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biology 0610printed 245 · PDF 245
328 pagesrevision 46933
printed 245 · PDF 245
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Causes of variation
● Discontinuous variation is usually caused by genes only, like
blood groups are decided by genes only.
● Continuous variation is often dependent on both genes and
environment such as height, which depends on both genes and the
diet and habits of a person.
● A mutation, put simply, is a random change in a gene.
● Mutations are how new alleles are formed.
● A gene mutation occurs when there is a random change in the
base sequence of DNA.
● It is unpredictable and can happen for no apparent reason but it
can be accelerated by ionizing radiation or by heavy metals such
as lead and mercury.
Why variation occurs
● Genetic variation can occur for a number of reasons. This
includes mutations, meiosis, random mating and random
fertilization.
● Mutations cause variation because they lead to change in
an allele.
● Meiosis causes variation because the pairs of homologous
chromosomes exchange genes and so the gametes formed can
have many different combinations of alleles.
● In a population, any male can mate with any female and at
fertilization, any two gametes can fuse together so there are a huge
number of combinations of alleles that can be produced in the
zygote.
What is an adaptive feature?
● An adaptive feature is an inherited feature that helps an
organism to survive and reproduce in its environment.