Natural Selection. Population Dynamics. The Origins of Genetic Variation. The Origins of Genetic Variation. Intergenerational Mutation Rate

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1 Natural Selection Population Dynamics Humans, Sickle-cell Disease, and Malaria How does a population of humans become resistant to malaria? Overproduction Environmental pressure/competition Pre-existing individual variation Heritable traits Happens over generations (time) Happens in populations (not single individuals) Offspring must be viable and fertile The Origins of Genetic Variation Offspring of sexual reproduction are genetically different from their parents and from one another. Meiosis Random mutations Crossing over Independent assortment of chromosomes Random fertilization Meiosis and comparing it to Mitosis The Origins of Genetic Variation Offspring of sexual reproduction are genetically different from their parents and from one another. Meiosis Random mutations Crossing over Independent assortment of chromosomes Random fertilization Intergenerational Mutation Rate By how many mutations does your genome differ from your parents genome? Roach, et al., Science (2010) found about 60 mutations, 30 from each parent, that occurred during meiosis. Hemophilia in the Royal Family: Hypothesis - hemophilia allele arose through mutation in gamete of Queen Victoria s mother or father. 1

2 Crossing Over In crossing over, Homologous chromosomes exchange genetic information. Genetic recombination occurs. Random Fertilization The human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote. Independent Assortment of Chromosomes In independent assortment, every chromosome pair orients independently of the others during meiosis. Natural Selection Overproduction Environmental pressure/competition Pre-existing individual variation Heritable traits Happens over generations (time) Happens in populations (not single individuals) Offspring must be viable and fertile Heritable Variation and Patterns of Inheritance - Ch 9 Gregor Mendel Was the first person to analyze patterns of inheritance. Deduced the fundamental principles of genetics. Figure 9.6a 2

3 Monohybrid Crosses A monohybrid cross is a cross between parent plants that differ in only one characteristic. Mendel s law of segregation The two members of an allele pair segregate (separate) from each other during the production of gametes. Shown using a Punnett square. Genetic Alleles and Homologous Chromosomes Phenotype An organism s physical traits; what it looks like. Genotype An organism s genetic makeup; what genes it has. Figure 9.7 Independent Assortment of Chromosomes In independent assortment, every chromosome pair orients independently of the others during meiosis. Mendel developed four hypotheses from the monohybrid cross: There are alternative forms of genes, called alleles. For each characteristic, an organism inherits two alleles, one from each parent. Alleles can be dominant or recessive. Gametes carry only one allele for each inherited characteristic. 3

4 Dihybrid cross Is the mating of parental varieties differing in two characteristics. Figure 9.5 Mendel s law of independent assortment states that Each pair of alleles segregates independently of the other pairs during gamete formation. Figure 9.23 Using a Testcross to Determine an Unknown Genotype A testcross is a mating between Family Pedigrees Shows the history of a trait in a family. Allows geneticists to analyze human traits. An individual of unknown genotype and a homozygous recessive individual. 4

5 Human Disorders Controlled by a Single Gene Variations On Mendel s Laws Some patterns of genetic inheritance are not explained by Mendel s laws. Incomplete dominance Codominance Pleiotropy Polygenic Inheritance Incomplete Dominance in Plants and People In incomplete dominance, F 1 hybrids have an appearance in between the phenotypes of the two parents. Incomplete Dominance in Plants and People In incomplete dominance, F 1 hybrids have an appearance in between the phenotypes of the two parents. ABO Blood Type: An Example of Multiple Alleles and Codominance The immune system produces blood proteins That may cause clotting when blood cells of a different type enter the body. The ABO blood groups in humans are an example of multiple alleles. 5

6 Pleiotropy and Sickle-Cell Disease Pleiotropy is the impact of a single gene on more than one characteristic. Sickle-cell disease is an example. Figure 9.20 Polygenic Inheritance Polygenic inheritance is the additive effects of two or more genes on a single phenotype. Figure 9.21 The Role of Environment Many human characteristics result from a combination of heredity and environment. Figure

7 The Chromosomal Basis of Inheritance The chromosome theory of inheritance states that Genes are located at specific positions on chromosomes. The behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns. Figure 9.23 Linked genes Linked Genes Are located close together on a chromosome. May be inherited together. The Process of Science: Are Some Genes Linked? Using the fruit fly Drosophila melanogaster, Thomas Hunt Morgan determined That some genes were linked based on the inheritance patterns of their traits. Genetic Recombination: Crossing Over Two linked genes Can give rise to four different gamete genotypes. Can sometimes cross over. Crossing Over Figure

8 Figure 9.25 Figure 9.26 Linkage Maps Early studies of crossing over were performed using the fruit fly Drosophila melanogaster. Studies using Drosophila Developed a method for mapping gene loci. Resulted in linkage maps. Sex Chromosomes and Sex- Linked Genes Sex chromosomes Influence the inheritance of certain traits. Figure

9 Sex Determination in Humans and Fruit Flies Sex chromosomes Are designated X and Y. Determine an individual s sex. Figure 9.28 Sex-Linked Genes Sex-linked genes Are any genes located on a sex chromosome. Were discovered during studies on fruit flies. Figure 9.29 Inheritance patterns of a sex-linked gene Figure

10 Sex-Linked Disorders in Humans A number of human conditions result from sex-linked (X-linked) genes. Red-green color blindness Is characterized by a malfunction of lightsensitive cells in the eyes. Hemophilia Is a blood-clotting disease. Figure 9.31 Hemophilia Is a blood-clotting disease. Duchenne muscular dystrophy Is characterized by a progressive weakening and loss of muscle tissue. Figure

11 The Rules of Probability The rule of multiplication states that The probability of a compound event is the product of the separate probabilities of the independent events. Figure