Chromosome variability consists of changes in the nucleotide sequences or changes in the number of chromosomes. Similarly to the case of genes, damages in chromosomes are often repaired by proteins. There exists an enzymatic system capable of recreating the chromosome. However, errors can occur during chromosome repair. For example, a broken-off fragment of one chromosome can be attached to another, nonhomologous chromosome, or two identical fragments can be put next to one another. If a chromosome breaks in two places, the broken-off segment can be attached back to front.

The number of chromosomes can change due to perturbations during mitosis or meiosis. For example, homologous chromosomes instead of separating into two forming cells, stay in one of them. Such faulty division of the 21^st chromosome in people causes a disease called the Down syndrome.

The Down syndrome is an incurable disease caused by the existence of an additional chromosome in the 21^st pair. Normal people have the diploidal chromosome number equal to 46. People with the Down syndrome have 47. Trisomal babies are born with frequency of one in about 700.(8) (11) It has been shown, that the likelihood of a Down syndrome birth increases with the mother’s age.

If, during the cell division, chromosomes do not separate, a cell with two sets is created. A gamete with the double set of chromosomes joined during fertilization with a normal one will produce a polyploid with three sets of chromosomes.

Chromosome changes can be caused by different factors. For example, an some alkaloids by disorganizing of the mitotic spindle causes chromosome mutations producing polyploids. This chemical compound is used in agriculture. Poliploidal mutations played a major role in plant evolution.