Why are individuals with an extra chromosome 21, which causes Down syndrome, more numerous than individuals with an extra chromosome 3 or chromosome 16

Answer: Extra copies of the other somatic chromosomes are usually fatal.

Explanation:

In genetics, the term aneuploidy refers to the change in chromosome number 23, which can lead to genetic diseases. An aneuploid is an individual whose number of chromosomes differs from the wild or euploid type in part of its chromosomal makeup, due to an extra or missing chromosome, which is always associated with a deficiency in physical or mental development or both. This occurs during errors in meiosis, which is the type of cell division that occurs during the formation of gametes, which are sex cells that during fertilization give rise to a zygote. Human beings are diploid, which means that we have two copies of each chromosome.

Non-disjunction indicates a failure in this process of meiosis, in which two chromosomes or chromatids go together and the other pole receives nothing. A pair of homologous chromosomes that does not separate correctly during meiosis I, gives rise to two abnormal cells as a product of meiosis I: one cell with an extra chromosome and one with a missing chromosome. In meiosis II, the chromatids in the chromosomes separate and it could also give rise to abnromal cells.

In the case of trisomy on chromosome 21, this is more common as the disease it because is not lethal. But in many other cases, trisomy on another pair of chromosomes can be lethal. Having an extra chromosome changes the way a baby develops both physically and intellectually. These changes can present the baby and subsequent adult with a number of mental and physical challenges. This is because it alters the amount of protein produced that is encoded by the DNA in those chromosomes.

Aneuploidy is almost always deleterious because of gene imbalance: the ratio between genes is different than in euploids, and this difference interferes with the normal functioning of the genome. In general, the amount of transcript produced by a gene is directly proportional to the number of copies of that gene in the cell. Thus, for a given gene, the rate of transcription is directly related to the number of DNA moulds available. Thus, the more copies there are of a gene, the greater the number of transcripts produced, and a greater amount of that protein will be obtained. This relationship between the number of copies of a gene and the amount of gene product produced is called the gene dose effect. Usually, this could be lethal but in Down syndrome it is not. So that's why there are more individuals with Down syndrome than with other trisomies, because they survive and the others don't.