Down's syndrome in caused by an extera copy of chromosome number 21 . What percentage of offspring is produced by an affected mother and a normal father .

Options

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

Answer

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

### Step-by-Step Solution:

1. **Understanding Down's Syndrome**:
- Down's syndrome is caused by an extra copy of chromosome 21, resulting in a total of 47 chromosomes instead of the normal 46. This condition is known as trisomy 21.

2. **Parental Genotypes**:
- The affected mother has Down's syndrome, so her genotype can be represented as 21(+) (indicating the extra chromosome).
- The normal father has a typical genotype of 21 (indicating he has the normal number of chromosomes).

3. **Gamete Formation**:
- The affected mother can produce two types of gametes (ova):
- Normal gamete (with 21 chromosomes) - 50% chance
- Abnormal gamete (with 22 chromosomes, due to the extra chromosome 21) - 50% chance
- The normal father can only produce normal gametes (with 21 chromosomes).

4. **Punnett Square Analysis**:
- We can set up a simple Punnett square to visualize the possible combinations of gametes from the parents:
```
Mother (Gametes):
Normal (21) | Abnormal (22)
----------------------------
Father (21) | Normal (21) | Down's (22)
```

5. **Offspring Genotypes**:
- The possible offspring genotypes from the combinations are:
- Normal offspring (from normal mother gamete and normal father gamete) - 50%
- Down's syndrome offspring (from abnormal mother gamete and normal father gamete) - 50%

6. **Conclusion**:
- Therefore, the percentage of offspring that will have Down's syndrome is 50%.

### Final Answer:
The percentage of offspring produced by an affected mother (with Down's syndrome) and a normal father that will have Down's syndrome is **50%**.

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