Concept Explanation: Y Chromosome and Genetic Traits

The Y chromosome is one of the two sex chromosomes in humans (X and Y). It is male-determining, meaning individuals with one X and one Y chromosome (XY) develop as males. The Y chromosome carries genes that are crucial for male sexual development, such as the SRY gene (Sex-determining Region Y), which triggers testis formation.

Key points about Y-linked inheritance:

• Y-linked traits are passed only from father to son.
• They do not skip generations and affect only males.
• There are very few known Y-linked disorders because the Y chromosome contains relatively few genes.

Step-by-Step Analysis of the Options:

Step 1: Evaluate "Hypertrichosis of ears" – This is a rare condition characterized by excessive hair growth on the outer ear. It is Y-linked, meaning it is passed from father to son and is associated with the Y chromosome.

Step 2: Evaluate "Night blindness" – Night blindness can be caused by vitamin A deficiency or genetic factors. Genetic forms are often autosomal recessive (e.g., retinitis pigmentosa) or X-linked recessive, not Y-linked.

Step 3: Evaluate "Haemophilia" – Haemophilia is a bleeding disorder caused by mutations in genes on the X chromosome (e.g., Factor VIII or IX). It is X-linked recessive and affects males more frequently, but it is not Y-linked.

Step 4: Evaluate "Colour blindness" – Colour blindness is most commonly due to mutations in genes on the X chromosome (e.g., red-green colour blindness). It is X-linked recessive and not associated with the Y chromosome.

Final Answer: Hypertrichosis of ears is associated with the Y chromosome.

Related Topics and Formulae:

Inheritance Patterns:

• Autosomal Dominant: Affected individuals have at least one affected parent; both sexes equally affected.
• Autosomal Recessive: Skips generations; both sexes equally affected.
• X-linked Dominant: Affected males pass to all daughters; no male-to-male transmission.
• X-linked Recessive: More common in males; affected females usually have affected father and carrier mother.
• Y-linked: Only males affected; passed from father to all sons.

Probability in Genetics (for X-linked recessive):

For a carrier female (X^cX) and normal male (XY), the probability of an affected son is:

Key Formulae:

Hardy-Weinberg Principle for allele frequencies:

where $p$ is frequency of dominant allele, $q$ is frequency of recessive allele.