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← Theoretical B

Q25 — X-Linked Trait Allele Mixing and Pedigree Probability

Theoretical B Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license
Sex Determination & Sex-Linked Inheritance Population Genetics & Hardy-Weinberg Equilibrium

A unique phenotype associated with enhanced performance in labyrinth tasks has been identified in mice. The trait is X-linked.

Scientist A introduces 500 male and 500 female mice to an island. The initial allele frequencies in this population are p = 0.7 (dominant allele) and q = 0.3 (recessive allele). Subsequently, Scientist B brings another 500 male and 500 female mice which were drawn from a larger population with allele frequencies p=0.8, and q=0.2. Both the initial and final populations are assumed to be in Hardy-Weinberg equilibrium. After several generations, the combined population on the island is 2,000 mice (Male/Female=1000/1000).

A female mouse was randomly picked up from Scientist A’s mice population before mixing and was crossed with Mouse 3 from the pedigree in Figure 1 (Cross Q).

A five-generation pedigree of mice showing the “performant” (black) phenotype passed through several families, with individuals 1, 2, and 3 labeled, and two unknown-phenotype individuals A and B at the bottom right. Figure 1. Inheritance mode of “Performant” phenotype (black-colored). Mice have the same XX/XY system as humans.

On your answer sheet, indicate “T” for true statements and “F” for false ones.

A. In the new population of an island, the ratio of "performant" male to "performant" female mice is 5:1
B. In the Cross Q, the probability that the male offspring will not have a "performant" phenotype is 70%
C. If a cross between mice 1 and 2 from Figure 1 produces 4 offspring, the probability that all are "performant" male mice is less than 0.2 %
D. The probability of having a "performant" phenotype is the same for mouse A and mouse B
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Question reproduced from IBO 2024, Theoretical Exam Part B, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF