16-1 Genes and Variation
How Common is Genetic Variation
Many genes have at least two forms, or alleles.
All organisms have genetic variation that is “invisible” because it involves small differences in biochemical processes.
An individual organism is heterozygous for many genes.
Variation and Gene Pools
A population is a group of individuals of the same species that interbreed.
A gene pool consists of all genes, including all the different alleles, that are present in a population.
The relative frequency of an allele is the number of times the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur.
Relative frequency is often expressed as a percentage.
Gene Pool Mice
In genetic terms, evolution is any change in the relative frequency of alleles in a population.
Sources of Genetic Variation
The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction.
Mutations
A mutation is any change in a sequence of DNA.
Mutations occur because of mistakes in DNA replication or as a result of radiation or chemicals in the
environment.
Gene Shuffling
Most heritable differences are due to gene shuffling.
Both Crossing-over and sexual reproduction are important
Single-Gene and Polygenic Traits
The number of phenotypes produced for a given trait depends on how many genes control the trait.
A single-gene trait is controlled by one gene that has two alleles. Variation in this gene leads to only two possible phenotypes.
The allele for a widow’s peak is dominant over the allele for a hairline with no peak.
Since widow's peak is dominant does that mean that it will be more common in the population?
Many traits are controlled by two or more genes and are called polygenic traits.
One polygenic trait can have many possible genotypes and phenotypes.
Height in humans is a polygenic trait. What are some other examples?
•A bell-shaped curve is typical of polygenic traits.
•A bell-shaped curve is also called normal distribution.
The Hardy-Weinberg principle
The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.
When allele frequencies remain constant it is called genetic equilibrium.
Five conditions are required to maintain genetic equilibrium from generation to generation:
•there must be random mating,
•the population must be very large,
•there can be no movement into or out of the population,
•there can be no mutations, and
•there can be no natural selection.