Question

More men suffer from colour blindness than women because _____________.

• A. Women are more resistant to diseases
• B. Male sex hormone testosterone causes the disease
• C. Colour blindness gene occurs on Y-chromosome
• D. Men are hemizygous and one defective allele is enough to cause the disease

Answer 1

The correct option is D Men are hemizygous and one defective allele is enough to cause the disease

• Homozygous is a word that refers to a particular gene that has identical alleles on both homologous chromosomes. It is referred to by two capital letters (XX) for a dominant trait, and two lowercase letters (xx) for a recessive trait.

• Hemizygous: Having only a single copy of a gene instead of the customary two copies. All the genes on the single X chromosome in the male are 'hemizygous.

For a recessive mutation to express itself, the mutated copies must be present on both the chromosomes. Color Blindness is X-linked recessive, then the female should have homozygous condition whereas male should have a hemizygous condition where one defective allele is enough to cause the disease.
So, the correct answer is 'Men are hemizygous and one defective allele is enough to cause the disease'.