A child born of parents heterozygous for sickle-cell disease is screened at birth for the disease and is diagnosed as heterozygous by allele-specific oligonucleotide screening based on the genotypes of the parents.Thus, it is an unpleasant surprise when the child exhibits the symptoms of a homozygous individual.The doctor concludes that the screening test failed to pick up the second abnormal HBB allele due to a _______ mutation.DNA sequencing later reveals a de novo loss-of-function mutation elsewhere in the HBB gene.

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The fact that the child inherited an abnormal HBB allele from each heterozygous parent indicates that the screening test failed to detect the presence of one of the abnormal alleles. A reversion mutation occurs when a previously mutated gene reverts back to its normal form. In this case, the child may have inherited a reversion mutation in one of the HBB alleles from one of the parents, which would have made that allele appear normal during the screening test. However, DNA sequencing later revealed a de novo loss-of-function mutation in the other HBB allele, which was not related to the initial screening test.

A Child Born of Parents Heterozygous for Sickle-Cell Disease Is