genetics doubt - senorita18

[ArchivalUser]

A 33-year-old primigravid woman requests a consultation with a genetic counselor. She and her husband are both heterozygous for an autosomal dominant disorder that has 80% penetrance in all affected individuals. Which of the following is the probability that their child will be genotypically normal?
A. 20%
B. 25%
C. 40%
D. 60%
E. 80%


what if we were asked to calculate genotypically normal child??? so we also include the child who does has receive any mutant allele or calculate the probability for carriers and homozygous mutant child??

[ArchivalUser]

U have 3/4 chance of being affected in this case so probability of having an affected child is 0,75*0,8=0,6 so probabilitz of having a normal child is 0,4 i.e 40%

In a dominant disease onlz 1 allele is enough to express disease

Can u reformulate yur question plz im nt sure i understood?

[ArchivalUser]

yea so when u took the probability of 0.75, that actually includes tha carriers and hommozygous affected ryt? so if they ask to calculate phenotypically normal child we shouldnt include the normal one???


srry my ques was abt phenotypically normal, not genotypically

[ArchivalUser]

If the question specifies the disease and tells u that only homozygotes manifest it phenotypically then you don't include heterozygotes.
Also at times u have to take in account diseases where homoyzgotes die in utero like achondrolplasia so then u r dealing only with heterozygotes and normal genotypes

[ArchivalUser]

B
25%

That is GENOTYPICALY .

(phenotypically would be: 25% + 5% + 5% + homozygous autosomal dominant (which is small percent))

[ArchivalUser]

I think littleturtles calculations are correct and in autosomal dominant disease only a genotypically dominant child can be phenotypically normal too

[ArchivalUser]

This actually becomes an easy question, if you realize that they provided penetrance information just to confuse you. Penetrance only matters if we are asked about the phenotype (not genotype). Because by definition, penetrance gives us an estimation of how often phenotype matches the genotype, basically. If the question asks about the probability of any kind of genotype (disease or normal) we can completely ignore the info they provided on penetrance and just get the correct answer simply from a Punnet square. We we would only use penetrance in calculation if they asked about the probability of normal or disease phenotype.

[ArchivalUser]

So just construct a regular Punnet square and you will get 25%.

[ArchivalUser]

so for genotypically normal it should be 25% and for phenotypically normal it should be 40% .What do u think ?

[ArchivalUser]

Yup, exactly!