Select the incorrect statement with regard to hemophilia:
Correct answer: A. It is a dominant disease.
- A. It is a dominant disease.
- B. A single protein involved in the clotting of blood is affected.
- C. It is a sex-linked disease.
- D. It is a recessive disease.
Explanation
They asked the WRONG statement about haemophilia in option A. Haemophilia is a sex-linked disease which is also known as bleeder's disease as the patient will continue to bleed even from a minor cut since he or she does not possess the natural phenomenon of blood clotting due to the absence of anti-haemophiliac globulin or factor VIII (hemophilia-A) and plasma thromboplastin factor IX (hemophilia-B, Christmas disease) essential for it. As a result of continuous bleeding, the patient may die of blood loss. It is genetically due to the presence of a recessive sex-linked gene h, carried by the X chromosome. A female becomes a hemophiliac only when both of her X-chromosomes carry the gene (Xh Xh). However, such females generally die before birth because the combination of these two recessive alleles is lethal. A female having only one allele for hemophilia (XXh) appears normal because the allele for normal blood clotting present on the other X-chromosome is dominant. Such females are known as carriers. In the case of males, a single gene for the defect is able to express itself as the Y chromosome is devoid of any corresponding allele (Xh Y).
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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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