IB DP Biology- D3.2 Inheritance - IB Style Questions For SL Paper 1A -New Syllabus

Question

A pedigree diagram is provided.

Based on the inheritance pattern shown, determine the most plausible mode of inheritance.

(A) Sex-linked recessive
(B) Sex-linked dominant
(C) Autosomal recessive
(D) Autosomal dominant

▶️ Answer/Explanation

Detailed solution

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)

Question

The pedigree chart shows the incidence of colour blindness in three generations of a family. Colour blindness is caused by a sex-linked recessive allele. What could the genotype of individual Z be?

(A) X^BX^B only
(B) X^BX^b only
(C) X^BX^B or X^BX^b
(D) X^bX^b or X^BX^b

▶️ Answer/Explanation

Correct Answer: \( \boxed{\mathbf{C}} \)

Since Z is an unaffected female, she cannot have the genotype X^bX^b. Her affected brother (X^bY) shows that their mother must be a carrier (X^BX^b), while the father is unaffected (X^BY). Therefore, Z could be either X^BX^B or X^BX^b.

Question

Which statements are characteristics of alleles?

I. Alleles differ significantly in number of base pairs.
II. Alleles are specific forms of a gene.
III. New alleles are formed by mutation.

A. I and II only
B. I and III only
C. II and III only
D. I, II and III

▶️ 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)

IB DP Biology- D3.2 Inheritance - IB Style Questions For SL Paper 1A -New Syllabus

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