A colour blind man marries a woman with normal sight who has no history of colour blindness in her family.What is the probablity of their grandson being colour blind ?

Options

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

Answer

To determine the probability of the grandson being color blind, we need to analyze the genetic inheritance of color blindness, which is a sex-linked recessive trait. Here’s a step-by-step solution:

### Step 1: Identify the Genotypes
- A color-blind man has the genotype X^cY (where X^c represents the X chromosome with the color blindness allele).
- The woman with normal sight, who has no history of color blindness in her family, is likely to have the genotype XX (homozygous normal).

### Step 2: Determine the Offspring's Genotypes
When the color-blind man (X^cY) marries the normal sight woman (XX), we can set up a Punnett square to determine the possible genotypes of their children.

| | X | X |
|-------|-----|-----|
| X^c | X^cX | X^cX |
| Y | XY | XY |

From the Punnett square, we can see the possible offspring:
- 50% (X^cX) - Female carriers of color blindness (normal sight)
- 50% (XY) - Normal sight males

### Step 3: Identify the Next Generation
The next generation consists of:
- 2 daughters (carriers, X^cX)
- 2 sons (normal sight, XY)

### Step 4: Analyze the Grandson's Probability
Now, we need to consider the potential mating of these offspring. The only way for a grandson to be color blind is if one of the daughters (carriers) has children with a normal sight male (XY).

#### Possible Cross:
- Carrier female (X^cX) with a normal male (XY):

| | X | Y |
|-------|-----|-----|
| X^c | X^cX | X^cY |
| X | XX | XY |

From this cross, the possible offspring are:
- 50% (X^cX) - Female carriers (normal sight)
- 25% (XX) - Normal sight females
- 25% (X^cY) - Color blind males

### Step 5: Calculate the Probability of a Color Blind Grandson
- Out of the possible offspring from the carrier female and normal male, there is a 25% chance that a male child (grandson) will be color blind (X^cY).

### Conclusion
Thus, the probability of their grandson being color blind is **0.25** or **25%**.