How Many Chromosome Do Humans Have?

How Many Chromosome Do Humans Have? The chromosomal number and pattern, called karyotype, is usually constant within each animal and plant species. Somatic cells of the human species have 23 pairs of chromosomes; They are diploid cells and have 23 chromosomes but with two copies of each, making a total of 46 individual chromosomes. Each pair of chromosomes is made up of a chromosome inherited from each parent.

Reproductive cells, also called gametes, are haploid and contain only one copy of each chromosome. That is, eggs and sperm have only 23 chromosomes instead of 46. When a sperm and an egg unite, the 23 chromosomes of each are conjugated and will give rise to the 23 pairs that the new individual will have.

Chromosomes can be divided into two main types: autosomes and allosomes. Allosomes carry sexual information and autosomes are all the others. The human species has a single allosome, chromosome 23, with a different shape and size than the rest, which is why it is also known as a heterochromosome.

The particular pair of allosomes that an individual inherits will determine her sex. The human species shares with the rest of mammals the determination of sex by the XX-XY system. If pair 23 is composed of two XX allosomes, the sex of the individual will be female, while it will be male with an XY karyotype.

The number of genes each chromosome contains is different. In some, such as chromosome 1, more than 2,000 genes have been identified, while in others, such as chromosome 18 or 21, the number of genes is around 200. The Y chromosome, with 50 genes, is the chromosome it contains fewer genes.

Aneuploidies

In the field of genetics, alterations in the number of chromosomes are known collectively as aneuploidies. They can lead to genetic diseases and some can be inherited, although most are due to errors during cell division in the formation of gametes or during the early embryonic stages.

For example, trisomy X, characterized by the presence of 3 X chromosomes, is a congenital disease, since it is present from the moment of birth, and it is of genetic origin but it is not hereditary. The same happens with Down syndrome, one of the most common aneuploidies and which is due to the trisomy of chromosome 21 and is not considered a hereditary disease either.

In general, trisomies and monosomies can cause disease but are viable, while nullisomies, or the complete absence of one of the chromosomes, are lethal. Aneuplodies are also often seen in cancer cells.

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