The chances of having an unaffected baby, when both parents have achondroplasia, are - (AI 08, AIIMS May 10)
a. First see general features of autosomal dominant disorders -
i. Autosomal dominant disorders are manifested in heterozygous state.
ii. So, at least one of the parent should be affected to transmit the disease to child.
iii. If an affected person marries an unaffected one, ie if one of the parents is affected than every child has 50% of chance of having the disease and 50% of chance not having the disease.
Note: ‘a’ represent normal allel and ‘A’ represent abnormal allel.
b. If both the parents are affected than the chance of having unaffected baby is 25%.
i. In this situation it is tricky to calculate the percentage of having affected baby.
ii. Most of you must be thinking 75%, (very easy).
iii. But, it is not the case, as already explained homozygous state is not compatible with life and fetus dies in utero.
iv. So, the chances of having affected baby will be 50% because 25% (AA) will die in utero.
c. Now see the question
i. Achondroplasia is an autosomal dominant condition.
ii. Both the parents are affected.
iii. So, chances of having an unaffected baby are 25%.