A normal visioned girl has a normal visioned brother and a colour blind sister. Their parents are

Options

A. Colour blind father, carrier mother
B. Normal visioned father, Colour blind mother
C. Carrier mother, normal visioned father
D. Colour blind father, colour blind mother

Answer

To solve the problem regarding the inheritance of color blindness in a family with a normal visioned girl, a normal visioned brother, and a color blind sister, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Genetic Basis of Color Blindness**:
- Color blindness is an X-linked recessive trait. This means that the gene responsible for color blindness is located on the X chromosome. Males (XY) have one X chromosome, while females (XX) have two.

2. **Identify the Siblings' Genotypes**:
- The normal visioned girl (let's denote her as G) must have at least one normal X chromosome (X^N). She could be either homozygous normal (X^N X^N) or a carrier (X^N X^c, where X^c represents the color blind allele).
- The normal visioned brother (B) also has normal vision, so he has the genotype X^N Y.
- The color blind sister (C) must have the genotype X^c X^c, as she needs two copies of the color blind allele to express the trait.

3. **Determine the Parents' Genotypes**:
- Since the color blind sister has the genotype X^c X^c, the mother must be a carrier (X^N X^c) because she can pass on the color blind allele to her daughter. The father must have normal vision (X^N Y) since he has normal vision and can only pass on a normal X chromosome.

4. **Construct a Punnett Square**:
- To visualize the inheritance, we can set up a Punnett square with the mother's alleles (X^N and X^c) and the father's alleles (X^N and Y).
- The possible combinations from this cross are:
- X^N X^N (normal vision girl)
- X^N X^c (carrier girl)
- X^N Y (normal vision boy)
- X^c Y (color blind boy)

5. **Analyze the Offspring**:
- The offspring from this cross will have:
- 50% chance of being normal visioned girls (X^N X^N)
- 50% chance of being carrier girls (X^N X^c)
- 50% chance of being normal visioned boys (X^N Y)
- 50% chance of being color blind boys (X^c Y)

6. **Conclusion**:
- The parents must be a carrier mother (X^N X^c) and a normal visioned father (X^N Y). This explains the presence of a color blind sister and the normal visioned siblings.

### Final Answer:
The parents are a carrier mother (X^N X^c) and a normal visioned father (X^N Y).