A colour blind girl is rare because she will be born only when:
Correct answer: B. Her father and maternal grandfather were colour blind
- A. Her mother and maternal grandfather were colour blind
- B. Her father and maternal grandfather were colour blind
- C. Her mother is colour blind and father has normal vision
- D. Parents have normal vision but grandparents were colour blind
Explanation
The correct option is B: Her father and maternal grandfather were colour blind. Color blindness is an X-linked recessive disorder, meaning a female must inherit two copies of the recessive allele (Xc) to express the trait. In this scenario, the daughter's father must be colour blind (XcY) to pass on an Xc, and the mother must at least be a carrier (XcX) or colour blind (XcXc) to pass on the second Xc. The maternal grandfather's condition indicates that the mother has at least one Xc, fulfilling the condition.Option A is incorrect because it does not ensure the father passes on an Xc. Option C is incorrect as it lacks information on the father's Xc contribution. Option D is incorrect because normal vision parents are less likely to pass two Xc alleles, despite the grandparents' condition.
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About Inheritance
Mendel's laws explain segregation, independent assortment and the prediction of genetic ratios using alleles, genotypes and phenotypes. The chapter also covers linked genes, recombination and crossing over during meiosis, then applies pedigree reasoning to X-linked recessive traits, which show different inheritance patterns in males and females.
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