The cause of combinative variability is the formation of new combinations of existing genes.
Combinative variability is based on three processes:
- exchange of sections of homologous chromosomes (crossing over);
- independent divergence of homologous chromosomes during the formation of germ cells;
- random combination of gametes during fertilization.
Thanks to these processes, there is a constant “rearrangement” of genes and new combinations of them appear. In this case, the structure of genes does not change.
Combinative variability obeys the laws of inheritance. An example is the inheritance of blood groups. So, parents with the second (A) and third (B) groups may have children with any of the four blood groups.
Combinative variability in the population also serves as a mechanism for the propagation of mutations. So in human populations, the spread of some diseases caused by mutations, such as hemophilia, occurs.
Combinative variability provides genotypic and phenotypic diversity of organisms in a population. The genes are preserved, but new combinations of them appear, and the genotype of each organism turns out to be unique. Allele frequencies do not change in this case, but the variety of phenotypes serves as material for natural selection.
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