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Introduction to Genetics Chapter 11 The Work of Gregor Mendel A. Gregor Mendel’s peas 1. Genetics – study of heredity 2. Mendel studied pea plants in the monastery gardens 3. Fertilization – male and female reproductive cells join Work of Mendel 4. True-breeding – produce offspring identical to themselves. B. Genes and Dominance 1. Trait – specific characteristic, such as seed color, or plant height, that varies from one individual to another. Work of Mendel 2. Mendel crossed plants with different traits a. Example: yellow seed with a green seed 3. Original pair of plants – P generation Work of Mendel 4. F1 – Offspring to the P generation 5. Hybrids – cross between parents with different traits 6. Genes – chemical factors that determine traits. Work of Mendel 7. Alleles – different forms of a gene a. Example: Gene is hair color, the different alleles are red, blond, brown, black, etc. Work of Mendel 8. Principle of dominance – some alleles are dominant and others are recessive 9. Dominant allele for a trait will always show up. Work of Mendel 10. Recessive allele will only show up if the dominant is not present. C. Segregation Work of Mendel 1. Mendel took 2 plants, each with a different trait and crossed them a. Example: tall x short 2. F1 generation had all tall plants Work of Mendel 3. Allowed F1 to self-pollinate 4. F2 generation had ¾ tall and ¼ short 5. Segregation – separation of alleles Work of Mendel 6. Gametes – sex cells 7. Capital letter represents a dominant allele 8. Lower case letter represents a recessive allele Probability and Punnett Squares A. Genetics and Probability 1. Probability – likelihood that a certain event will occur a. Example: chances of flipping heads on a coin is 50% Probability and Punnett Squares b. Chance of tails three times in a row? ½ x ½ x ½ = 1/8 2. Past outcomes do not affect future ones. Probability and Punnett Squares B. Punnett Squares – diagram that shows gene combinations 1. Types of gametes produced by the parent are on the top and left side. 2. Possible gene combinations appear in the squares. Probability and Punnett Squares 3. Punnett squares are used to predict and compare the genetic variations that will result from a cross. 4. Homozygous – organism that has 2 identical alleles for a particular trait Probability and Punnett Squares a. Example: TT or tt 5. Heterozygous – organism that has two different alleles for the same trait a. Example: Tt Probability and Punnett Squares 6. Phenotype – Physical characteristic 7. Genotype – genetic makeup C. Probabilities predict averages 1. More likely to get the expected ratio the more times the test is done. Mendelian Genetics A. Independent Assortment 1. Two-Factor cross (F1) a. Crossed true breeding round yellow peas (RRYY) with wrinkled green (rryy) Mendelian Genetics b. All offspring were round and yellow (RrYy) 2. Two-Factor Cross (F2) a. Mendel wanted to know what these heterozygous F1 plants would produce. Mendelian Genetics b. Crossed RrYy x RrYy c. Independent assortment – alleles for one trait are not influenced by the other alleles. Mendelian Genetics B. Summary of Mendel’s Principles 1. The inheritance of characteristics is determined by genes. Genes are passed from parent to offspring. Mendelian Genetics 2. Some forms of the gene are dominant and others recessive. 3. Organism has 2 copies of each gene – one from each parent. Genes are segregated during gamete production. Mendelian Genetics 4. Alleles for different genes usually segregate independently of one another. C. Beyond Dominant and Recessive Alleles Mendelian Genetics 1. Some alleles are neither dominant or recessive. Many traits are controlled by multiple alleles or multiple genes. Mendelian Genetics 2. Incomplete Dominance – one allele is not completely dominant over another, heterozygous shows a blending a. Example: four o’clock flowers b. RR (red) x rr (white) Mendelian Genetics 3. Codominance – both alleles contribute to the phenotype, heterozygous shows both phenotypes a. Example: chicken feathers b. BB (black feathers) x bb (white feathers) Mendelian Genetics 4. Multiple Alleles – more than 2 alleles exist to form several different combinations of genes a. Example: coat color in rabbits, blood types b. Each organism only has 2 alleles, but more than 2 alleles exist in a population. Mendelian Genetics c. I – Type A AIA, IAi IBIB, IBi – Type B IAIB – Type AB ii – Type O d. Heterozygous type A x type AB Mendelian Genetics 5. Polygenic Traits – traits controlled by 2 ore more genes a. Example: more than 4 different genes control skin color Mendelian Genetics D. Genetics and The Environment 1. Genes provide a plan, but how it unfolds depends on the environment. Meiosis A. Chromosome Number 1. Homologous – 2 sets of chromosomes (one set from mom, a set from dad) a. Humans – 23 mom, 23 dad Meiosis 2. Diploid – a cell that contains both sets of homologous chromosomes a. Humans 2N = 46 Meiosis 3. Haploid – a cell that contains only a single set of chromosomes a. Humans N = 23 b. Sperm and egg cells Meiosis B. Phases of Meiosis 1. Meiosis – process of reduction division in which the number of chromosomes per cell is cut in half. Meiosis a. Has 2 divisions – Meiosis I, Meiosis II b. Start with 2N and at the end each cell has N Meiosis 2. Meiosis I a. Chromosomes have been doubled b. Homologous chromosomes pair up and form a tetrad Meiosis c. Crossing over – parts of homologous chromosomes are exchanged i. Crossing over increases genetic variety Meiosis d. Homologous chromosomes separate and from two cells e. Each cell only has half the genetic information Meiosis 3. Meiosis II a. The chromosomes have sister chromatids b. The chromosomes line up down the middle c. Sister chromatids break apart Meiosis d. Produces 4 cells with the haploid number of chromosomes C. Gamete Formation 1. Male animals – produce sperm cells Meiosis 2. Plants – pollen contains sperm cells 3. Female plant and animal – egg cells Meiosis a. In female animals the cell divisions are uneven in Meiosis, producing one egg and three polar bodies (not used in reproduction) Meiosis D. Comparing Mitosis and Meiosis 1. Mitosis – 2 cells – identical Meiosis – 4 cells – different 2. Mitosis – 2N (diploid) Meiosis – N (haploid) Meiosis 3. Mitosis – Cells used in asexual reproduction Meiosis – cells used in sexual reproduction Linkage and Gene Maps A. Gene Linkage 1. In flies for example: reddish-orange eyes and miniature wings are almost always found together. Linkage and Gene Maps 2. Chromosome is a group of linked genes 3. Chromosomes are associated independently, not individual genes Linkage and Gene Maps 4. Gene Map – relative locations of known genes on the chromosome a. Farther apart 2 genes are, the more likely they are of being separated by crossing over.
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