The chances of having an unaffected baby, when both parents have achondroplasia, are- (AIIMS Nov 2012)
First see general features of autosomal dominant disorders -
o Autosomal dominant disorders are manifested in heterozygous state.
o So, at least one of the parent should be affected to transmit the disease to child.
o Homozygous state does not exist as it is not compitable with life, i.e. homozygous fetus usually dies in utero.
o So, all the affected individuals will be heterozygous.
o If an affected person marries an unaffected one, i.e. if one of the parents is affected than every child has 50% of chance of having the disease and 50Yo of chance not having the disease.
Aa (affected parent) aa(normal parent)
AA Aa aa aa
Affected Affected not Affected Not affected
Note - a. represent normal allel and ‘A’represent abnormal allel.
o lf both the parents are affected than the chance of having unaffected baby is 25%.
AA Aa Aa aa
In this situation it is tricky to calculate the percentage of having affected baby.
Most of you must be thinking 75% (very easy).
But, it is not the case, as already explained homozygous state is not compitable with life and fetus dies in utero.
So, the chances of having affected baby will be 50% because 25% (AA) will die in utero.
Now see the question
o Achondroplasia is an autosomal dominant condition.
o Both the parents are affected.
o So, chances of having an unaffected baby are 25%