Question

A colour blind girl is rare because she will be born only when

• A. her mother and maternal grand father were colour blind
• B. her father and maternal grand father were colour blind
• C. her mother is colour blind and father has normal vision
• D. parents have normal vision but grand parents were colour blind.

Answer 1

The Correct Option is B

To understand why a color blind girl is rare and under what genetic conditions this occurs, let's delve into the genetics behind color blindness.

Genetic Explanation:

• Color blindness is typically a sex-linked recessive trait, found on the X chromosome.
• Males (XY) only have one X chromosome, so if they inherit an X chromosome with the color blindness gene, they will express the trait because they lack a second X chromosome to mask the effect.
• Females (XX), however, must inherit two copies of the color blindness gene (one from each parent) to express the trait, because they have two X chromosomes.

Conditions for a Color Blind Girl:

1. The girl must inherit the color blindness allele from her mother. This means her mother must be either a carrier (X^cX) or color blind herself (X^cX^c).
2. The girl must also inherit the color blindness allele from her father. Since a father passes his X chromosome to his daughters, he must be color blind (X^cY) to pass the color blindness gene to her.

Evaluating the Options:

• Option 1: Her mother and maternal grandfather were color blind.
  This situation isn't sufficient. While it confirms the mother has at least one color blindness gene (X^c), without a color blind father, the girl cannot inherit another X^c from the father.
• Option 2: Her father and maternal grandfather were color blind.
  This option ensures that the girl can inherit an X^c from her father (as he is color blind) and the mother passes on the X^c from her color blind father (resulting in X^cX^c for the girl).
• Option 3: Her mother is color blind and father has normal vision.
  This results in the girl being a carrier (X^cX) as the father would pass a normal X chromosome.
• Option 4: Parents have normal vision but grandparents were color blind.
  Neither parent would pass on a color blindness allele necessary for the girl to express the trait.

Conclusion: Correct Option

The correct answer is: Her father and maternal grandfather were color blind. This situation provides the necessary genes for both X chromosomes in the daughter to be carriers of the color blindness gene, resulting in the expression of color blindness.