Genotype is a combination of alleles (gene variants) borne by a specific individual in his cells (cell – see Cells). In diploid organisms, each individual has a pair of alleles from each gene in his cells, where this can be a pair of identical alleles (homozygote) or a pair of different alleles (heterozygote). Because the number of genes in the genome (see Genome) of organisms is enormous and a large percent of them occur in many variants in a given species, the number of possible combinations of alleles – number of different genotypes – is unimaginably large and practically no pair of individuals in the population of a sexually reproducing species, with the exception of identical twins, has an identical genotype.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more