A normal female whose father was colourblind marries a normal male whose father was also colour-blind. What is the probability that their son will be colour-blind?

Options

A. `0%`
B. `25%`
C. `50%`
D. `75%`

Answer

To solve the problem of determining the probability that the son of a normal female (whose father was colorblind) and a normal male (whose father was also colorblind) will be colorblind, we can follow these steps:

### Step 1: Determine the Genotypes of the Parents
- The normal female has a father who is colorblind. Since colorblindness is an X-linked recessive trait, her father must have the genotype XcY (where Xc represents the colorblind allele). The mother of the female is normal and can be homozygous (XX) or a carrier (XcX). However, since the problem states she is normal and does not specify she is a carrier, we will assume she is homozygous normal (XX).

- The normal male has a father who is colorblind. Thus, his father is XcY. The mother of the male must be normal, and we will assume she is also homozygous normal (XX).

### Step 2: Determine the Possible Genotypes of the Offspring
- For the female (XX) and her father (XcY), the possible offspring genotypes are:
- Daughters: XX (normal) and XcX (carrier)
- Sons: XY (normal) and XcY (colorblind)

- For the male (XY) and his father (XcY), the possible offspring genotypes are:
- Daughters: XX (normal) and XcX (carrier)
- Sons: XY (normal) and XcY (colorblind)

### Step 3: Create a Punnett Square for the Cross
- Now we will cross the genotypes of the female (XX or XcX) with the male (XY):

| | X | Y |
|------|----|----|
| X | XX | XY |
| Xc | XcX| XcY|

### Step 4: Analyze the Offspring
- From the Punnett square, we can see the possible outcomes:
- Daughters: XX (normal), XcX (carrier)
- Sons: XY (normal), XcY (colorblind)

### Step 5: Calculate the Probability of a Colorblind Son
- Out of the possible male offspring (XY and XcY), there are 2 outcomes:
- 1 son is colorblind (XcY)
- 1 son is normal (XY)

Thus, the probability that their son will be colorblind is:
\[ \text{Probability} = \frac{\text{Number of colorblind sons}}{\text{Total number of sons}} = \frac{1}{2} = 50\% \]

### Final Answer
The probability that their son will be colorblind is **50%**.

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