Transcript Gene Pools
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 Sample Population Frequency of Alleles allele for brown fur 48% heterozygous black 36% homozygous brown 16% homozygous black allele for black fur 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: 1. What two processes can lead to inherited variation in populations? 2. How does the range of phenotypes differ between single-gene traits and polygenic traits? 3. What is a gene pool? How are allele frequencies related to gene pools? 4. 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 • Natural selection – does not act directly on genes, but on phenotypes. – affects which individuals having different phenotypes survive and reproduce and which do not. – determines which alleles are passed from one generation to the next. – can change the relative frequencies of alleles in a population over time. 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)