Define Aneuploidy. What is the cause of Down's syndrome and Turner's syndrome?

Aneuploidy

Aneuploidy is a type of chromosomal abnormality in which the number of chromosomes in a cell is not exactly 46, which is the normal diploid number in humans. In this condition, the cell may have either one extra chromosome or one missing chromosome. So, the total number of chromosomes becomes either 45 or 47.

This abnormality happens mostly due to non-disjunction during meiosis, which means that chromosomes fail to separate properly during the formation of gametes (sperm or egg). Because of this, the gamete may carry an extra or missing chromosome. When such a gamete fuses with a normal gamete, the resulting zygote has an abnormal chromosome number.

There are two major types of aneuploidy:
  1. Trisomy – where one extra chromosome is present (total becomes 2n + 1, that is 47)
  2. Monosomy – where one chromosome is missing (total becomes 2n − 1, that is 45)

Down's Syndrome

Down's syndrome is a genetic disorder that occurs due to trisomy of chromosome number 21, which means the person has three copies of chromosome 21 instead of the normal two. So, the total number of chromosomes becomes 47 instead of 46. This condition is also known as Trisomy 21.

It was first described in 1866 by a British physician Dr. John Langdon Down, who noticed a pattern of similar physical and mental features among some patients. Later, in 1959, the chromosomal cause of the syndrome was discovered by a French geneticist Jerome Lejeune, who identified the extra chromosome 21 as the reason behind the disorder.

The condition results from non-disjunction during meiosis, where chromosome 21 does not separate properly. As a result, one of the gametes carries two copies of chromosome 21, and when it combines with a normal gamete, the zygote ends up with three copies of chromosome 21.

People with Down's syndrome usually show a group of signs like a flat face, upward slanting eyes, small ears, short neck, poor muscle tone and intellectual disability. Some also have congenital heart defects or other medical conditions. The severity of symptoms can vary from person to person.

Turner's Syndrome

Turner's syndrome is a genetic disorder that affects only females, caused due to the absence of one X chromosome, leading to a total of 45 chromosomes instead of 46. The genetic formula becomes 45, XO, where "O" means the second sex chromosome is missing. This condition is a type of monosomy involving the sex chromosomes.

It was first described by Dr. Henry Turner in 1938, who observed a set of common physical features in certain female patients and linked them to a chromosomal issue.

The main reason for Turner's syndrome is non-disjunction during meiosis, which leads to the formation of gametes that lack one sex chromosome. When such a gamete is involved in fertilisation, the resulting zygote ends up with only one X chromosome.

Girls with Turner's syndrome usually have short stature, a broad chest with widely spaced nipples, webbed neck, low hairline at the back of the neck, and underdeveloped secondary sexual characteristics. Most of them are infertile due to the absence of ovaries (gonadal dysgenesis), and they may also have heart and kidney defects.






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