146. Parents who appear normal have a child with sickle cell anemia, which is an autosomal recessive trait. The
woman becomes pregnant again and is told that she is carrying fraternal twins. What is the probability that
both the twins will develop sickle cell anemia?
1. 1/16
2. 1/4
3. 1/2
4. 9/16

Question

Parents who appear normal have a child with sickle cell anemia, which is an autosomal recessive trait. The woman becomes pregnant again and is told that she is carrying fraternal twins. What is the probability that both the twins will develop sickle cell anemia?

1. 1/16

2. 1/4

3. 1/2

4. 9/16

Detailed Explanation

Sickle Cell Anemia is an autosomal recessive genetic disorder, meaning the disease occurs only when an individual inherits two copies of the sickle cell allele, one from each parent. In autosomal recessive inheritance, individuals who are carriers (i.e., have one normal allele and one sickle cell allele) do not exhibit symptoms of the disease but can pass the sickle cell allele to their offspring.

Understanding the Inheritance Pattern

• Parents’ Genotypes: Since both parents are normal but have a child with sickle cell anemia, they must both be carriers of the sickle cell allele, meaning their genotype is AS (where A is the normal allele and S is the sickle cell allele).

• Probability of Child’s Genotype: When two carriers (AS) have a child, the possible genotypes of the child are:

  • AA (normal, 25% probability)

  • AS (carrier, 50% probability)

  • SS (sickle cell anemia, 25% probability)

Thus, there is a 1 in 4 (25%) chance that any child born to these parents will have sickle cell anemia (SS).

Fraternal Twins

Fraternal twins develop from two separate fertilized eggs, each having an independent genetic probability of inheriting the sickle cell allele. The events of inheritance for each twin are independent of each other.

For both twins to have sickle cell anemia, each twin must independently inherit the SS genotype. Since the probability for one child to have sickle cell anemia is 1/4, the probability that both twins will have sickle cell anemia is calculated by multiplying the probabilities for each twin:

• Probability that first twin has sickle cell anemia = 1/4

• Probability that second twin has sickle cell anemia = 1/4

Thus, the combined probability for both twins to have sickle cell anemia is:

14×14=116\frac{1}{4} \times \frac{1}{4} = \frac{1}{16}41​×41​=161​

Answering the Question

The correct answer is:

1. 1/16

This represents the probability that both fraternal twins will inherit the sickle cell anemia gene from both parents and develop the disease.

Conclusion

When dealing with autosomal recessive genetic traits like sickle cell anemia, the probability of inheriting the disease depends on both parents being carriers of the sickle cell allele. In the case of fraternal twins, the probability that both twins will develop sickle cell anemia is the product of their independent chances of inheriting the disease, which in this case is 1/16