Genetic Variation Essay

859 words - 3 pages

Genetic Variation

Species refers to a population of organisms which are potentially
capable of interbreeding to produce fertile offspring. Variation can
be shown within a species. Variation is the difference between
individuals within a species. Variation can be caused by genetic
factors, environmental factors or a combination of both. Environmental
factors which affect variation include nutrition, climate and
pollution. Organisms that reproduce sexually display genetic
variation. Genetic variation is a result of meiosis. Meiosis allows
the independent assortment of chromosomes which contributes to
variation.. Independent assortment happens at metaphase I, when the
bivalents line up on the equator. Each bivalent is made up of two
homologous chromosomes, one of which is maternal and one which is
paternal. The orientation of the bivalents across the spindle is
completely random so the maternal and paternal chromosomes can be
mixed up in the final gametes. Each homologous pair aligns on the
spindle completely independently to any other pair. This contributes
to genetic variation.

Another contributing factor to genetic variation is ‘crossing over.’
Crossing over occurs at prophase I, when the bivalents first form.
When the two chromatids come together to form a synapsis, parts of one
chromosome can be exchanged with the corresponding parts of the other
chromosome. The points at which the chromosomes cross over are called
chiasmata. There are usually many chiasmata in a bivalent and it is
the chiasmata which actually hold the bivalent together. Ultimately,
crossing over means that maternal and paternal alleles can be mixed
resulting in different allele combinations.

The fusion of gametes at fertilization to form a zygote is completely
random. Each gamete has a unique combination of genes. Any of the
numerous male gametes can fertilise any of the numerous female
gametes, therefore every zygote is unique. In mitosis (a-sexual
reproduction) the only source of variation is mutation. If a mutation
occurs in a germ line it will be inherited by the offspring.

The members of a population share the same gene pool and their genes
occur within particular frequencies. Natural Selection and isolation
can result in changes in the allele and phenotype frequency which
leads to the formation of a new species. Natural selection is when
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