1/4/2016. Mendel learned about heredity by conducting experiments. There are multiple-gene traits too? Chapter 7: Genes and Inheritance.

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1 Chapter 7: Genes and Inheritance Family resemblance: your mother and father contribute equally? to your genetic makeup. Heredity the passing of characteristics from parent to offspring through their genes Selective Breeding: Observing Heredity Mendel learned about heredity by conducting experiments. Some traits are controlled by a single gene. Traits that are determined by the instructions a person carries at one gene are called single-gene traits. There are multiple-gene traits too? First we examine the mechanism by which single-gene traits pass from parent to child. 1

2 True-Breeding Segregation: you have two copies of each gene but only one copy in each sperm or egg. A dominant trait masks the effect of a recessive trait. Three Ideas Mendel Used for Explaining This Pattern of Inheritance 1) Each parent puts into every sperm or egg a single set of instructions for building the trait. 1) Offspring thus find themselves with two copies of the instructions for any trait (called alleles). 1) The actual trait produced by an individual depends on the two copies of the gene that they inherit from their parents. homozygous and heterozygous Gene A Alleles A or a Dominate/Recessive Genetic Symbols Monohybrid Cross Aa x Aa A a A AA Aa a Aa aa Demonstrates Mendel s segregation of alleles 3:1 Phenotypic ratio 1:2:1 Genotypic ratio 2

3 Dihybrid Cross AaBb x AaBb AB Ab ab ab Observing an individual s phenotype may not tell you its genotype. AB Ab AABB AABb AaBB AaBb AABb AAbb AaBb AaBb Demonstrates Mendel s Independent assortment of genes ab AaBB AaBb aabb aabb ab AaBb Aabb aabb aabb 9:3:3:1 Phenotypic ratio 1:2:1:2:4:2:1:2:1 Genotypic ratio Phenotypes and Genotypes The outward appearance of an individual is called their phenotype. Genotype. This is an organism s genetic composition. Genotypes Homozygous dominant Heterozygous Homozygous recessive Chance is important in genetics. Probability has a central role in genetics for two reasons: The first is a consequence of segregation. The second reason is that fertilization, too, is a chance event. 3

4 Probabilities Any gamete produced by an individual heterozygous for a trait has a 50% probability of carrying the dominant allele and a 50% probability of carrying the recessive allele. If a male is heterozygous for albinism (Aa) and a female is homozygous for albinism (aa), what is the probability that their child will be homozygous for albinism (aa)? You would like to produce white alligators via a mating program. The problem is that you cannot be certain of the genotype of your alligators. Analyzing Which Individuals Manifest the Trait and Which Do Not They might be homozygous dominant, MM, or they might be heterozygous, Mm. In either case their phenotype is normal coloration. How can you figure out which of these two possibilities is the actual genotype? 4

5 Sex-Linked Traits A Trait s Mode of Inheritance Is Not Always Completely Obvious Complete dominance or the influence of the environment Incomplete dominance, in which the heterozygote appears to be intermediate between the two homozygotes. Codominance, in which the heterozygote displays characteristics of both homozygotes. Multiple Allelism A single gene may have more than two alleles each individual still carries only two alleles Inheritance of the ABO Blood Groups A, B, and O alleles The A and B alleles are both completely dominant to O. The A and B alleles are codominant to each other. Individuals can be one of four different blood types: A, B, AB, and O. 5

6 Why are people with type O blood considered universal donors? Why are those with type AB considered universal acceptors? Polygenic Trait A trait that is influenced by many different genes Additive Effects what happens when the effects of alleles from multiple genes all contribute to the ultimate phenotype Sometimes one gene influences multiple traits What is the benefit of almost having sickle cell disease? The SRY Gene Sex-determining Region on the Y- chromosome Causes fetal gonads to develop as testes shortly after fertilization. Following the gonads secretion of testosterone, other developmental changes also occur. 6

7 Pleiotrophy In pleiotropy, one gene influences multiple traits. Many genes may be pleiotropic. Phenylketonuria Can t break down phenylalanine Affects many systems Brain development, seizures, metabolic problems Albinism? Affects many areas Why are more men than women colorblind? Sex-linked traits differ in their patterns of expression in males and females. If a man is colorblind, did he inherit this condition from his mother, his father, or both parents? If a boy is colorblind but his parents are not, then he got the allele causing colorblindness from 1. his mom 2. his dad 3. his grandfather 4. his sister Environmental effects: identical twins are not identical. Many traits are passed on as independent features: Mendel s law of independent assortment. But some genes are linked together. 7

8 Red Hair and Freckles Genes on the same chromosome are sometimes inherited together. Redheads tend to have pale skin Linked genes: Genes on the same chromosome, may be right next to each other. Chapter 8: Evolution and Natural Selection Learning Objectives Be able to explain evolution in action. Be able to explain Darwin s journey to an idea. Be able to describe and explain the four mechanisms that can give rise to evolution. Be able to explain how populations of organisms can adapt to their environment through natural selection. Be able to explain the evidence for the occurrence of evolution. What is your opinion? The human population on our planet is evolving. 1. Strongly Agree 2. Agree 3. Neutral 4. Disagree 5. Strongly disagree Evolution is an ongoing process. 8

9 Could you breed fruit flies who could live longer than 20 hours on average? When these eggs hatch, do you think the flies in this new generation will live longer than 20 hours without food? Make a prediction: A population of fruit flies was starved until 80% of the flies were dead. The remaining flies were fed and reproduced (offspring). What do you expect to see in the next generation if you repeated the starvation experiment? 1. More flies would be alive after 20 hours. 2. Fewer flies would be alive after 20 hours. 3. Fruit flies fed after 80% of the population died will lay more eggs. 4. No change in the average number of fruit flies that were alive after 20 hours. After 60 generations the average starvation resistance of fruit flies was 160 hours! What has happened to this population of fruit flies? 1. The current population is genetically identical to the original population. 2. The current population is genetically different from the original population. 9

10 What happened? Does evolution occur? Evolution a genetic change in the population Natural selection the consequence of certain individual organisms in a population being born with characteristics that enable them to survive better and reproduce more than the offspring of other individuals in the population YES. We can watch it happen in the lab. What trait in dogs (listed below) has evolved due to interactions with humans (think of wolves their closest cousins)? 1. Floppy ears 2. Adults behave like puppies 3. Large range of body sizes 4. 1 and 2 5. All of the above Evolution How does evolution occur? What types of changes can evolution cause in a population? Five primary lines of evidence Evolution by natural selection Darwin s Journey to an Idea Before Darwin, some people believed that all species had been created separately and were unchanging. But the ideas of evolution predate Darwin by at least 100 years 10

11 Jean-Baptiste Lamarck Charles Lyell Biologist, early 1800s Living species might change over time. Geologist 1830 book Principles of Geology Geological forces had shaped the earth and were continuing to do so. Gradual but constant change Passage on a around-the-world survey ship allowed Darwin to indulge and advance his love of nature. Age 16, University of Edinburgh, medical studies Studied theology at Cambridge University His real love: study of nature Observing geographic similarities and differences among fossils and living plants and animals, Darwin developed a theory of evolution by natural selection. Lyell s Principles of Geology The Galapagos Islands Two important and unexpected patterns: 1. Traits exhibited by species 2. Similarity between the fossils of extinct species and the living species in that same area Glyptodonts and armadillos 11

12 Economist Thomas Malthus Essay on the Principle of Population Darwin realized that favorable variations are preserved Which idea did not support the observations Darwin made during his trip on the HMS Beagle? 1. The earth is shaped by gradual forces (Lyell). 2. Species are fixed and unchanging (Aristotle). 3. The earth is older than 6,000 years (Buffon). 4. Populations could grow beyond the ability of the environment to support them (Malthus). In 1859, after decades of mulling and procrastinating, Darwin published his thoughts on natural selection first draft Witnessing Evolution Alter the population Increase the white phenotype through breeding. As the generations go by Higher proportion of white tigers Evolution = change of allele frequencies of the population 14 years in a drawer Alfred Russel Wallace, Individuals do NOT evolve. Populations evolve Allele frequencies It is helpful to think of each allele as having some market share of all of the alleles. Four mechanisms can give rise to evolution. Evolution occurs when the allele frequencies in a population change. 1. Mutation 2. Genetic drift 3. Migration 4. Natural selection 12

13 Mutation An alteration of the base-pair sequence of an individual s DNA When this alteration occurs in a gene, the change in the DNA sequence may change the allele. Mutation as an Agent of Evolution Which are inherited: Somatic cells or Reproductive cells? What causes mutations? The process of cells dividing can go awry. Environmental phenomena mutagens Mutation is the ultimate source of genetic variation in a population. Mutations are random Beneficial? Detrimental? Nearly all mutations reduce an organism s fitness. Genetic drift is a random change in allele frequencies in a population. Suppose that you have written a 10-page paper. Randomly select one letter in the paper and change it to another letter. Is the change more likely to make your paper better or worse? 13

14 The important factor that distinguishes genetic drift from natural selection: The change in allele frequencies is not related to the alleles influence on reproductive success. The impact of genetic drift is much greater in small populations than in large populations. Fixation Genetic drift can lead to fixation for one allele for a gene in a population. If this happens, there is no more variability in the population for this gene. Genetic drift reduces the genetic variation in a population. Two special cases of genetic drift, the founder effect and population bottlenecks, are important in the evolution of populations. Founder Effect A small number of individuals may leave a population and become the founding members of a new, isolated population. The founders may have different allele frequencies than the original source population, particularly if they are a small sample. Why are Amish people more likely to have extra fingers and toes? Population Bottlenecks 14

15 Which answer is not a form of genetic drift? 1. A large number of people of Irish descent have fair skin with freckles and red hair. 2. The American bison was almost hunted to extinction, the remaining populations are increasing again. 3. Small population of Ecuadorian finches populated islands where no finches lived before them (Galapagos). 4. People can leave there native country and become part of the population of the United States. Migration into or out of a population may change allele frequencies. When three simple conditions are satisfied, evolution by natural selection occurs. 1. There must be variation for the particular trait within a population. 2. That variation must be inheritable. 3. Individuals with one version of the trait must produce more offspring than those with a different version of the trait. Most agricultural pests evolve resistance to pesticides. How does this happen? A trait does not decrease in frequency simply because it is recessive. The Hardy-Weinberg Law If p = frequency of the dominant allele If q = frequency of the recessive allele p + q = 1 15

16 Use the Hardy-Weinberg equation to predict how common each genotype in the population will be. p 2 = frequency of homozygous dominants q 2 = frequency of homozygous recessives 2pq = frequency of heterozygotes P 2 +2pq+q 2 =1 Hardy-Weinberg Equilibrium Applies when there is: No natural selection No mutations No migration No genetic drift Random mating Boreal Toads in Colorado are being decimated by a fungus. About 20% of the population is resistant to the fungus. Use the Hardy-Weinberg equation to predict the allelic frequency for this dominant allele % 2. 36% 3. 80% 4. 64% 5. 4% Through natural selection, populations of organisms can become adapted to their environments. Traits causing some individuals to have more offspring than others become more prevalent in the population. 16

17 Reproductive Success Fitness A measure of the relative amount of reproduction of an individual with a particular phenotype compared with the reproductive output of individuals of the same species with alternative phenotypes Fruit Fly Example One fly carries the genes for a version of a trait that allows it to survive a long time without food. The other fly has the genes for a different version of the trait that allows it to survive only a short while without food. Which fly has the greater fitness? There are three important elements to an organism s fitness: 1. An individual s fitness is measured relative to other genotypes or phenotypes in the population. 2. Fitness depends on the specific environment in which the organism lives. 3. Fitness depends on an organism s reproductive success compared to other organisms in the population. A field is treated with pesticide, killing 99% of the insects. Which of the survivors listed are the most fit, evolutionarily speaking? 1. An insect survives, but is sterile. 2. An insect survives, but is weak and is unable to evade predators. 3. An insect survives and has many offspring that are fertile. 4. An insect survives and has many offspring that are sterile. Organisms in a population can become better matched to their environment through natural selection. Natural selection does not lead to perfect organisms. If the environment changes, the alleles causing the traits favored by natural selection may change as well. 17

18 Factors that Prevent Populations from Progressing Inevitably Toward Perfection Natural selection can change the traits seen in a population in several ways. 1. Environments change quickly. 2. Variation is needed as the raw material of selection. 3. There may be multiple different alleles for a trait, each causing an individual to have the same fitness. Directional Selection Individuals with one extreme from the range of variation in the population have higher fitness. Turkeys on poultry farms have such large breast muscles that they can t get close enough to each other to mate. Artificial selection is a special case of natural selection. Stabilizing Selection Individuals with intermediate phenotypes are most fit. Disruptive Selection Individuals with extreme phenotypes experience the highest fitness, and those with intermediate phenotypes have the lowest. 18

19 Most mammals do not drink milk after they are weaned because they lose the ability to digest lactose. This is true for about 60% of people. But, most people of northern European descent can drink milk (areas where dairy farming is historically prevalent). What type of evolution would this exemplify? 1. Directed selection 2. Stabilizing selection 3. Disruptive selection Which answer below is another example of directional evolution? 1. Spectacular plumage on the male Bird of Paradise. 2. Human selection of traits resulting in the Labrador Retriever. 3. Changes in beak size of Galapagos finches depends on seed size from year to year. 4. All of the above Natural selection causes the evolution of complex traits and behaviors. How can a wing evolve if 1% of a wing doesn t help an organism fly or glide at all? Often, structures appear because they serve some other purpose. Functional Shifts Humans have image-forming eyes. Three types of primitive eyes are listed below. Which answer puts the steps of eye evolution in the most parsimonious order? 1. A B C 2. B A C 3. A C B 4. C B A 5. B C A A. Eye spots (sense light) B.Pinhole camera eye (shape discrimination) C.Eye cup (sense direction of light) 19

20 Five primary lines of evidence: 1. The fossil record The fossil record documents the process of natural selection. 2. Biogeography 3. Comparative anatomy and embryology 4. Molecular biology 5. Laboratory and field experiments Geographic patterns of species distributions reflect their evolutionary histories. 20

21 History Matters Who arrived first? Are numerous different habitats available? Comparative anatomy and embryology reveal common evolutionary origins. Homologous Structures Vestigial Structures The human appendix serves no function. Convergent Evolution Analogous structures all developed from different original structures. Which example below is an analogous structure formed by convergent evolution? 1. Humans have an appendix while Gorillas have an extension of their intestine for cellulose digestion. 2. Image-forming eyes evolved separately in the squid and humans. 3. Fish, chicken, and human embryos all have gills. 4. The bones in the wing of a bat and the flipper of the whale are similar. 21

22 Molecular biology reveals that common genetic sequences link all life forms. The genetic code provides our fourth line of evidence that evolution occurs. DNA Similarities and Differences Related vs. unrelated individuals The more distantly you and another individual are related, the more your DNA differs. Compare DNA sequences between two species. In Rhesus monkeys, 138 amino acids are the same as those found in human hemoglobin. Recency of Common Ancestry Estimates of evolutionary relatedness made from: Comparative Anatomy Embryology The Fossil Record Molecular Clocks A short sequence of amino acids from human hemoglobin was compared to other vertebrates. Which answer is most closely related to the human amino acid sequence (sequence differences are highlighted in red): ser-thr-ala-gly-asp-glu-val-glu-asp-thr 1. asn-thr-thr-gly-asp-glu-val-asp-asp-ser 2. ala-thr-ser-gly-glu-lys-val-glu-asp-ser 3. gln-thr-gly-gly-ala-glu-ilu-ala-asn-ser 4. asp-ser-gly-gly-lys-his-val-thr-asn-ser Changes in the Frequency Laboratory and field of Alleles experiments enable us to watch evolution in progress. A fifth line of evidence for the occurrence of evolution comes from multigeneration experiments and observations. 22

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