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Genetics Vocabulary Trait - specific characteristic that varies from one individual to another Gene - sequence of DNA that codes for a protein and thus determines a trait Allele - one of a number of different forms of a gene Hybrid - offspring of crosses between parents with different traits The principle of dominance states that some alleles are dominant and others are recessive. Gregor Mendel – Father of Modern Genetics True-breeding term used to describe organisms that produce offspring identical to themselves if allowed to selfpollinate. Mendel’s Work Mendel had truebreeding pea plants. He asked the question: What would happen if he breed pea plants with different traits? Dominant and Recessive Traits & Gregor Mendel’s Peas P – Parent generation F1 – first generation of offspring (f – filial from latin filius “son”) Punnett Square diagram showing the gene combinations that might result from a genetic cross Cross YY and yy Y Y y Yy Yy y Yy Yy Crossing true-breeding parent generation P Generation Trait Green pea recessive Genes (alleles) yy Gametes formed y and y Trait Yellow pea dominant Cross YY and yy Y Y y Yy Yy y Yy Yy F1 Generation Genes (alleles) YY Gametes formed Y and Y Crossing the F1 generation F1 Gen. Trait Yellow pea Genes (alleles) Yy Gametes formed Y and y Trait Yellow pea Cross Yy and Yy Y Y YY Yy y Yy yy y F2 Generation Genes (alleles) Yy Gametes formed Y and y Probability and Genetics Probability likelihood that a particular event will occur Cross YY and yy Y Y y YY Yy Yy yy y Crossing the F1 generation Segregation - separation of alleles during gamete formation When each F1 plant flowers, the two alleles are segregated from each other so that each gamete carries only a single copy of each gene. Therefore, each F1 plant produces two types of gametes—those with the allele for tallness and those with the allele for shortness. More vocabulary… Homozygous - term used to refer to an organism that has two identical alleles for a particular trait Heterozygous - term used to refer to an organism that has two different alleles for the same trait Phenotype - physical characteristics of an organism Genotype - genetic makeup of an organism Genotypes and Phenotypes Phenotypes and Genotypes Although these plants have different genotypes (TT and Tt), they have the same phenotype (tall). What are the ratios for genotype and phenotype? Cross Tt and Tt T t T TT Tt t Tt tt Genotype ratio - 1:2:1 1 HoZ tall, 2 HeZ, 1 HoZ short Phenotype ratio – 3:1 3 tall pea plants, one short pea plant Using ratios from Punnett Squares Ratio of tall to short pea plants is 3:1 If we breed heterozygous tall pea plants with each other and in one generation we made 1000plants, How many tall pea plants and short pea plants should we have? Law of Independent Assortment The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. In other words, genes (alleles) of different traits separate independent of one another. Genes (alleles) of one trait do not affect genes of another trait. This allows us to cross genes for different traits at the same time. Cross of heterozygous yellow and round peas. First, what is the genotype of the parents? Second, how many different gametes can be formed? Third, what are the different gametes? Parent: RrYy RY Ry rY ry Cross of heterozygous yellow and round peas. How many different phenotypes do we have? 4 • What is the ratio for the phenotypes? 9:3:3:1 Summary of Mendel’s Principles The inheritance of biological characteristics is determined by individual units known as genes. In organisms that reproduce sexually, genes are passed from parents to their offspring. In cases in which two or more forms of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive. In most sexually reproducing organisms, each adult has two copies of each gene—one from each parent. These genes are segregated from each other when gametes are formed. The alleles for different genes usually segregate independently of one another. 1. List the four basic principles of genetics that Mendel discovered in his experiments. Briefly describe each of these principles. 2. What is probability? How does probability relate to genetics? 3. In pea plants, the allele for yellow seeds is dominant to the allele for green seeds. Predict the genotypic ratio of offspring produced by crossing two parents heterozygous for this trait. Draw a Punnett square to illustrate your prediction. 4. Designing Experiments In sheep, the allele for white wool (A) is dominant over the allele for black wool (a). How would you determine the genotype of a white ram, or male sheep? 5. Inferring Suppose Mendel crossed two pea plants and got both tall and short offspring. What could have been the genotypes of the two original plants? What genotype could not have been present? 6. Applying Concepts In guinea pigs, the allele for a rough coat (R) is dominant over the allele for a smooth coat (r). A heterozygous guinea pig (Rr) and a homozygous recessive guinea pig (rr) have a total of nine offspring. The Punnett square for this cross shows a 50 percent chance that any particular offspring will have smooth coats. Explain how all nine offspring can have smooth coats. Beyond dominant and recessive alleles. Most genes do not follow the simple patterns of dominant and recessive alleles. Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes. Beyond dominant and recessive alleles incomplete dominance situation in which one allele is not completely dominant over another There is no white no red: new phenotype pink Beyond dominant and recessive alleles Co dominance situation in which both alleles of a gene contribute to the phenotype of the organism Beyond dominant and recessive alleles multiple alleles - three or more alleles of the same gene Beyond dominant and recessive alleles polygenic trait trait controlled by two or more genes Linkage and Gene Maps It’s easy to see how genes located on different chromosomes assort independently, but what about genes located on the same chromosome? Wouldn’t they generally be inherited together? Thomas Hunt Morgan’s studies back in 1910 helped us to answer this question. Linkage and Gene Maps Just because two genes are located on the same chromosome does not mean that they are linked together forever. Crossing-over Also, the further apart they are the more likely they are to separate. The closer they are the less likely they are to separate.