Variation - Variation and Its Causes (GCSE Biology)

Variation and Its Causes

Genotypes and Phenotypes

Key Terms

A genotype is the collection of genes an organism has. This is inherited from the parents.

A phenotype is the physical characteristics of an organism. All organisms all have different appearances and characteristics – i.e. different phenotypes. The phenotype is determined by environmental and genetic factors.

Variation is the differences in phenotypes in a population. There can be variation in physical appearance (e.g. different weight), social factors (e.g. personality), academic factors (e.g. natural talents), and much more.

Continuous variation results in a range of phenotypes between two extremes e.g. weight and height in humans.

Discontinuous variation results in a limited number of phenotypes with no intermediates but instead distinct groups e.g. ability to roll your tongue. It is mostly caused by one gene without environmental influence e.g. A, B, AB and O blood groups in humans.

Genotypic variation is the differences in genotype.

Phenotypic variation is the differences in the phenotypes expressed. Genotypic variation can cause phenotypic variation and environmental factors can also cause phenotypic variation.

Environmental variation is the differences in phenotype causes by the environment. Hence, they are acquired characteristics e.g. your accent.

Reasons for Variation

There are many reasons for variation:

1. Genes – the set of genes that have been inherited from the mother and father can control many characteristics. For example, blood type is controlled by genes only, there is no environmental influence. Other examples of genetic phenotypes are natural eye colour and the ability to roll your tongue.
2. Environment – the environment in which an organism grows and develops will play a large role in phenotype. For example, an accent is controlled solely by the environment.
3. Genes and Environment– the majority of factors are not solely controlled by either variable. Weight for example is a cross of both – even if both parents of a child have a low metabolism and are overweight, they may still have a very light child, as the weight is dependent also on environmental factors such as diet and exercise.

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Variation in Species

Variation Between and Within Species

Variation can occur either:

• Between species – individuals in different species tend to be very different and have lots of variation. For example, a lion has a tail whereas a human does not.
• Within species – individuals of the same species tend to have more similarities and have less variation than individuals of different species. They tend to produce similar proteins and have a similar body anatomy, but genetic variation is almost always present. These variations arise through mutations.

Mutation and Variation

Variation occurs within species due to mutations, which we learnt about earlier. Mutations are a spontaneous, random change in genetic material due to a mistake in replication and they can form new alleles.

When the genetic material is duplicated during reproduction, a mutation can occur leading to a change in genotype. However, this does not always lead to a change in the phenotype.

Sometimes the mutation is silent, which means that it doesn’t affect the final phenotype. Sometimes mutations have a small effect on the phenotype and, rarely, a single mutation will significantly affect the phenotype. These mutations can lead to the new organism having different features. If the mutation is beneficial, it can give the new organism an advantage and can therefore be passed on to then next generation. We will learn more about this in the next tutorial on natural selection.

Sexual Reproduction and Variation

• Sexual reproduction can also cause variation. The mixing of the maternal and paternal chromosomes during meiosis in the formation of gametes gives rise to variation.
• Random fertilisation of gametes also produces genetic variation of offspring. This is because any female gamete can fuse with any male gamete e.g. an egg or sperm in humans.