Q38 - A Pedigree with Two X-Linked Disorders

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Sex Determination & Sex-Linked Inheritance

Two human genetic disorders, haemophilia and red-green colour blindness, are both located on chromosome X. The pedigree in the Fig. shows a family with both disorders. Pedigree of a family suffering from both haemophilia and red-green colour blindness. Blue symbols = haemophilic individuals; red symbols = colour blind individuals; purple symbol (IV-1) = haemophilic+colour blind individuals; white symbols = unaffected individuals. It is assumed that no new mutations related to the two disorders occur in the family.

Figure 1. Figure 1.

Using the information and data, determine which of the statements are true or which are false.

A. Person II-2 is a carrier of both disorders

B. At least two individuals in the pedigree carry recombinations due to genetic crossover

C. If IV-2 (unborn) has Klinefelter's syndrome (XXY) and also has colour blindness, then a non-disjunction must have taken place in the first meiotic division of the mother's egg cell

D. III-5 marries a man from a population in which the frequency of the allele for colour blindness is 1%. The probability that their firstborn child is a colourblind daughter is 0.25%

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Question reproduced from IBO 2015, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)