AIIMS 2006 Botany Sex-linked Inheritance MCQ Question

To tackle this question, we need to understand the nature of sex-linked inheritance and the specific conditions mentioned in the options.

Explanation of the Correct Answer (C: haemophilia and red-green colour blindness)

Both haemophilia and red-green colour blindness are well-known examples of sex-linked traits, specifically located on the X chromosome.

1. Haemophilia: This is a genetic disorder that impairs the body's ability to make blood clots, a process needed to stop bleeding. The gene responsible for haemophilia is located on the X chromosome. Because males have only one X chromosome (XY), if they inherit the allele for haemophilia, they will express the condition. Females (XX) would need to inherit two copies of the allele to express the disease, making it less common among females.

2. Red-Green Colour Blindness: This is another X-linked recessive condition that affects the perception of red and green colors. Similar to haemophilia, males are more frequently affected because they have only one X chromosome. If a male inherits the allele for red-green colour blindness, he will exhibit the trait. Females require two copies of the allele to be colour blind.

Since both conditions are carried on the X chromosome and are examples of X-linked inheritance, Option C is the correct answer.

Clarification of Why Other Options Are Incorrect

Option A: Colour blindness and body height

• Colour Blindness: This is an X-linked trait, similar to red-green colour blindness.
• Body Height: This trait is polygenic and not linked to the sex chromosomes. It is influenced by multiple genes located on various chromosomes, making this option incorrect.

Option B: Attached earlobe and Rhesus blood group

• Attached Earlobe: This trait is not sex-linked; it's a common autosomal trait that can be influenced by multiple genes.
• Rhesus Blood Group: This is determined by the Rhesus factor gene, which is also not located on the sex chromosomes. Hence, this option is incorrect.

Option D: Phenylketonuria and haemophilia

• Phenylketonuria (PKU): This is an autosomal recessive disorder caused by a mutation in the PAH gene located on chromosome 12, not an X-linked trait.
• Haemophilia: As previously noted, this is an X-linked trait. However, since PKU is not sex-linked, this option is also incorrect.

Summary of Relevant Concepts

• Sex-linked Inheritance: Refers to genes located on the sex chromosomes, primarily the X chromosome. Traits that are linked to the X chromosome often exhibit different patterns of inheritance in males and females.
• X-linked Recessive Traits: Males are more likely to express these traits because they have only one X chromosome. Females must have two copies of the allele to express the trait.

Conclusion

Thus, the only option that correctly identifies both traits as sex-linked is C) haemophilia and red-green colour blindness. This understanding allows us to analyze genetic inheritance patterns accurately, especially in the context of sex-linked traits.