Question

problem 1
sickle cell anemia is a genetic disorder of the blood.
the pedigree below shows its inheritance in one family.

1. what is the relationship between ii-1 and ii-2?
2. how many children do i-3 and i-4 have?
3. is sickle cell anemia dominant or recessive? explain how you know.
4. iii-1 and iii-2 are having a baby. work out the probability of it having sickle cell anemia.

use the letter \a\ to represent the disease allele.
individual iii-1 genotype:
individual iii-2 genotype:

• star any offspring that would have the disease.

probability of the child having sickle cell anemia:

1. iii-3 and iii-4 are also having a baby. work out the probability of it having sickle cell anemia.

use the letter \a\ to represent the disease allele.
individual iii-3 genotype:
individual iii-4 genotype:
probability of the child having sickle cell anemia:

Explanation:

Step1: Confirm relationship (Q1)

II-1 and II-2 share parents I-1 and I-2, so they are siblings.

Step2: Count children (Q2)

I-3 and I-4 have only II-3, so count = 1.

Step3: Determine inheritance (Q3)

Recessive: Affected individuals (I-1, I-4, III-1, III-3) have unaffected parents (e.g., II-2 and II-3 are unaffected but have affected child III-3). For a recessive trait, carriers (unaffected) can pass the allele to produce affected offspring.

Step4: Define genotypes (Q4)

III-1 is affected, so genotype = $aa$. III-2 is unaffected but has affected parents (II-2 and II-3, both carriers), so genotype = $Aa$.

Step5: Set up Punnett square (Q4)

	$a$	$a$
$a$	$aa$	$aa$

Step6: Calculate probability (Q4)

2 out of 4 offspring are $aa$ (affected). Probability = $\frac{2}{4} = \frac{1}{2}$

Step7: Define genotypes (Q5)

III-3 is affected, so genotype = $aa$. III-4 is a carrier (half-shaded symbol), so genotype = $Aa$.

Step8: Set up Punnett square (Q5)

	$a$	$a$
$a$	$aa$	$aa$

Step9: Calculate probability (Q5)

2 out of 4 offspring are $aa$ (affected). Probability = $\frac{2}{4} = \frac{1}{2}$

Answer:

1. Siblings
2. 1
3. Recessive. Unaffected parents (e.g., II-2 and II-3) can have an affected child (III-3), which only occurs if the trait is recessive (parents are carriers of the hidden recessive allele).
4. Probability of the child having sickle cell anemia: $\frac{1}{2}$
5. Probability of the child having sickle cell anemia: $\frac{1}{2}$