3.19–3.20Variation
Differences in the characteristics of individuals in a population is called variation, and it may be due to:
• the genes they have inherited — genetic causes • the conditions in which they have developed — environmental causes • a combination of genes and the environment.
The third is the commonest and the one students leave out. Height is a good example: your genes set a range, and diet and health decide where in that range you end up. The genome and its interaction with the environment together influence the development of the phenotype.
There is usually extensive genetic variation within a population of a species, and all variants arise from mutations. Of those mutations:
• most have no effect on the phenotype • some influence the phenotype • very few determine the phenotype.
Mutations occur continuously. Very rarely a mutation will lead to a new phenotype, and if that new phenotype is suited to an environmental change, it can lead to a relatively rapid change in the species.
- Variation
- differences in the characteristics of individuals in a population, caused by their genes, by the conditions in which they developed, or by a combination of the two
- Mutation
- a change in the genetic material from which all variants arise; most have no effect on the phenotype, some influence it and very few determine it
- Phenotype
- the characteristics an organism actually shows, influenced by the genome and its interaction with the environment
3.21–3.23The Human Genome Project and mutation
The Human Genome Project set out to read the order of every base in human DNA. It was completed in 2003, and the sequence it produced is what most modern genetics is built on. Four uses are worth being able to name:
• searching for the genes linked to particular diseases, by comparing the DNA of people who have a condition with people who do not • understanding and treating inherited disorders, because knowing which gene is at fault is the first step to doing anything about it • tracing patterns of human migration in the past, by comparing the genomes of populations living in different parts of the world • tailoring medicine to the individual, choosing a drug and a dose that suits a particular person's genes.
Two facts about variation sit alongside it, and both are commonly misremembered.
First, most features of the phenotype are the result of several genes working together, not one gene acting alone. Height, mass and skin colour are all like this, which is why they vary continuously rather than falling into distinct groups.
Second, there is usually extensive genetic variation within a population of a species, and that variation arises through mutation. Mutations happen continuously and at random. Most have no effect on the phenotype at all. Some have a small effect. Very rarely, a single mutation is enough to determine a feature of the phenotype on its own — and it is that rare case, not the common one, that leads to an inherited disorder.
Common mistake
A mutation is not automatically harmful and not automatically useful. The great majority change nothing a student or a doctor would notice; natural selection only has anything to act on in the minority that do.
Before you practise
- Variation has three sources: genes, the environment, and a combination of the two.
- Most phenotypic features are controlled by several genes, not one, which is why they vary continuously.
- All genetic variation in a population arises originally through mutation.
- Most mutations have no effect on the phenotype; a few have a small effect; very rarely one determines it.
- Name the uses of the Human Genome Project: finding disease genes, treating inherited disorders, tracing migration, and tailoring medicine.