Variation and mutation

Last time

Before we start: last time, in Protein synthesis, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

No two alike

Look around any classroom: different heights, different hair, different blood groups. These differences are variation. Today: the two kinds of variation, where variation comes from, and mutation, the source of all new alleles.

Variation

Variation is the differences between individuals of the same species. It can be caused by genes, by the environment, or by both.

Continuous variation

Continuous variation gives a range of phenotypes between two extremes. Examples: height, body mass, and leaf length. It is caused by both genes and the environment, and is shown on a histogram, often a bell-shaped curve.

Discontinuous variation

Discontinuous variation gives a limited number of distinct phenotypes, with no in-between. Examples: A B O blood groups, and seed shape in peas, round or wrinkled. It is caused by genes alone, and is shown on a bar chart.

Compare them

Here is the comparison. The cause and the graph are the usual marks. A quick test: can there be an in-between? If yes, it is continuous.

Mutation

A mutation is a genetic change. A gene mutation is a random change in the base sequence of D N A. Mutation is the way new alleles are formed. It is the only source of brand new variation.

What increases mutation

Mutations happen randomly, but some things make them more likely. Ionising radiation, such as X-rays, gamma rays and ultraviolet light. Some chemicals, such as those in tobacco smoke.

Sources of genetic variation

Mutation creates new alleles. Meiosis shuffles chromosomes into different gametes. Random mating: any individual can mate with any other. Random fertilisation: any gamete can fuse with any other.