Presentation on theme: "Do Now Write the answers in your notebooks. 1. What is the difference between a genotype and a phenotype? 2. What are alleles?"— Presentation transcript:
Do Now Write the answers in your notebooks. 1. What is the difference between a genotype and a phenotype? 2. What are alleles?
What have we learned about evolution so far? Evolution is change in the traits of a population over time Natural selection is the mechanism of evolution Evidence of evolution comes from the fossil record, DNA and proteins, comparative anatomy, and embryology Isolation can lead to speciation. The major types of isolation are geographic, behavioral, temporal, and reproductive
What are we going to learn about evolution today? Variation within a species increases the probability that some members of the species will survive A great diversity of species increases the chance that at least some organisms will survive great changes in the environment Natural selection acts on the phenotype of an organism, not its genotype How do lethal alleles stay in the gene pool?
Variation and Gene Pools Genetic variation is studied in populations of species. Because populations interbreed (mate with each other), they share a gene pool. A gene pool is made up of all the genes, including all the different alleles, that are present in a population.
Allele Frequency Allele frequency is the percentage of an allele in the gene pool. For example, if a group of pea plants is 50% heterozygous (Bb), 30% homozygous dominant(BB), and 20% homozygous recessive (bb), the allele frequency of the dominant allele would be 55% and the allele frequency of the recessive allele would be 45%.
Calculating Allele Frequency Here’s how I calculated that: We start with the 30% of the total that is homozygous dominant (BB). We know 50% of the population is heterozygous (Bb). Only half of the alleles in the heterozygotes are dominant, so we add half of 50%, or 25%, to 30%. Here is the formula I used to calculate the allele frequency of dominant alleles: % BB individuals+ half the % of the Bb individuals = allele frequency 30% + (½ x 50%) = allele frequency 30% + 25% = 55% Then, to find the frequency of the recessive allele, just subtract 55% from 100% 100% - 55% = 45%
Another Definition of Evolution Using genetic terms, evolution can be defined as any change in the allele frequency in a population. For example, in beetles, if the frequency of the recessive green allele was 75%, but over time the frequency changed to 71%, evolution has occurred.
Practice Problem In pea plants, purple flowers (P) are dominant over white flowers (p). In one population of pea plants, 15% of the plants are homozygous dominant, 60% are heterozygous, and 25% are homozygous recessive. What is the allele frequency of the dominant purple allele? What is the allele frequency of the recessive white allele?
Variation Within a Species Increases Chance of Survival Let’s say you are studying a bird that is very well adapted to its environment, but all of the individuals are genetically very similar. If a deadly virus starts infecting these birds, they could all die because they are all susceptible to the virus! Now, if there was more genetic variation in this population of birds, there is a chance that some of the birds might have a natural immunity to the virus. These immune birds would survive and be able to pass on their genes.
Biodiversity Increases the Chance of Survival We just looked at how variation within species can increase the chance that some members of the species will survive even if there is a change in the environment. Having a lot of biodiversity (many different species present) increases the chances that some species will survive a major change in the environment. For example, the asteroid that scientists believe was the reason dinosaurs went extinct did not kill all of the life on earth because there was a great diversity of living species. If large dinosaurs were the only species on earth at the time, all life on earth might have been killed! But, since there were many different species all adapted to different environments and niches, some animals were able to survive and reproduce.
Natural Selection acts on the Phenotype of an Organism, not its Genotype Natural selection never acts directly on genes. Organisms, not genes, survive and reproduce or die without reproducing, not genes. Phenotypes contribute to fitness, not genotypes. For example, a green beetle might be more fit than a brown beetle, but if the green color is dominant, the green beetle could have the genotype GG or Gg and still be equally fit. The green beetles that survive and reproduce may pass on either the green allele (G) or the brown allele (g).
How are Lethal Alleles Maintained in the Gene Pool? If there are alleles that can cause lethal (deadly) genetic disorders, why are they still in the gene pool? Wouldn’t they be removed because organisms that have them die before they can reproduce? Not necessarily! We are about to find out how!
Lethal Allele Maintenance Example 1: Some lethal alleles are recessive, so an organism would have to have two of the alleles to get the lethal effect. Example 2: Other lethal alleles are dominant, but are only lethal in an individual that is homozygous dominant. Heterozygotes can survive and pass on their genes. An example of this is coat colors in mice. Mice with two dominant alleles for coat color die before they are born, but heterozygotes survive.
Lethal Allele Maintenance Example 3: Huntington’s disease is a lethal, dominant genetic disorder in humans. People with Huntington’s disease always die from the disease (unless something else killed them first). However, the symptoms of Huntington’s disease do not appear until the affected person is 30-50 years old. Many people have already had children by this age, so they have already had a chance to pass on the lethal allele to their offspring.
Lethal Allele Maintenance and the Heterozygote Advantage Sometimes, alleles that cause genetic disorders can give an advantage to heterozygotes. Sickle cell anemia is a genetic disorder that causes red blood cells to be shaped like sickles. People who are homozygous for the sickle cell allele have a severe form of the disease and have a life expectancy of about 40-50 years with treatment. Sickle shaped cells usually live about 10-20 days. Normal red blood cells live about 120 days. People who are heterozygous for the sickle cell gene have some red blood cells that are normal, and some that are sickle-shaped. They do not have a severe form of the disease.
Lethal Allele Maintenance and the Heterozygote Advantage Malaria is a disease that is caused by a parasite, and it can be deadly. The malaria parasite infects red blood cells. When the parasite infects normal red blood cells, it has enough time to reproduce, creating many baby parasites that then burst out and infect more red blood cells. When the parasite infects sickle shaped cells, the cells die before the parasite has a chance to reproduce. This gives the heterozygotes an advantage: protection from malaria!