Alleles and base sequences
Definition
Allele
A different version of a gene.
- A variant is a difference in the base sequence of a gene.
- Different alleles of a gene are simply different variants of that gene.
- Variants arise through mutations, which are changes to the DNA.
- This is a source of the genetic variation found within a species.
Note
Every allele is a variant: a version of the gene with a slightly different base sequence.
Example
The gene for a flower's colour may exist as several variants, giving alleles for different colours.
How coding variants affect phenotype
Definition
Phenotype
The observable characteristics of an organism, produced by its genotype and its environment.
- A variant in the coding DNA can change the protein made, and so change the characteristic.
- A variant in the non-coding DNA can change how much protein is made, and so change the characteristic.
- Most variants have no effect on the phenotype.
- Some variants have a small effect on the phenotype.
- Very few variants, on their own, determine the phenotype.
- Together, all these variants produce the range of phenotypes seen in a population.
Hint
To judge a variant, first ask whether it is in coding or non-coding DNA, then ask how big its likely effect is.
Key Idea
Alleles are variants, and it is the mix of many variants that gives a population its range of features.
Self review
- What is a variant?
- How are alleles related to variants?
- How can a variant in coding DNA affect the phenotype?
- Out of all variants, how many actually determine the phenotype?
