There are two basic types of variability: recombinational and mutational. As is implied by its name, the recombinational variability consists of different combinations of existing alleles in the descendent population. The result of this variability is the enormous diversity of individuals capable of sexual reproduction. Random union of haploidal gametes during fertilization produces a huge diversity of genotypes. A two-gene union of individuals with two alleles produces nine genotypes. A three-gene union produces as much as 81 genotypes.

As can be seen from these calculations, it is extremely unlikely that two people with the same genotypes can be born, except for monozygous twins. Monozygous twins are born, due to perturbations in zygote division, from one egg and one spermatozoon. Twins are very similar not only with respect to their appearance. Where do doubles of famous people come from? How similar are they genetically?

It would be interesting to see if, among people looking at this page, are individuals similar to its authors. If you want to check it out, take a look at our photos. We are waiting for the photos of our "separated at birth" twins. You can mail the photos to this address (22446@advanced.org ).

Mutational variability consists of changes in genes and changes in chromosomes.

Maintaining genetic variability

It is important for organisms not to lose information already produced. It can be useful in different unexpected situations. For example, during cataclysms, normally advantageous traits favored by natural selection can turn out to be completely useless, whereas normally disadvantageous traits can turn out to be the ones deciding upon the survival. Genetic variability is maintained, for example, thanks to the selectional superiority of heterozygotes. Individuals with different alleles are favored and the genetic information is not lost. In some sense, an example of such mechanism is the genetic superiority of heterozygotes with sickle-cell anemia. Another mechanism of maintaining genetic variability is the frequency related selection.