A colourblind man marries a woman with bormal sight who has no histroyh of colour blindess in her family. What is the probability of their grandson becoming blind

Options

A. 0.5
B. 1
C. nil
D. 0.25

Answer

To solve the problem of determining the probability of the grandson becoming colorblind, we will follow these steps:

### Step 1: Understand the Genetics 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, and a male (XY) needs only one copy of the recessive allele to express the trait, while a female (XX) needs two copies.

### Step 2: Determine the Genotypes of the Parents
- The colorblind man has the genotype **X^cY** (where X^c represents the X chromosome with the color blindness allele).
- The woman with normal sight and no family history of color blindness has the genotype **XX** (both X chromosomes are normal).

### Step 3: Determine the Possible Genotypes of Their Children
When these two individuals have children, we can set up a Punnett square to determine the possible genotypes of their offspring.

- The man can pass on either **X^c** or **Y**.
- The woman can pass on either **X**.

The possible combinations are:
- From the man: X^c (colorblind) or Y (normal male)
- From the woman: X (normal)

The potential offspring genotypes will be:
1. **X^cX** (female carrier of color blindness)
2. **XY** (normal male)

### Step 4: Determine the Probability of the Grandson Being Colorblind
The grandson will be the child of one of the daughters (X^cX) and a normal male (XY). The possible combinations for the grandson would be:
- If the daughter (X^cX) passes on X^c and the normal male passes on Y, the grandson will be **X^cY** (colorblind).
- If the daughter passes on X and the normal male passes on Y, the grandson will be **XY** (normal).

Thus, the probabilities are:
- 50% chance of being colorblind (X^cY)
- 50% chance of being normal (XY)

### Conclusion
The probability of their grandson becoming colorblind is **50%**.

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