Chromosome - a structural and functional element of the cell nucleus containing genes. The name "chromosome" comes from Greek words (chrōma - color, color and sōma - body), and is due to the fact that during cell division they are intensively stained in the presence of basic dyes (for example, aniline).
Many scientists, since the beginning of the 20th century, have thought about the question: “How many chromosomes does a person have?” So until 1955, all the "minds of mankind" were convinced that the number of chromosomes in humans is 48, i.e. 24 pairs. The reason was that Theophilus Painter (a Texas scientist) incorrectly counted them in preparative sections of the testes of people who were neutered by court order (1921). In the future, other scientists, using different methods of counting, also came to this opinion. Even having developed a chromosome separation method, the researchers did not dispute the result of Painter. The mistake was discovered by scientists Albert Levan and Jo-Hin Tho in 1955, who accurately calculated how many pairs of chromosomes a person has, namely - 23 (more modern techniques were used to calculate them).
Somatic and germ cells contain a different chromosome set in biological species, which cannot be said about the morphological signs of chromosomes that are constant. Somatic cells have doubled (diploid set), which are divided into pairs of identical (homologous) chromosomes, which are similar in morphology (structure) and size. One part is always paternal, the other is of maternal origin. Human sex cells (gametes) are represented by a haploid (single) set of chromosomes. When the egg is fertilized, they are combined in one nucleus of the zygote of the haploid sets of female and male gametes. At the same time, double dialing is restored. It can be said with accuracy how many chromosomes a person has - 46, with 22 pairs of them autosomes and one pair - sex chromosomes (gonosomes). Genders have differences - both morphological and structural (gene composition). In the female body, the gonosal pair contains two X chromosomes (XX pair), and in the male, one X and Y chromosome (XY pair).
Morphologically, chromosomes change during cell division, when they double (with the exception of germ cells, in which duplication does not occur). This is repeated many times, however, a change in the chromosome set is not observed. The most noticeable chromosomes at one of the stages of cell division (metaphase). In this phase, the chromosomes are represented by two longitudinally-split formations (sister chromatids), which are narrowed and combined in the region of the so-called primary constriction, or cenomeres (an essential element of the chromosome). The telomeres are called the ends of the chromosome. Structurally, human chromosomes are represented by DNA (deoxyribonucleic acid), which encodes the genes that make up their composition. Genes, in turn, carry information about any particular trait.
From how many chromosomes a person will depend on his individual development. There are such concepts as: aneuploidy (a change in the number of individual chromosomes) and polyploidy (the number of haploid sets is greater than diploid). The latter can be of several types: loss of a homologous chromosome (monosomy), or the appearance of extra chromosomes (trisomy - one extra, tetrasomy - two extra, etc.). All this is a consequence of genomic and chromosomal mutations, which can lead to pathological conditions such as: Down's disease, Kleinfelter syndromes, Shereshevsky-Turner and other diseases.
Thus, only the 20th century gave answers to all questions, and now every educated resident of planet Earth knows how many chromosomes a person has. It is on the composition of 23 pairs of chromosomes (XX or XY) that the sex of the unborn child depends, and this is determined during fertilization and the fusion of the female and male reproductive cells.