A woman with two genes for haemophilia and two gene for colour blindness on of the 'X' chromosomes marries a normal man. How will the progeny be?
Correct answer: C. All daughters carriers of haemophilia and colour blindness
- A. 50% haemophilic and colour-blind sons and 50% normal sons
- B. 50% haemophilic daughters (carrier) and 50% colour-blind daughters (carrier)
- C. All daughters carriers of haemophilia and colour blindness
- D. Haemophilic and colour-blind daughters
Explanation
The correct answer is that all daughters will be carriers of both haemophilia and colour blindness. In X-linked inheritance, males inherit one X chromosome from their mother and one Y chromosome from their father, while females inherit two X chromosomes, one from each parent. In this case, the mother has two X chromosomes with the genes for both haemophilia and colour blindness. Therefore, all daughters will inherit one of these affected X chromosomes and one normal X chromosome from their father, making them carriers of both traits without expressing them. The incorrect options fail to recognize this inheritance pattern. For example, sons cannot inherit X-linked traits from their father and cannot be affected by these conditions, and the suggestion that only 50% of daughters will be affected is misleading as all daughters will be carriers.
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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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