Biology Lesson 2: Applying Mendel's Principles PDF

BIOLOGY Lesson 2: Applying Mendel’s Principles  OBJECTIVES 1. Explain how we can use probability to predict traits. 2. Describe how alleles segregate when more than one gene is involved. 3. Describe the contribution Gregor Mendel made to our understanding of genetics...

BIOLOGY Lesson 2: Applying Mendel’s Principles  OBJECTIVES 1. Explain how we can use probability to predict traits. 2. Describe how alleles segregate when more than one gene is involved. 3. Describe the contribution Gregor Mendel made to our understanding of genetics.  PROBABILITY & HEREDITY Probability: likelihood that a particular event will occur. ○ Example: flipping a coin--- there’s two possible outcomes: heads up or tails up. The chance, or probability, of either outcome is equal. Probability that a single coin flip will land heads up is 1 chance in 2 (or ½ or 50%). If you flip a coin three times in a row, each coin flip is an independent event with a ½ probability of landing heads up. ½x½x½=⅛  PROBABILITY & HEREDITY Using segregation to predict outcomes: ○ The way alleles segregate during gamete formation is as random as a coin flip. ○ In Mendel’s F1 cross, each F1 plant had one green pod allele and one yellow pod allele (Gg), so ½ of the gametes produced by the plants would carry the yellow allele (g). The only way to produce a plant with yellow pods (gg) is for two gametes carrying the recessive g allele to combine. Each F2 gamete has a one in two, of ½, chance of carrying the g allele. With two gametes, the probability of both gametes carrying the g allele is ½ x ½ = ¼ (¼ of the F2 offspring should have yellow pods and the remaining ¾ should have green pods).  PROBABILITY & HEREDITY Using segregation to predict outcomes (continued): ○ Not all organisms with the same characteristics have the same alleles, as shown in the F2 generation. GG and Gg resulted in green pods. Organisms that have two identical alleles for a gene are called homozygous (GG or gg). Organisms that have two different alleles for the same gene are heterozygous (Gg).  PROBABILITY & HEREDITY Probabilities Predict Averages ○ Probabilities predict the average outcome of a large number of events. Statistically, the larger the number of offspring, the closer the results will be to the predicted values. If an F2 generation has only three or four offspring, it may not match Mendel’s ratios. Genotype & Phenotype ○ Phenotype: physical traits (color, height, etc.) ○ Genotype: genetic makeup (heterozygous dominant, etc.) ○ In Mendel’s experiment, all pea plants with green pods had the same phenotype, but not the same genotype. There were three different genotypes among the F2 plants: GG, Gg, and gg. Plants with GG or Gg combinations of alleles have different genotypes, but the same phenotype (green pods).  PROBABILITY & HEREDITY Using Punnett Squares ○ Punnett Squares use mathematical probability to help predict the genotype and phenotype combinations in genetic crosses. ○ You begin with a square and all possible combinations of alleles in the gametes produced by one parent are written along the top edge of the square. ○ The other parent’s alleles are then segregated along the left edge. ○ Next, every possible genotype is written inside the boxes within the square, as they might appear in the F2 generation.  INDEPENDENT ASSORTMENT Mendel wondered if the segregation of one pair of alleles affects another pair. ○ Example: Does the gene that determines the shape of a seed affect the gene for seed color? ○ Mendel followed two different genes as they passed from one generation to the next--- two-factor, or dihybrid, cross. Single-gene crosses are monohybrid crosses.  INDEPENDENT ASSORTMENT The Two-Factor Cross: F1 ○ First, Mendel crossed true-breeding plants that produced only round yellow peas with plants that produced only wrinkled green peas. The round yellow peas had the genotype RRYY. The wrinkled green peas had the genotype rryy. All of the F1 offspring produced round yellow peas. Showed that the alleles for yellow and round peas are dominant. The genotype in each of these F1 plants were all heterozygous for both seed shape and color.  INDEPENDENT ASSORTMENT The Two-Factor Cross: F2 ○ Next, Mendel crossed the F1 plants to produce F2 offspring. Would the two dominant alleles always stay together, or would they segregate independently, so that any combination of alleles was possible? Mendel’s F2 plants produced 556 seeds--- 315 were round and yellow, 32 were wrinkled and green, and 209 were a combination of phenotypes not found in either parent. This meant alleles for seed shape segregated independently of those for seed color (in other words, genes that segregate independently don’t influence each other’s inheritance).  INDEPENDENT ASSORTMENT The Two-Factor Cross: F2 (continued) ○ Mendel’s results were very close to a 9:3:3:1 ratio. ○ Principle of Independent Assortment: genes for different traits can segregated independently during the formation of gametes.  SUMMARY OF MENDEL’S PRINCIPLES Mendel’s principles of heredity, observed through patterns of inheritance, form the basis of modern genetics. ○ The inheritance of biological characteristics is determined by individual units called genes, which are passed from parents of offspring. ○ Where two or more forms (alleles) of the gene for a single trait exist, some alleles 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 segregate from each other when gametes are formed.  SUMMARY OF MENDEL’S PRINCIPLES Mendel’s principles of heredity, observed through patterns of inheritance, form the basis of modern genetics. (continued) ○ Alleles for different genes usually segregate independently of each other. ○ Mendel’s principles don’t only apply to plants. Geneticist Thomas Hunt Morgan wanted to use a model organism of another kind to advance the study of genetics. He used the common fruit fly, Drosophila melanogaster.