A 26-year-old woman comes to the office with her husband for genetic counseling. She is pregnant with their second child, whose gender is unknown. Both parents are asymptomatic, but their firstborn 3-year-old son has recurrent episodes of anemia, jaundice, and painful swelling of the hands and feet. A blood sample is obtained from the boy, and hemoglobin electrophoresis is performed at alkaline pH. The results are shown in the image below. What is the probability that the unborn child will inherit one or more mutant alleles from the parents?

Question

A 26-year-old woman comes to the office with her husband for genetic counseling.  She is pregnant with their second child, whose gender is unknown.  Both parents are asymptomatic, but their firstborn 3-year-old son has recurrent episodes of anemia, jaundice, and painful swelling of the hands and feet.  A blood sample is obtained from the boy, and hemoglobin electrophoresis is performed at alkaline pH.  The results are shown in the image below.  What is the probability that the unborn child will inherit one or more mutant alleles from the parents?

Quizplus · Accepted Answer

11ec160b_376a_09aa_9249_4b930f540db8_MD0001_00 The couple's 3-year-old son has sickle cell anemia, an autosomal recessive disorder that causes red blood cells to assume a sickle shape during times of hypoxic stress, resulting in painful and potentially life-threatening vasoocclusive crises. The disease can be diagnosed using hemoglobin electrophoresis, which is able to determine the different types of hemoglobin in a blood sample based on their electrical charge and the speed by which they move through the medium. The patient's electrophoresis results show a predominance of hemoglobin S (HbS), which is diagnostic for sickle cell disease. In this case, both parents are unaffected and must be heterozygous carriers (Aa) of the disorder because they have an affected child. This means that each parent has a 50% chance of passing either a normal allele (A) or a mutant allele (a) to the offspring. The possible genotypes of the unborn child can be calculated using a Punnett square and are as follows: Only the last 2 options result in the child inheriting a mutant allele, so the probability that the unborn child is carrying one or more mutant alleles is 75%. (Choice B) The probability of the unborn child being affected is equivalent to the chance of inheriting two mutant alleles from the carrier parents, which is 25%. However, this question asks for the probability that the unborn child will inherit one or more mutant alleles (ie, chance of having the disease or being a carrier). Educational objective: Sickle cell anemia is an autosomal recessive disease that results in recurrent episodes of anemia, jaundice, and painful swelling of the hands and feet. Offspring of carrier parents have a 25% chance of being affected and a 50% chance of being heterozygous carriers, resulting in a 75% chance of inheriting at least one mutant allele. __________ References: Hematologic Disorders: Sickle Cell Disease. (http://www.ncbi.nlm.nih.gov/pubmed/26080456)

A 26-Year-Old Woman Comes to the Office with Her Husband