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16-1 Genes and Variation
16-1 Genes and Variation
Photo credit: ©MURRAY, PATTI/Animals Animals Enterprises
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2. How Common Is Genetic Variation?
Many genes have at least two forms, or________.
All organisms have genetic variation that is “_______” because it involves small differences in biochemical processes.
An individual organism is heterozygous for many genes.
Typically, insects are heterozygous for as many as ____% of genes while fish, reptiles, and mammals are heterozygous for between ______% of genes.
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3. Variation and Gene Pools
Genetic variation is studied in populations.
A population is a group of individuals of the same species that interbreed.
A ____________consists of all genes, including all the different alleles, that are present in a population.
Relative_________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 ___________________.
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4. Variation and Gene Pools
Gene Pool for Fur Color in Mice
Sample Population
Frequency of Alleles
allele for brown fur
allele for black fur
When scientists determine whether a population is evolving, they may look at the sum of the population’s alleles, or its gene pool. This diagram shows the gene pool for fur color in a population of mice. 
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5. Variation and Gene Pools
Gene pools are important to evolutionary theory, because evolution involves changes in populations over time
In genetic terms, evolution is any change in the ___________________of alleles in a population.
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6. Sources of Genetic Variation
The two main sources of genetic variation are __________ and the genetic shuffling that results from sexual reproduction.
Mutations
A mutation is any change in a sequence of _____.
Mutations occur because of mistakes in DNA replication or as a result of ________or chemicals in the environment.
Mutations do not always affect an organism’s ________________.
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7. Sources of Genetic Variation
Gene Shuffling
Most heritable differences are due to ___________ that occurs during formation of gametes.
The chromosomes in human can produce 8.4 ___________ different combinations of genes!
Crossing-over further increases the number of genotypes that can appear in offspring.
Sexual reproduction produces different phenotypes, but it ___________change the relative frequency of alleles in a population.
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8. Single-Gene and Polygenic Traits
The number of phenotypes produced for a given trait depends on how many genes control the trait.
A _____________ trait is controlled by one gene that has two alleles. Variation in this gene leads to only two possible phenotypes.
In humans, a single gene with two alleles controls whether a person has a widow’s peak (left) or does not have a widow’s peak (right). As a result, only two phenotypes are possible. The number of phenotypes a given trait has is determined by how many genes control the trait.
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9. Single-Gene and Polygenic Trait
The allele for a ______________is dominant over the allele for a hairline with no peak.
However, the presence of a widow’s peak may be less common in a population.
In real populations, phenotypic ratios are determined by the frequency of alleles as well as by whether the alleles are dominant or recessive.
Allele frequencies may not match __________ ratios.
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10. Single-Gene and Polygenic Trait
Many traits are controlled by two or more genes and are called _______________. Each gene of a polygenic trait has two or more alleles.
One polygenic trait can have many possible genotypes and phenotypes.
Height in humans is a polygenic trait.
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11. Single-Gene and Polygenic Trait
A bell-shaped curve is typical of polygenic traits.
A bell-shaped curve is also called____________ distribution.
The graph below shows the distribution of phenotypes that would be expected for a trait if many genes contributed to the trait.
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