Genes and Variation 16-1

Objectives What are the main sources of heritable variation in a population? How is evolution defined in genetics terms? What determines the numbers of phenotypes for a given trait?

Darwin and Today Darwin was unaware of Mendel’s work with genes and heredity. What did Mendel study again? Today’s understanding of genes, DNA, variation, and mutations is central to our understanding of how evolution works.

How Common is Genetic Variation? Many genes have more than one form or alleles Examples? There is also additional variation that is “invisible” because it involves small variations in biochemical processes

Variation and Gene Pools It is important to study entire populations for genetic variation Population: group of the same species that interbreed. Interbreeding populations share common groups of genes, including all the different alleles, in their gene pool.

Relative Frequency The relative frequency of an allele is the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. Has nothing to do with whether the allele is dominant or recessive. EVOLUTION in genetics terms: change in the relative frequency of alleles in a population.

Relative Frequency

Main Sources of Genetic Variation in a Population Mutations Gene Shuffling: 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. Some mutations do not always affect an organisms phenotype, some do. Some mutations do not affect an organism’s fitness, some do.

Gene Shuffling Most heritable differences are due to gene shuffling that occurs during the production of gametes during meiosis. Crossing-over further increases the number of different genotypes that can appear in offspring. Sexual reproduction produces different phenotypes, but it does not change the relative frequency of alleles in a population. i.e. no matter how many times you shuffle a deck of cards, the probability of drawing a particular card will remain the same.

What determines how many phenotypes there are for a given trait? Question What determines how many phenotypes there are for a given trait?

Single Gene Traits Single-gene traits are controlled by a single gene that has two alleles. Variation only leads to two possible phenotypes.

Polygenic Traits Polygenic traits are controlled by two or more genes. Polygenic traits have many possible genotypes and phenotypes. Represented with a bell shaped or “normal distribution” curve.

Answer The number of phenotypes produced for a given trait depends on how many genes control the trait.