Question

Briefly explain the inheritance of sex-linked recessive character in humans.

Hint:

Sex-linked recessive traits affect males more than females. Classic examples: haemophilia and colour blindness.

Answer 1

Step 1: Definition.
Sex-linked recessive traits are those traits whose genes are located on the X chromosome and express themselves only when present in a homozygous condition in females or a hemizygous condition in males.

Step 2: Characteristics of inheritance. \[\begin{array}{rl} \bullet & \text{More common in males, as they have only one X chromosome (XY).} \\ \bullet & \text{Females (XX) are affected only if they inherit the defective gene on both X chromosomes.} \\ \bullet & \text{Carrier females (heterozygous) usually do not show symptoms but can pass the defective allele to offspring.} \\ \end{array}\]

Step 3: Pedigree pattern.
\[\begin{array}{rl} \bullet & \text{Affected father never transmits the trait to his sons (as he gives Y chromosome).} \\ \bullet & \text{Affected father transmits the defective gene to all daughters, making them carriers (if mother is normal).} \\ \bullet & \text{Carrier mother has a 50% chance of passing the defect to sons (affected) and daughters (carriers).} \\ \end{array}\]

Step 4: Examples of sex-linked recessive disorders. \begin{enumerate} \item Haemophilia: Blood fails to clot normally due to absence of clotting factors. \item Colour blindness: Inability to distinguish between red and green colours. \item Duchenne muscular dystrophy: Severe muscle degeneration. \end{enumerate}

Step 5: Conclusion.
Sex-linked recessive inheritance demonstrates how X-chromosome plays a key role in the transmission of genetic disorders, with males being more vulnerable than females.