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IB DP Biology- D3.2 Inheritance- IB Style Questions For HL Paper 1A -FA 2025

IBDP Biology HL Paper 1 -FA 2025- All Chapters

Question

A Mirabilis jalapa (four o’clock) plant bearing red flowers was crossed with one that had white flowers. The offspring in the F₁ generation all showed pink flowers. What phenotypic ratio would most likely be observed in the F₂ generation?

A. 100% pink
B. 50% red and 50% white
C. 25% white, 50% pink and 25% red
D. 75% red and 25% white

▶️ Answer/Explanation
This cross demonstrates incomplete dominance. When red-flowered and white-flowered plants are crossed, the F₁ generation shows pink flowers (a blend of the two). In the F₂ generation, the segregation of alleles produces a phenotypic ratio of 1 red : 2 pink : 1 white, or equivalently, 25% red, 50% pink, and 25% white.
✅ Answer: (C) 25% white, 50% pink and 25% red

Question 

Huntington’s disease is a neurodegenerative genetic disorder caused by a dominant allele.
The pedigree chart for a family shows the inheritance of Huntington’s disease.
What can be concluded from this pedigree chart?
 
A. Huntington’s disease is sex-linked, since most affected individuals are male.
 
B. Huntington’s disease is not sex-linked, since individuals IV-1 and IV-3 are not affected.
 
C. Huntington’s disease is not sex-linked, since individual III-3 is affected.
 
D. Individual IV-1 is a carrier, since individual III-2 is affected
▶️Answer/Explanation
Answer: C. Huntington’s disease is not sex-linked, since individual III-3 is affected.
Explanation:

Huntington’s disease is autosomal dominant, meaning anyone who inherits one copy of the mutant gene will develop the disease. It affects both males and females equally because it’s not linked to sex chromosomes.

Evaluation of Options:

A. Incorrect: Huntington’s disease is not sex-linked. Both males and females can be affected equally.

B. Incorrect: The fact that individuals IV-1 and IV-3 are unaffected doesn’t prove whether the disease is sex-linked or not.

C. Correct: Individual III-3 is a female affected by Huntington’s disease. If it were sex-linked, females wouldn’t typically be affected unless they had two copies of the mutant allele. This means the disease is autosomal, not sex-linked.

D. Incorrect: Huntington’s disease is dominant, so there are no carriers. If an individual is unaffected, they simply don’t carry the allele.

 
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