Presentation on theme: "Ch. 16 Sec. 1 Genes and Variation. I. How Common Is Genetic Variation? Many genes have at least two forms, or alleles.Many genes have at least two forms,"— Presentation transcript:
Ch. 16 Sec. 1 Genes and Variation
I. How Common Is Genetic Variation? Many genes have at least two forms, or alleles.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.All organisms have genetic variation that is “invisible” because it involves small differences in biochemical processes. An individual organism is heterozygous for many genes.An individual organism is heterozygous for many genes.
II. Variation and Gene Pools A. Genetic variation is studied in populations 1. Group of individuals of the same species that interbreed 2. Gene pool consists of all genes, including all the different alleles, that are present in a population
B. Relative frequency - the number of times the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur 1. Expressed as a percentage
Gene Pool for Fur Color in Mice 2. Example Fig. 16 – 2 Page 394 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.
In genetic terms, evolution is any change in the relative frequency of alleles in a population.
III. Sources of Genetic Variation The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction.
A. Mutations 1.Any change in a sequence of DNA a. Mistakes in DNA replication b. Radiation or chemicals in the environment 2.Do not always affect an organism’s phenotype a.Can effect fitness b.Ability to survive and reproduce
B. Gene Shuffling 1.Most heritable differences a. 23 pairs of chromosomes b. 8.4 million gene combinations 2.Crossing-over a. Increases number of genotypes b. Sexual reproduction
IV. Single-Gene and Polygenic Traits The number of phenotypes produced for a given trait depends on how many genes control the trait.
A. A single-gene trait is controlled by one gene that has two alleles. Variation in this gene leads to only two possible phenotypes. Fig Page 395 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.
B. Polygenic traits 1. Can have many genotypes and phenotypes a. Height in humans is a polygenic trait b. Blood Type c. Skin color
2. A bell-shaped curve is typical of polygenic traits a. AKA normal distribution Fig Page 396 The graph shows the distribution of phenotypes that would be expected for a trait if many genes contributed to the trait.
16–1 Genes and Variation Darwin’s theory of evolution by natural selection explained how life on Earth changed, or evolved, over many generations. What Darwin did not know was how heritable traits were passed down through each generation. The study of genetics helps scientists understand the relationship between inheritance and evolution. Genetics supports Darwin’s ideas. Scientists know that genes control traits and that many genes have at least two forms, or alleles. They also know that members of all species are heterozygous for many genes.
In genetic terms, evolution is any change in the relative frequency of alleles in a population. A population is a group of individuals of the same species that can interbreed. Members of a population share a gene pool. A gene pool is all the genes, and their alleles, in the population. The number of times the allele occurs in a gene pool compared to the number of times that other alleles for the same gene occur is the relative frequency of the allele. The two main sources of genetic variation are mutations and gene shuffling. Amutation is any change in a sequence of DNA. Gene shuffling occurs during gamete formation. It can produce millions of different gene combinations. Both mutations and gene shuffling increase genetic variation by increasing the number of different genotypes.
The number of phenotypes for a trait depends on how many genes control the trait. A single-gene trait is a trait controlled by only one gene. If there are two alleles for the gene, two or three genotypes are possible. An example in humans of a single-gene trait is the presence of a widow’s peak—a downward dip in the center of the hairline. The allele for a widow’s peak is dominant over the allele for a hairline with no peak. As a result, there are only two phenotypes—having a widow’s peak or not having one. A polygenic trait is controlled by two or more genes. Each gene of a polygenic trait may have more than one allele. Polygenic traits form many phenotypes. Variation in a polygenic trait in a population often forms a bell-shaped curve with most members near the middle. An example of a polygenic trait is height in humans.