A colour blind girl can be born when

Options

A. her parents have normal visions but grand parents were colour blind
B. her mother is colour blind and father normal
C. her mother is colour blind and father normal but grand father is colour blind
D. her father is colour blind and mother normal but mother's father is colour blind

Answer

To determine when a colorblind girl can be born, we need to analyze the genetic inheritance of color blindness, which is a sex-linked trait primarily carried on the X chromosome.

### Step-by-Step Solution:

1. **Understanding Color Blindness**:
- Color blindness is usually inherited as a recessive trait linked to the X chromosome. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

2. **Genotypes of Parents**:
- For a girl to be colorblind, she must inherit two copies of the colorblind allele (one from each parent). This means:
- If her father is colorblind, his genotype must be XcY (where Xc represents the colorblind allele).
- Her mother must carry at least one colorblind allele. This means her mother could either be colorblind (XcXc) or a carrier (XcX).

3. **Analyzing the Given Scenarios**:
- **Scenario 1**: Parents have normal vision, but grandparents are colorblind.
- This scenario does not guarantee that the girl will be colorblind, as both parents can be carriers without showing symptoms.
- **Scenario 2**: Mother is colorblind and father is normal.
- In this case, the girl will definitely be colorblind as she inherits Xc from her mother and Y from her father.
- **Scenario 3**: Mother is colorblind and father is normal, but grandfather is colorblind.
- This is similar to Scenario 2; the girl will be colorblind.
- **Scenario 4**: Father is colorblind, and mother is normal, but mother's father is colorblind.
- Here, the mother could be a carrier (XcX), allowing the girl to inherit Xc from her father and Xc from her mother, making her colorblind.

4. **Conclusion**:
- The scenarios that can lead to the birth of a colorblind girl are:
- Mother is colorblind and father is normal.
- Father is colorblind and mother is normal, with the mother's father being colorblind (indicating she could be a carrier).

Thus, the correct answer is that a colorblind girl can be born when her father is colorblind and her mother is normal but has a colorblind father (option D).