Genes and Variation

Learning Objectives

▪ Define evolution in genetic terms.
▪ Identify the sources of genetic variation.
▪ Explain what determines the phenotypes for a given trait.

Genetics Joins Evolutionary Theory

Genotype: particular combination of alleles
Phenotype: physical, physiological, and behavioral characteristics

Natural selection acts on an organism’s characteristics, not on its
alleles.

Allele Frequency

Number of times an allele occurs in a gene pool, as a percentage
of the total occurrence of all alleles

In 50 alleles:

20 alleles are B (black)

30 alleles are b (brown)

In 100 alleles:

are B (black)

are b (brown)

40

60

Genetic Variation

Three evolutionary mechanisms
that generate genetic variation:
• mutation
• genetic recombination
• lateral gene transfer

Single-Gene Traits

Traits controlled by only one gene

With bands

Without bands

Single-Gene Traits

Phenotypic ratios are determined by the frequency of alleles and
by whether the alleles are dominant or recessive.

Polygenic Traits

Traits controlled by two or more genes

Polygenic Traits

Height in humans is an example of a polygenic trait.