17.1 genes and variation - wikispaceskeystonescience.wikispaces.com/file/view/genes+and... · 17.1...

Lesson Overview 17.1 Genes and Variation

Lesson Overview Genes and Varia+on

THINK ABOUT IT Darwin developed his theory of evolu+on without knowing how heritable traits passed from one genera+on to the next or where heritable varia+on came from. What would happen when gene+cs answered ques+ons about how heredity works?


Genetics Joins Evolutionary Theory How is evolu+on defined in gene+c terms?


Genetics Joins Evolutionary Theory How is evolu+on defined in gene+c terms? In gene+c terms, evolu+on is any change in the rela+ve frequency of alleles in the gene pool of a popula+on over +me.


Genetics Joins Evolutionary Theory Researchers discovered that heritable traits are controlled by genes. Changes in genes and chromosomes generate varia+on. For example, all of these children received their genes from the same parents, but they all look different.


Genotype and Phenotype in Evolution

An organism’s genotype is the par+cular combina+on of alleles it carries. An individual’s genotype, together with environmental condi+ons, produces its phenotype. Phenotype includes all physical, physiological, and behavioral characteris+cs of an organism.


Genotype and Phenotype in Evolution

Natural selec+on acts directly on phenotype, not genotype. Some individuals have phenotypes that are beMer suited to their environment than others. These individuals produce more offspring and pass on more copies of their genes to the next genera+on.


Populations and Gene Pools

A popula+on is a group of individuals of the same species that mate and produce offspring. A gene pool consists of all the genes, including all the different alleles for each gene that are present in a popula+on.



Researchers study gene pools by examining the rela*ve frequency of an allele. The rela+ve frequency of an allele is the number of +mes a par+cular allele occurs in a gene pool, compared with the number of +mes other alleles for the same gene occur.


For example, this diagram shows the gene pool for fur color in a popula+on of mice.



Evolu+on is any change in the rela+ve frequency of alleles in the gene pool of a popula+on over +me. Natural selec+on operates on individuals, but resul+ng changes in allele frequencies show up in popula+ons. Popula+ons, rather than individuals, evolve.


Sources of Genetic Variation What are the sources of gene+c varia+on?


Sources of Genetic Variation What are the sources of gene+c varia+on? Three sources of gene+c varia+on are muta+on, gene+c recombina+on during sexual reproduc+on, and lateral gene transfer.


Mutations

Muta+ons that produce changes in phenotype may or may not affect fitness. Some muta+ons may be lethal or may lower fitness; others may be beneficial. Muta+ons maMer in evolu+on only if they can be passed from genera+on to genera+on. The muta+on must occur in the germ line cells that produce either eggs or sperm.


Genetic Recombination in Sexual Reproduction

Muta+ons that produce changes in phenotype may or may not affect fitness. Some muta+ons may be lethal or may lower fitness; others may be beneficial. Muta+ons maMer in evolu+on only if they can be passed from genera+on to genera+on. The muta+on must occur in the germ line cells that produce either eggs or sperm.


Lateral Gene Transfer

Lateral gene transfer occurs when organisms pass genes from one individual to another that is not its offspring. It can occur between organisms of the same species or organisms of different species. Lateral gene transfer can increase gene+c varia+on in a species that picks up the “new” genes.


Single-Gene and Polygenic Traits What determines the number of phenotypes for a given trait?


Single-Gene and Polygenic Traits What determines the number of phenotypes for a given trait? The number of phenotypes produced for a trait depends on how many genes control the trait.


Single-Gene Traits A single-‐gene trait is a trait controlled by only one gene. Single-‐gene traits may have just two or three dis+nct phenotypes. The most common form of the allele can be dominant or recessive.


Dominance of an allele for a single-‐gene trait does not necessarily mean that the dominant phenotype will always appear with greater frequency in a given popula+on. An example of a single-‐gene trait is the presence of dark bands that appear on the shells of a certain species of snails. Even though the allele for shells without bands is dominant, a popula+on may show a greater frequency of the “with bands” phenotype.


Polygenic Traits

Polygenic traits are traits controlled by two or more genes. Each gene of a polygenic trait oUen has two or more alleles. A single polygenic trait oUen has many possible genotypes and even more different phenotypes.


Polygenic Traits

Human height, which varies from very short to very tall, is an example of a polygenic trait. The bell-‐shaped curve in the graph is typical of polygenic traits.

17.1 genes and variation - wikispaceskeystonescience.wikispaces.com/file/view/genes+and... · 17.1...

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