Sickle cell anaemia is an inherited autosomal recessive condition caused by a mutation in the haemoglobin
subunit beta (HBB) gene. A male and female who are
phenotypically normal, discover through genetic testing
that they are both carriers of a HBB mutant allele. What is
the probability that their first child will be biologically
female and phenotypically normal?

1/8 (12.5%)

Explanation:

An autosomal recessive disease is an inherited disease in which an individual need to receive both defective alleles at the same gene locus to be expressed in the phenotype. In this case, both parents are carriers of the recessive mutant allele associated with the sickle cell anaemia trait, thereby both parents are heterozygous, ie., each parent has one copy of the normal allele 'H' and one copy of the defective mutant allele 'h' associated with this condition. In consequence, their first child has a 1/4 (25%) chance of having sickle-cell anaemia. Moreover, the chance of having a girl is 1/2 and the chance of having a boy is 1/2, thereby the final chance of having a girl sickle cell anaemia individual is 1/4 x 1/2 = 1/8 (12.5%).

- Parental cross for sickle cell anaemia trait = Hh x Hh >>

- F1 = 1/4 HH (normal); 1/2 Hh (normal); 1/4 hh (sickle cell anaemia) >>  

- Sex proportion of sickle cell anaemia individuals =  1/8 female sickle cell anaemia individuals + 1/8 male sickle cell anaemia individuals (1/8 + 1/8 = 1/4)