Biology - Genetic variation
In organisms which reproduce sexually there are several causes of genetic
variation. These include:
* Independent assortment  (segregation)  of chromosomes during
meiosis; homologous chromosomes separate independently of each other
therefore there is a wide variety of different gametes due to the
particular combination of chromosomes which end up with each other, for
example if n is the haploid number of chromosomes then the number of
possible combinations is 2n. In humans the haploid number of chromosomes is
23, therefore the possible combinations of chromosomes in gametes would be
* Crossing over and recombination  results in new combinations of
alleles in chromosomes of gametes; the number and location of chiasmata 
relative to the sequence of genes, determines the different combinations
* Random fusion of gametes in fertilisation results in variety between
* Partial dominance (a range between complete and no dominance) will
affect the phenotype.
It is important to remember that:
* these causes of variation are due to reshuffling of existing genetic
material, NOT the introduction of new genetic information;
* variations caused by environmental factors are not heritable.
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