1.2: Most Genes Do Not Function Alone Essential Question: How do allele frequencies vary between populations

Gene Pools: All members of a population can interbreed, they share a common group of genes, called a gene pool. A gene pool is the combined genetic information of all the members of a particular population. Typically contains two or more alleles—or forms of a certain gene—for each inheritable trait. The relative frequency of an allele is the number of times that allele occurs in a gene pool compared with the number of times other alleles occur.

Relative Frequencies of Alleles Section 16-1 Frequency of Alleles Sample Population allele for brown fur allele for black fur 48% heterozygous black 16% homozygous black 36% homozygous brown

Sources of Genetic Variation The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction. Sexual reproduction can thus produce many different phenotypes, but this does not change the relative frequency of alleles in a population. (Card deck analogy)

Evolution as Genetic Change Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to evolution. Ex: Color Mutations

Most Genes Do Not Function Alone: Multifactorial Traits- Traits that are determined by one or more genes and the environment. Some illnesses occur in different forms Inherited or not Caused by one gene or more than one Alzheimer Disease, Breast Cancer, Parkinson Disease, Familial Hypercholesterolemia Knowing whether a trait or illness is single-gene or multifactorial (polygenic) is important for predicting the risk of recurrence.

Single-Gene and Polygenic Traits Inheritable variation can be expressed in a variety of ways.     The number of phenotypes produced for a given trait depends on how many genes control the trait

Single-gene trait Trait controlled by a single gene Variation in this gene leads to only two distinct phenotypes The number of phenotypes a given trait has is determined by how many genes control the trait. In humans, having a widow’s peak or not having a widow’s peak is controlled by a single gene with two alleles. As a result, only two phenotypes are possible.  

Polygenic Traits: Most traits are controlled by two or more genes and are, therefore, called polygenic traits. Each gene of a polygenic trait often has two or more alleles. As a result, one polygenic trait can have many possible genotypes and even more possible phenotypes. EX: height (A bell-shaped curve is also called a normal distribution)

Checkpoint Questions:  What two processes can lead to inherited variation in populations? How does the range of phenotypes differ between single-gene traits and polygenic traits? What is a gene pool? How are allele frequencies related to gene pools? How could you distinguish between a species in which there is a lot of variation and two separate species?

Genes & Disease Risk: Environmental factors & population groups influence the different incidence of disease associated with inheriting the same abnormal gene.

Warm-up . . . All the Help I Can Get Natural selection operates on traits in different ways. You might be able to predict which traits natural selection would favor if you think about the demands of an organism’s environment.

Evolution as Genetic Change determines which alleles are passed from one generation to the next. can change the relative frequencies of alleles in a population over time. Natural selection does not act directly on genes, but on phenotypes. affects which individuals having different phenotypes survive and reproduce and which do not.

Exactly what factors change the relative frequencies of alleles in a population? In genetic terms, any factor that causes alleles to be added to or removed from a population will change the relative frequencies of alleles. Evolution is any change in the relative frequencies of alleles in a population’s gene pool. Evolution acts on populations, not on individuals.

Natural Selection on Single-Gene Traits  Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to evolution. EX: Color Mutations (organisms of one color may produce fewer offspring than organisms of another color.

Genetic Determinism: The fact that the environment modifies gene actions counters the idea of genetic determinism, which is that an inherited trait is inevitable. Harmful or Helpful Public Policy (Harmful) Identifying the genetic component to a trait (Helpful)