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Introduction to Genetic Variation in Humans

Genetic variation, simply put, is variation in alleles of genes. This variation is

produced by the crossing over of chromosomes during meiosis that results in different
combination of alleles from both parents. Variation is caused by many factors, a large part
of it derived from the previous generation, and the remaining is slightly altered by the
environment. This genetic diversity is what makes every individual unique.
There are two main types of variation; continuous and discontinuous variation.
Continuous variation is the combined effect of polygenic inheritance, where a
number of genes are responsible for the particular phenotype. For example, height is a
form of continuous variation. There is a complete range of measurements from one
extreme to another. There are people with height 65inches, and also 76inches. But there
are also individuals with height 70inches that falls in the middle of the two former
extreme values. Thus, analysis of a type of continuous variation is often presented in a
normal distribution graph with a bell-shaped curve.

Continuous variation can be significantly influenced by environmental factors.

Children that grow up consuming a European diet are given a different amount of
nutrition compared to children that grew up eating Asian meals. Difference in lifestyles
may affect their height and other traits such as weight, foot span and IQ that are classified
under continuous variation.

Discontinuous variation is controlled by a single gene or a very small number of

genes. It is characterized as either having either having a certain trait or not. This
variation is where individuals fall into distinct categories, based on features that do not
have a range. For example, blood type. An individual can be either an O+ or an A-, there
is no in between. These discrete characteristics are classified under discontinuous

Phenotypes classified under discontinuous variation cannot be influenced by

environmental factors. A large difference in lifestyles do not bring a change to what blood
type one has, it will stay the same throughout that individuals life. Other traits in this
category are eye color, ability to roll tongue, attached or non-attached ear lobes and skin
To investigate the variety in humans, be it continuous or discontinuous, our group
has done a small survey on 15 of our classmates on their height, weight, foot span, IQ and
blood type. We have then drawn up graphs to analyse the difference and similarities in the
two types of genetic variation.