Family has 9 girls, Probability of son at 10th birth is :-

Options

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

Answer

To solve the question regarding the probability of having a son at the 10th birth in a family that already has 9 girls, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Genetic Basis**:
- In humans, the sex of a child is determined by the combination of sex chromosomes contributed by the parents. Females have two X chromosomes (XX), and males have one X and one Y chromosome (XY).

2. **Gamete Formation**:
- During reproduction, females produce eggs that all carry an X chromosome. Males produce sperm that can carry either an X or a Y chromosome. Thus, there is a 50% chance that a sperm will carry an X chromosome and a 50% chance that it will carry a Y chromosome.

3. **Probability Calculation**:
- When an egg (X) is fertilized by a sperm (X), the result is a female (XX). When an egg (X) is fertilized by a sperm (Y), the result is a male (XY). Therefore, for each birth, the probability of having a boy (XY) is 50%, and the probability of having a girl (XX) is also 50%.

4. **Independence of Events**:
- The outcome of each birth is independent of previous births. This means that the fact that the family has already had 9 girls does not affect the probability of the 10th child being a boy.

5. **Conclusion**:
- Therefore, the probability of having a son at the 10th birth remains 50%.

### Final Answer:
The probability of having a son at the 10th birth is **50%**.

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