Not all failures of meiosis result in early miscarriage. There are late miscarri

Question

Not all failures of meiosis result in early miscarriage. There are late miscarriages and stillbirths of severely malformed fetuses with wrong chromosome numbers. In addition, about one live-born human baby in 200 has the wrong number of chromosomes, often accompanied by severe abnormalities.

One of the most common chromosomal abnormalities is Trisomy 21, or Down's Syndrome resulting from three copies of chromosome 21. This occurs in about one baby in 600. Characteristics of Down syndrome include rounded features, an enlarged tongue that often protrudes, and various internal disorders.

Trisomy 21 occurs more frequently among babies born to women over 35 years old. The age of the father has little or no effect.

What do you think are possible reasons leading to this meiotic nondisjunction in female eggs? It is interesting to note that the total amount of women's eggs are already set by the time she is born, while a man's sperm is continuosly produced. Do you think that there could be some other environmental problems affecting this situation?

Explanation / Answer

The egg cells in females are formed when they are in their mother's womb. The number of egg cells a female could have is fixed at the time of their birth itself. The egg cells remain in prophase of Meiosis I. The homologous chromosomes are held by a protein complex known as cohesin. They remain in this state still they are ovulated.

Over time, the cohesin breaks down, and it's unclear whether it is replaced or not. If it gets replaced, then the replacement system appears to break down over time.

This means, as the woman gets ageing the egg's chromosomes may lose their cohesion and break apart. If this happens, it greatly increases the chance of nondisjunction, which is the causing the diseases like Down syndrome.

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