Variation and MutationAQA GCSE Biology: Revision notes

Variation describes the differences between individuals of the same species. Variation can be caused by:

• Genetic causes — differences in the genes an individual inherits
• Environmental causes — differences in the conditions an organism develops or lives in (e.g. diet, climate, exercise)
• A combination of genes and environment — most characteristics, such as height, are affected by both

All genetic variants ultimately arise from mutations — random changes to an organism's DNA.

• Most mutations have no effect on the phenotype
• Some mutations influence the phenotype
• Very few mutations determine the phenotype on their own

There is usually extensive genetic variation within a population of any species, arising from the accumulation of these mutations over many generations. The genome interacts with the environment to determine an organism's final phenotype — genes alone rarely tell the whole story.

The order of bases in DNA controls the order in which amino acids are assembled during protein synthesis, which determines the protein produced. If a mutation changes the DNA base sequence, it may change the amino acid sequence of the protein, which can change or destroy the protein's function. Genetic variants can occur in coding DNA (which directly specifies protein structure) or non-coding DNA (which can affect how genes are switched on or off) — both types of variant can influence phenotype.

The type of reproduction an organism uses affects how much variation appears in its offspring:

• Sexual reproduction involves the fusion of male and female gametes, mixing genetic information from two parents — this leads to genetic variation in the offspring
• Asexual reproduction involves only one parent, no fusion of gametes and no mixing of genetic information — offspring are genetically identical clones of the parent