Chapter 17, Problem 2CS

The Joneses were referred to a clinical geneticist because their 6-month-old daughter was failing to grow adequately and was having recurrent infections. The geneticist took a detailed family history (which was uninformative) and a medical history of their daughter. He discovered that their daughter had a history of a constant cough and wheeze that was becoming progressively worse, had difficulty gaining weight (failure to thrive), and had an extensive history of yeast infection (thrush) in her mouth. The geneticist did a simple blood test to check their daughter’s white blood count and determined that she had severe combined immunodeficiency (SCID).

The geneticist explained that SCID is an immune deficiency that causes a marked susceptibility to infections. The defining characteristic is usually a severe defect in both the T- and B-lymphocyte systems. This results in one or more infections within the first few months of life that are serious and may even be life-threatening. Based on the family history, it was possible that their daughter had inherited a mutant allele from each of them and therefore was homozygous for a gene that causes SCID. If so, each time the Joneses had a child, there would be a 25% chance that the child would have SCID. Prenatal testing is available to determine whether the developing fetus has SCID.

If the Joneses want to be certain that their next child will not have SCID, what types of reproductive options do you think they have?