A parent is homozygous and a parent heterozygous for an autosomal recessive gene. What will be the outcome- (LQ)
|A||75% children affected|
|B||No child affected, but all are carriers|
|C||50% children affected, rest are carriers|
|D||25% children affected, rest are carriers|
50% children are affected, Rest are carriers
Autosomal recessive disorders
o Autosomal recessive disorders are manifested in homozygous state,
i.e. when both alleles at a given gene locus are mutant.
o There may be following five situations -
1. One parent is carrier and the other is normal + 50% of child will be carrier.
Aa Aa aa aa
2. Both the parents are carrier + 25% children will be affected, 50% will be carrier and 25% will be normal.
AA Aa Aa aa
3. One parent is affected and other is carrier 50% of child will be affected, 50% will be carrier.
AA AA Aa Aa
4. Both parents are affect + 100% children will be affected.
AA AA AA AA
5. One parent is affected and one is normal All will be carrier
Aa Aa Aa Aa
Note - I have given the percentage of genotypically affected children.
o Affection of phenotype will be different because all carrier will be phenotypically normal (as autosomal recessive disease is manifested only in Homozygous state).
o So in 1st situation all child (100%) will be phenotypically normal, 2nd situation 75% child will be phenotypically normal and in 3rd situation 50% will be normal in 4th situation no child will be normal and in 5th situation all will be normal.
o But you remember the percentage of affected children for which there is no confusion.