Question

problem 2
polydactyly is a genetic disorder that causes extra digits.
the pedigree below shows its inheritance in one family.

1. who is iii-2’s maternal grandfather?
2. what is the relationship between iii-? and ii-3?

ii-3 is iii-1 uncle

1. how many people in the third generation have polydactyly?

1

1. is polydactyly a dominant or recessive disorder? use evidence from the pedigree to support your answer.
2. ii-1 and ii-2 are having a new baby. work out the probability of it having polydactyly.

use the letter “d” to represent the disease allele.
individual ii-1 genotype: dd

• star any possible offspring with polydactyly

probability of the child having polydactyly: 100%

Explanation:

Step1: Identify maternal grandfather

III-2's mother is II-1. II-1's father is I-1.

Step2: Confirm uncle-nephew relationship

II-3 is the brother of III-1's father (II-2), so he is III-1's uncle.

Step3: Count affected in generation III

Only III-2 (shaded) has polydactyly.

Step4: Determine dominance type

Affected individuals appear in every generation, and unaffected parents (none here) would not have affected offspring if recessive. I-1 (affected) has affected child II-2; I-4 (affected) has affected children II-2/II-3. Dominant alleles express in heterozygotes, so it is dominant.

Step5: Calculate offspring probability

II-1 genotype: $dd$ (unaffected), II-2 genotype: $Dd$ (since he has an unaffected child III-1, so he must carry a recessive allele). Punnett square:

$$\begin{array}{c|cc} & d & d \\ \hline D & Dd & Dd \\ d & dd & dd \\ \end{array}$$

Affected offspring are $Dd$, 2 out of 4. Probability = $\frac{2}{4} = 50\%$.

Answer:

1. I-1
2. II-3 is III-1's uncle
3. 1
4. Polydactyly is dominant. Evidence: Affected individuals are present in each generation, and an affected parent (e.g., I-1) has affected offspring, which aligns with dominant inheritance (dominant alleles are expressed in heterozygotes and passed to offspring).
5. 50%