Thalassaemia is an autosomal recessive disorder. In the given case, both the partners are carriers for thalassaemia i.e., are heterozygous. Persons homozygous for thalassaemia suffer from severe haemolytic anaemia. Heterozygous persons are also not normal, but show the defect in a less severe form (thalassaemia minor). The probability of their progeny being affected with the disease i.e., with homozygous genotype is $25 \%$ only as shown below:
$\sigma^\pi$ 우
Parents :
$\mathrm{Tt} \times \mathrm{Tt}$
Offspring :
$\begin{array}{cccc}\mathrm{g}: \mathrm{TT} & : \mathrm{Tt} & : & \mathrm{tt} \\ 1 & : 2 & : & 1 \\ \text { Normal } & \text { Carriers } & \text { Affected } \\ 25 \% & 50 \% & 25 \%\end{array}$