Question

Sickle cell anemia is an autosomal recessive trait in humans. A man whose sister has sickle cell anemia and a woman whose mother suffers from sickle cell anemia, marry and are considering having children. What is the probability that their first child will be normal?

Answer 1

The probability that the first child of a couple, both of whom are carriers of sickle cell anemia, will be phenotypically normal is 75%.

Step-by-step explanation:

Sickle cell anemia is an autosomal recessive trait, which means that two copies of the mutant gene are necessary for a person to express the disease. If a man has a sister with sickle cell anemia, he must be at least a carrier (has one mutant allele). The woman, whose mother suffers from sickle cell anemia, also has at least one mutant allele.

To calculate the probability that they will have a normal child—that is, a child without sickle cell anemia—we must consider all possible combinations of their alleles. Let's denote the normal hemoglobin allele by 'A' and the sickle cell allele by 's'. The man's genotype could either be AA (normal) or As (carrier). However, since his sister has sickle cell anemia (ss), we know he must be a carrier (As). The woman must also be a carrier (As) because she has an affected mother (ss) and an unknown father.

When two carriers have a child, there is a 25% chance (1 in 4) the child will have sickle cell anemia (ss), a 50% chance (2 in 4) the child will be a carrier like the parents (As), and a 25% chance (1 in 4) the child will have two normal alleles (AA). Therefore, the probability that their first child will be normal, which includes both non-carriers and carriers who are phenotypically normal, is 75% (3 in 4).

Answer 2

The probability that the first child of a couple, where the man's sister has sickle cell anemia and the woman's mother suffers from sickle cell anemia, will be normal depends on whether the man is a carrier of the sickle cell gene. If he isn't a carrier, the probability is 100%, but if he is, the probability is 75%.

Step-by-step explanation:

Sickle cell anemia is an autosomal recessive disorder, which means a child must inherit two copies of the mutant gene to express the disease. Since the student's question involves the inheritance pattern of sickle cell anemia, let's assess the genetic possibilities for their potential offspring. The man has a sister with sickle cell anemia, implying that each of his parents carried at least one sickle cell allele.

Therefore, he could either be a carrier (having one sickle cell gene and one normal gene) or could have two normal genes if he received the normal allele from each parent. The woman, whose mother has sickle cell anemia, must have received one sickle allele from her mother, thus she's a carrier. The genotype of the man could be 'AS' (carrier) or 'AA' (not a carrier), and the genotype of the woman is 'AS' (carrier).

To calculate the probabilities of their child being normal, we should consider the different scenarios. If the man is 'AA', the chance of a normal child (not having sickle cell anemia) is 100%, as he would only pass on the normal allele. If he's a carrier ('AS'), there's a 1 in 4 chance the child will have sickle cell anemia ('SS'), a 2 in 4 (or 50%) chance the child will be only a carrier ('AS'), and a 1 in 4 chance the child will be normal ('AA'). The probability of the child being normal is 75% in this situation.

Without genetic testing of the man, you cannot determine the exact probability but can only estimate that it is somewhere between 75% to 100%. For a definitive answer, the man should consider genetic testing to know his carrier status.