Down's syndrome is caused by an extra copy of chromosome number 21. What percentage of offspring produced by an affected mother and a normal father would be affected by this disorder

Options

A. `50%`
B. `25%`
C. `100%`
D. `75%`

Explanation

To solve the question regarding the percentage of offspring affected by Down's syndrome when an affected mother and a normal father reproduce, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Down's Syndrome**:
Down's syndrome is caused by the presence of an extra copy of chromosome 21, known as trisomy 21. This means that individuals with Down's syndrome have three copies of chromosome 21 instead of the usual two.

2. **Identify the Genotypes**:
- The affected mother has an extra chromosome 21, which can be represented as 2A + 1 (where A represents the normal chromosome 21).
- The normal father has the typical chromosome arrangement, which can be represented as 2A (normal chromosome 21).

3. **Gamete Formation**:
- The affected mother can produce two types of gametes:
- One with the extra chromosome 21 (A + 1)
- One with the normal chromosome 21 (A)
- The normal father can produce two types of gametes:
- One with chromosome 21 (A) and an X chromosome (X)
- One with chromosome 21 (A) and a Y chromosome (Y)

4. **Punnett Square Analysis**:
- Set up a Punnett square to visualize the combinations of gametes from the mother and father:
```
Mother (Gametes):
1. A + 1 (extra chromosome)
2. A (normal)

Father (Gametes):
1. A + X
2. A + Y
```
- The possible combinations from the Punnett square are:
- (A + 1) + (A + X) = A + A + 1 + X (affected)
- (A + 1) + (A + Y) = A + A + 1 + Y (affected)
- (A) + (A + X) = A + A + X (normal)
- (A) + (A + Y) = A + A + Y (normal)

5. **Count the Outcomes**:
- From the combinations, we can see that 2 out of the 4 possible offspring are affected by Down's syndrome.
- Therefore, the percentage of affected offspring is (2 affected / 4 total) * 100 = 50%.

6. **Conclusion**:
- The percentage of offspring produced by an affected mother and a normal father that would be affected by Down's syndrome is **50%**.