactosomic as both his mother and father are carrier. Thus the man's genotype is Gg.
The female's great grandfather had the disease, thus the female can be carrier or normal. The genotype of female is Gg Or GG.
When female is Gg
G | g | |
G | GG | Gg |
g | Gg | gg (disease) |
The probability of their first child to be galactosomic is 1/4 = 0.25.
When female is GG-
G | G | |
G | GG | GG |
g | Gg | Gg |
The probability here is zero.
Thus
Male | Female | Probability of child having condition | |
Case 1 | GG (normal) | GG (normal) | Zero |
Case 2 | Gg (carrier) | Gg(carrier) | 1/4= 0.25 |
Case 3 | GG (normal) | Gg (carrier) | zero |
Cas e 4 | Gg (carrier) | GG (normal) | zero |
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