This pedigree chart shows the inheritance of sickle cell anemia in three generations of a family. Each male is symbolized by a square and each female is symbolized by a circle. A person who has sickle cell anemia is symbolized by a dark square or circle. The couple labeled 1 and 2 had five children, including one daughter with sickle cell anemia (5). 7a. In the pedigree, write the genotypes of each person who is labeled with a number. 7b. If parents 3 and 4 had a fourth child, what is the probability that this child would have sickle cell anemia? 0% 25% 50% 75% 100% 7c. Explain your reasoning.
The Correct Answer and Explanation is:
To answer this pedigree question about sickle cell anemia, we need to understand how this disorder is inherited.
Background:
Sickle cell anemia is inherited in an autosomal recessive pattern. This means a person must inherit two copies of the sickle cell allele (s) — one from each parent — to have the disease (ss). A person with only one sickle cell allele (Ss) is a carrier and typically does not show symptoms.
7a. Genotypes of each labeled person:
Let’s assume the pedigree chart provides the following:
- 1 (Male): Father of 5 children, one of whom has sickle cell anemia.
- 2 (Female): Mother, also unaffected.
- 5 (Daughter): Has sickle cell anemia.
- 3 and 4: A couple who are both children of 1 and 2, and are parents to three children.
Now, based on this:
- 1 and 2: Both must be Ss. Why? They have a child (5) with the disease, meaning both must have passed on an s allele.
- 5: Must be ss (has the disease).
- 3 and 4: Both are children of 1 and 2, so they each have a 50% chance of inheriting s and 50% chance of inheriting S. But because they have had children together, and assuming their children are unaffected, the most likely genotype is Ss for both (we’ll assume this unless evidence says otherwise).
So:
- 1: Ss
- 2: Ss
- 3: Ss
- 4: Ss
- 5: ss
7b. Probability that parents 3 and 4 will have a child with sickle cell anemia:
Answer: 25%
7c. Explanation
Sickle cell anemia is a genetic condition caused by a mutation in the hemoglobin gene. It is inherited in an autosomal recessive manner, which means that a child must inherit two copies of the faulty gene—one from each parent—to be affected by the disease.
In the pedigree, both parents labeled 3 and 4 are children of carriers (1 and 2), and therefore are most likely carriers themselves. Carriers have one normal allele (S) and one mutated allele (s), giving them the genotype Ss. Carriers do not typically show symptoms of sickle cell anemia but can pass the mutated allele to their children.
When two carriers (Ss and Ss) have children, each child has a:
- 25% chance of inheriting SS (not a carrier and not affected),
- 50% chance of inheriting Ss (a carrier),
- 25% chance of inheriting ss (affected by sickle cell anemia).
These probabilities are derived from a simple Punnett square crossing Ss × Ss:
| S (from one parent) | s (from one parent) | |
|---|---|---|
| S | SS | Ss |
| s | Ss | ss |
So, for every pregnancy, there is a 25% chance the child will inherit both mutated alleles (ss) and have sickle cell anemia.
Therefore, if parents 3 and 4 have a fourth child, there is a 25% probability that the child will have sickle cell anemia, assuming both are carriers.
