How, any explanation pls pls... - comebackhero

[ArchivalUser]

Phenylketonuria is an autosomal recessive disease. What is the probability that the asymtomatic brother or sister of a child with disease is a heterozygous carrier?

a) 0
b) 1/2
c) 1/4
d) 2/3
e) 3/4

Ans is D , But how? plss....

[ArchivalUser]

is their parents heterozygous carriers???

[ArchivalUser]

normally :

AA : 1/4

Aa : 1/2

aa : 1/4


symptomatic : 1/4

asymptomatic : AA,aA,Aa : 3/4

IN asymptomatic : heterozygous is 2/3 ,homozygous is 1/3 , so answer is D

[ArchivalUser]

25%=affected
25%=normal (asympyomatic)
50%=carrier(asymptomatic)
25%+50%=75%,,,,,,,,so 50% out of 75%=2/3

[ArchivalUser]

thanks guysss

[ArchivalUser]

Aa(father)

Aa(mother) AA Aa
Aa aa

AA=NORMAL=25% (asympyomatic)
Aa=heterozgus carrier=50% (asymptomatic)
aa=25%=affected

25%=AA=NORMAL ASYMPTMATIC +50%Aa ASYMPTOMATIC CARRIER=75%
SO 50%(ASYMP. CARRIER) OUT OF 75%(ALL ASYMP,)=2/3

[ArchivalUser]

thank u very much everybody: GL