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Biology
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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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3. How Common Is Genetic Variation?
Many genes have at least two forms, or alleles.
All organisms have genetic variation that is “invisible” because it involves small differences in biochemical processes.
An individual organism is heterozygous for many genes.
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4. Variation and Gene Pools
Genetic variation is studied in populations.
A population is a group of individuals of the same species that interbreed.
A gene pool consists of all genes, including all the different alleles, that are present in a population.
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5. Variation and Gene Pools
The relative frequency 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 percentage.
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6. 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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7. Variation and Gene Pools
How is evolution defined in genetic terms?
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8. Variation and Gene Pools
In genetic terms, evolution is any change in the relative frequency of alleles in a population.
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9. Sources of Genetic Variation
What are the main sources of heritable variation in a population?
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10. Sources of Genetic Variation
The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction.
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11. Sources of Genetic Variation
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.
Mutations do not always affect an organism’s phenotype.
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12. Sources of Genetic Variation
Gene Shuffling
Most heritable differences are due to gene shuffling.
Crossing-over increases the number of genotypes that can appear in offspring.
Sexual reproduction produces different phenotypes, but it does not change the relative frequency of alleles in a population.
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13. Single-Gene and Polygenic Traits
What determines the numbers of phenotypes for a given trait?
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14. Single-Gene and Polygenic Traits
The number of phenotypes produced for a given trait depends on how many genes control the trait.
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15. Single-Gene and Polygenic Traits
A single-gene 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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16. Single-Gene and Polygenic Trait
The allele for a widow’s peak 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.
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17. Single-Gene and Polygenic Trait
Many traits are controlled by two or more genes and are called polygenic traits.
One polygenic trait can have many possible genotypes and phenotypes.
Height in humans is a polygenic trait.
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18. Single-Gene and Polygenic Trait
A bell-shaped curve is typical of polygenic traits.
A bell-shaped curve is also called normal 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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16-1
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16-1
Which of the following statements is TRUE?
The relative frequency of an allele is not related to whether the allele is dominant or recessive.
Mutations always affect an organism's phenotype.
Crossing-over decreases the number of different genotypes that appear in an offspring.
Evolution does not affect the frequency of genes in a gene pool.
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16-1
Most inheritable differences are a result of
gene shuffling.
frequency of alleles.
mutations.
DNA replication.
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16-1
The main sources of inherited variation are
gene shuffling and mutations.
gene pools and frequencies.
single-gene and polygenic traits.
genotypes and phenotypes.
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16-1
A widow's peak in humans is an example of a(an)
invariable trait.
single-gene trait.
polygenic trait.
mutation.
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16-1
A graph of the length of the little finger on the left hand versus the number of people having fingers of a particular length is a bell-shaped curve. This indicates that finger length is a
single-gene trait.
polygenic trait.
randomly inherited trait.
strongly selected trait.
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