MEDizzy - A couple living in the United States has four children:

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General Pathology (I)

A couple living in the United States has four children: three girls, and one boy, all of whom are under the age of 10. Two of the girls have been diagnosed with cystic fibrosis, but the boy, who is 7 years of age, appears normal. Neither of the parents has clinical signs of cystic fibrosis. What is the chance that if these parents have another child, that child will develop cystic fibrosis?

ExplanationCystic fibrosis is an autosomal recessive disorder that results from mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene located on chromosome 7. Therefore, to develop the disease, a child must inherit an abnormal chromosome 7 from each parent. The probability that a child will inherit a particular gene found on only one chromosome of a chromosome pair from one parent is 1 in 2 (eg, 50%). Therefore, the probability that a child will inherit the cystic fibrosis (CF) gene from a parent is 50% and the probability that they will inherit an abnormal gene from both parents and be homozygous for cystic fibrosis is 25% (eg, 1/2 * 1/2). This is the recurrence risk for an autosomal disorder. Note that this is the same probability (25%) that a child will inherit a normal gene from both parents, that is, the child is homozygous normal. The remaining possibility, which is 50%, is the chance that a child will be a heterozygous carrier of the disease.