A diseased man marries a normal woman. They have five daughters and three sons. All the daughters were diseased and the sons were normal. The gene of this disease is:
Correct answer: A. X-linked dominant
- A. X-linked dominant
- B. X-linked recessive
- C. Autosomal recessive
- D. Autosomal dominant
Explanation
The father transmits his "X-chromosome" to the daughter while the sons receive "Y-chromosome" from the father. Since only the daughter and none of the sons are affected, the trait is X-linked. Daughters get the second copy of X-chromosomes from the mother who is normal and hence does not carry the affected allele of the trait. The trait is expressed in daughters under heterozygous conditions, hence it is an X-linked dominant trait. The correct option is A.
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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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