Question

If both parents are carrier of thalassaemia which is an autosomal recessive disorder what are the chances of pregnancy resulting in an affected child

Answer 1

AA-normal
Aa- carrier
aa- affected
                                      Aa × Aa
                                           ↓
                                AA : Aa : aa
                                 1      2     1(dead)
                                           
Affected=1/4 so only 25%

Answer 2

Answer: Affected children: 25% or 1/4

Explanation:

Autosomal recessive disorder refers to a disorder in which an individual is affected only when both the chromosomes carry the affected gene.

Here AA- homozygous dominant ; Aa- heterozygous dominant ; aa- homozygous recessive( only in this case the manifestations of the disorder are seen)

A carrier means an individual who carries the affected gene but does not have the disorder so their genotype can be written as Aa( one is dominant, other is recessive so the dominant one is expressed).

When mating takes place:

Aa × Aa ( parental generation)

AA , Aa, Aa, aa ( possible genotypes of children)-------- out of these only children with the genotype 'aa' is affected.

Therefore, 1/4 or 25% of the children are probable to be affected by this disorder.