Question

1. the following pedigree demonstrates the expression of a trait with an unknown pattern of inheritance:

pedigree image

a) what is the most likely pattern of inheritance for this trait? how do you know? (2)

b) what must be the genotypes of individuals 1 & 2 of generation i? (1)

c) if individual 2 of generation ii is heterozygous, what is the probability that individual 1 of generation iii will inherit the trait? (use a punnett square to illustrate your answer) (2)

Explanation:

Part (a)

Step 1: Analyze Pedigree Features

• Both parents in Generation I (filled symbols) have children, most of whom show the trait (filled symbols).
• If it were recessive, two affected parents (who would be homozygous recessive) could only have recessive offspring, but here that’s consistent, but dominant is more likely because affected parents have affected kids, and there’s no skipping (recessive often skips generations as carriers pass it). Also, when an affected (filled) individual marries a non - affected (empty) in Generation II, some kids might be affected (if dominant, the affected has at least one dominant allele to pass).
• For dominant inheritance: Affected individuals have at least one dominant allele. Since both Generation I parents are affected and have mostly affected kids, autosomal dominant is likely (no sex bias, males and females affected). If it were X - linked dominant, all daughters of an affected father would be affected, but here the pattern doesn't show sex - linked bias (males and females affected in similar ratios). So autosomal dominant.

Step 2: Confirm with Examples

• Generation I: Both affected (filled). Generation II: Most children are affected (filled), which fits autosomal dominant (parents with dominant trait, passing dominant allele). The non - affected in Generation II (empty) is a recessive homozygote (if dominant is \(A\), then non - affected is \(aa\)).

Step 1: Recall Autosomal Dominant Genotypes

In autosomal dominant inheritance, an affected individual has at least one dominant allele (\(A\)). Since both Generation I individuals are affected and they have some affected and (in one case) an unaffected child (in Generation II, the empty circle), they must be heterozygous (\(Aa\)). If they were homozygous dominant (\(AA\)), all their children would be affected, but there is an unaffected child in Generation II (the empty circle). So for autosomal dominant, to have an unaffected child (\(aa\)), both parents must be heterozygous (\(Aa\)) (because \(Aa\times Aa\) can produce \(aa\) with probability \(1/4\)).

Step 1: Identify Genotypes

• Let's assume the dominant allele for the trait is \(A\) and the recessive is \(a\).
• Individual 2 of Generation II: Given it is heterozygous, so genotype \(Aa\) (since it is affected, and heterozygous as per the question).
• Individual 1 of Generation II (the non - affected one, empty circle): In autosomal dominant, non - affected individuals are homozygous recessive (\(aa\)) (because if they had a dominant allele \(A\), they would be affected).

Step 2: Set Up Punnett Square

	\(A\)	\(a\)
\(a\)	\(Aa\)	\(aa\)

• The cross is \(Aa\times aa\). The Punnett square shows the possible genotypes of the offspring (Generation III individual 1).

Step 3: Calculate Probability

• The genotypes from the Punnett square are \(Aa\) (affected, 2 out of 4) and \(aa\) (unaffected, 2 out of 4). But wait, no: Wait, individual 2 of Generation II is the affected one (heterozygous \(Aa\)) and the other parent (individual 1 of Generation II) is \(aa\) (unaffected). Wait, no, in the pedigree, the "individual 2 of Generation II" who is heterozygous: Wait, the question says "individual 2 of generation II is heterozygous". Let's re - clarify: The affected individual in Generation II (the one with the question mark's parent) – wait, the question is about individual 1 of Generation III. Let's re - track: The parents of Generation III individual 1 are an affected (heterozygous \(Aa\)) and a non - affected (\(aa\))? Wait, no, looking at the pedigree: The affected individual in Generation II (the one with the square, filled) is married to an empty circle (non - affected, \(aa\)). Wait, the question says "individual 2 of generation II is heterozygous" – let's assume that the affected individual (the one with the question mark's parent) is \(Aa\) (heterozygous) and the non - affected is \(aa\). Then the Punnett square is \(Aa\times aa\). The possible offspring: \(Aa\) (affected) and \(aa\) (unaffected), each with probability \(1/2\). Wait, no, the Punnett square has two \(Aa\) and two \(aa\)? Wait, no: \(Aa\) (parent 1) produces \(A\) and \(a\); \(aa\) (parent 2) produces \(a\) and \(a\). So the cross is:

	\(A\)	\(a\)
\(a\)	\(Aa\)	\(aa\)

So the probability of inheriting the trait (being \(Aa\)) is \(2/4 = 1/2\) or 50%.

Answer:

The most likely pattern is autosomal dominant inheritance. We know this because: (1) Affected parents (Generation I) have mostly affected offspring (Generation II), which is consistent with dominant inheritance (affected individuals pass the trait). (2) There is no sex - linked bias (males and females are affected in similar proportions), so it is autosomal (not X - or Y - linked). (3) The trait does not skip generations (a key feature of dominant inheritance, unlike recessive which often skips as carriers are unaffected).

Part (b)