A colour blind man has a colour blind sister butnormal brother .The phenoptype of parents is

Options

A. Normal father and colour blind mother
B. Both parents are normal
C. Both parents are colour blind
D. Father colour blind and mother normal

Answer

To determine the phenotypes of the parents based on the information provided, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Genetic Basis**: Color blindness is an X-linked recessive trait. This means that the gene responsible for color blindness is located on the X chromosome, and a male (XY) will express the trait if he has the recessive allele (Xc), while a female (XX) will express the trait only if she has two copies of the recessive allele (XcXc).

2. **Identify the Genotypes of the Children**:
- The colorblind man has the genotype XcY (colorblind).
- The colorblind sister has the genotype XcXc (colorblind).
- The normal brother has the genotype XY (normal vision).

3. **Determine the Genotype of the Parents**:
- Since the son is colorblind (XcY), he must inherit the Xc from his mother. Therefore, the mother must either be a carrier (XcX) or colorblind (XcXc).
- The colorblind sister also inherits one Xc from the mother, confirming that the mother must carry at least one Xc.
- The normal brother must inherit a normal X chromosome from the mother, which means the mother cannot be colorblind (XcXc) because she must have at least one normal X (X).

4. **Conclude the Mother's Genotype**: The mother must be a carrier (XcX) since she has one normal X chromosome and one colorblind X chromosome.

5. **Determine the Father's Genotype**: The father is colorblind (XcY), as he has only one X chromosome and it carries the recessive allele for color blindness.

6. **Summarize the Parental Phenotypes**:
- The father is colorblind.
- The mother is normal but a carrier of the colorblind allele.

### Final Answer:
The phenotypes of the parents are:
- Father: Colorblind
- Mother: Normal (Carrier)