Risk of Down’s syndrome in Elderly (>35 years)
**Core Concept**
The risk of Down's syndrome in offspring increases with the age of the mother, particularly after the age of 35. This is due to the increased chance of meiotic errors during oogenesis, which are more frequent in older eggs. The exact mechanisms are not fully understood, but it is thought to be related to the accumulation of genetic damage over time.
**Why the Correct Answer is Right**
The increased risk of Down's syndrome in older mothers is thought to be due to the increased rate of meiotic errors, such as non-disjunction, which occurs when a pair of homologous chromosomes fails to separate properly during meiosis. This can result in an extra copy of chromosome 21, leading to Down's syndrome. As women age, the rate of meiotic errors increases, resulting in a higher risk of Down's syndrome. Specifically, the risk of Down's syndrome is increased by 3-4 times in women over 35 years old.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the risk of Down's syndrome is not significantly increased in women under 35 years old.
* **Option B:** This option is incorrect because the increased risk of Down's syndrome in older mothers is not due to the increased rate of genetic mutations, but rather due to meiotic errors.
* **Option C:** This option is incorrect because the risk of Down's syndrome in women over 35 years old is not limited to those with a family history of the condition.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the risk of Down's syndrome increases significantly with the age of the mother, particularly after the age of 35. This is a critical consideration in prenatal counseling and screening.
**Correct Answer:** D.