IB DP Biology- D3.2 Inheritance - IB Style Questions For SL Paper 1A -FA 2025
▶️ Answer/Explanation
In the pedigree, two unaffected parents produce an affected offspring. This situation is only possible if both parents are carriers of a recessive allele. Additionally, both males and females appear unaffected/affected in a pattern consistent with a non-sex-linked trait. Therefore, the most likely inheritance pattern is autosomal recessive.
✅ Answer: (C)
▶️ Answer/Explanation
Correct answer: (C) \(75\%\)
Huntington’s disease is an autosomal dominant condition. A person needs only one dominant allele \(H\) to express the disorder.
If both parents are heterozygous \(Hh\), the possible genotypes for each child (via a Punnett square) are:
– \(HH\): affected
– \(Hh\): affected
– \(Hh\): affected
– \(hh\): unaffected
Out of four outcomes, three result in Huntington’s disease:
\[ \frac{3}{4} = 0.75 = 75\% \]
✅ Answer: (C)
Why the other options are incorrect:
A. \(25\%\) is the probability of being unaffected \((hh)\).
B. \(50\%\) would apply only if one parent were heterozygous and the other homozygous recessive.
D. \(100\%\) would require both parents to be \(HH\), which they are not.
▶️ Answer/Explanation
Correct answer: (C) II and III only
Alleles are different forms of the same gene, located at the same genetic locus. They differ by small changes in the DNA sequence, typically involving only a few base pairs rather than large differences in length.
Mutations such as base substitutions, insertions, or deletions can create new alleles, contributing to genetic variation within populations.
Evaluation of statements:
• I: Incorrect – Alleles differ slightly in sequence, not significantly in the number of base pairs.
• II: Correct – Alleles are specific versions of a gene.
• III: Correct – Mutations are the source of new alleles.
✅ Answer: (C)