What is the normal set of chromosomes in humans?
Human cells have 22 sex-independent chromosome pairs (autosomes) and two sex chromosomes (gonosomes), so a total of 46 chromosomes make up one set of chromosomes. Autosomes are usually present in pairs. The chromosomes of a pair are similar in shape and sequence of the genes and are therefore called homologous.
The two X chromosomes of women are also homologous, whereas men have one X and one Y chromosome. These differ in the shape and number of genes present in such a way that one can no longer speak of homology. Due to meiosis, germ cells, i.e.
egg and sperm cells, only have half the set of chromosomes, i.e. 22 individual autosomes and one gonosome each. Since the germ cells fuse during fertilisation and sometimes exchange whole segments (crossover), a new combination of chromosomes is created (recombination). All chromosomes together are called the karyotype, which with few exceptions (see chromosome aberrations) is identical in all individuals of one sex. Here you can find out everything about the topic: Mitosis – Simply explained!
Why are there always pairs of chromosomes?
Basically, this question can be answered in one sentence: Because it has proven to be beneficial. The presence of chromosome pairs and the principle of recombination are essential for heredity in the sense of sexual reproduction. In this way, the genetic material of two individuals can be randomly combined to form a completely new individual.
This system enormously increases the diversity of traits within a species and ensures that it can adapt to changing environmental conditions much more quickly and flexibly than would be possible through mutation and selection alone. The double set of chromosomes also has a protective effect: If a mutation of one gene would lead to a loss of function, there is still a kind of “safety copy” in the second chromosome. Although this is not always sufficient for the organism to compensate for the dysfunction, especially when the mutated allele is dominant, it does increase the chance of this happening. Furthermore, this way the mutation is not automatically passed on to all offspring, which in turn protects the species from overly radical mutations.
What is a chromosome mutation?
Genetic defects can be caused by ionising radiation (e.g. X-rays), chemical substances (e.g. benzopyrene in cigarette smoke), certain viruses (e.g. HP viruses) or, with a small probability, purely by chance.
Often several factors are involved in the development. In principle, such changes can occur in all body tissues, but for practical reasons the analysis is usually limited to lymphocytes (a special type of immune cells), fibroblasts (connective tissue cells) and bone marrow cells. A chromosomal mutation is a large structural change in individual chromosomes.
The absence or addition of whole chromosomes, on the other hand, would be a genome or ploidy mutation, while the term gene mutation refers to comparatively small changes within a gene. The term chromosomal aberration (lat. aberrare = to deviate) is somewhat broader and includes all changes that can be detected with the light microscope.
Mutations can have very different effects: The best known of the various forms of chromosomal aberrations are probably the numerical aberrations, in which individual chromosomes are present only once (monosomy) or three times (trisomy). If this applies to only a single chromosome, one speaks of aneuploidy, the entire set of chromosomes is affected by polyploidy (tri- and tetraploidy). In most cases, this maldistribution occurs during germ cell development due to non-separation (nondisjunction) of chromosomes during cell division (meiosis).
This leads to an uneven distribution of chromosomes to the daughter cells and thus to numerical aberration in the developing child. Monosomes of non-gender chromosomes (= autosomes) are not compatible with life and therefore do not occur in living children. Also autosomal trisomies, apart from trisomy 13, 18 and 21, almost always lead to a spontaneous miscarriage.
In any case, in contrast to the aberrations of the sex chromosomes, which can also be rather inconspicuous, there are always severe clinical symptoms and usually also more or less pronounced external abnormalities (dysmorphies). However, such a maldistribution can also occur later in life with mitotic cell division (all cells except germ cells). Since here, in addition to the affected cells, there are also unaltered cells, this is called a somatic mosaic.
Somatic (gr. soma = body) refers to all cells that are not germ cells. Since only a small part of the body cells is affected, the symptoms are usually much milder.
Mosaic types therefore often remain undetected for a long time. Here you can find out everything about the topic: Chromosome mutation
- Silent mutations, i.e. mutations where the change has no effect on the individual or their offspring, are rather atypical for chromosomal aberrations and are more often found in the area of gene or point mutations. – One speaks of a loss-of-function mutation when the mutation results in a misfolded and thus functionless or no protein at all. – So-called gain-of-function mutations change the type of effect or the amount of proteins produced in such a way that completely new effects are created. On the one hand, this is a decisive mechanism for evolution and thus for the survival of a species or the emergence of new species, but on the other hand it can also, as in the case of the Philadelphia chromosome, contribute decisively to the development of cancer cells.
