The recessive genes located on X-chromosomes in humans are always -
- Expressed in females
- Sub-lethal
- Expressed in males
- Lethal
Answer
(3) Expressed in males
Solution
Human males are XY, so they have only one X chromosome (hemizygous for X-linked genes).
⇒ The Y chromosome has no corresponding allele to mask an X-linked recessive gene.
∴ An X-linked recessive gene is always expressed in males, while females (XX) show it only when homozygous (e.g. haemophilia, colour blindness).
Answer: (3)