# 11 2 Probability and Punnett Squares Slide 1

11 -2 Probability and Punnett Squares Slide 1 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares Genetics and Probability The principles of probability can be used to predict the outcomes of genetic crosses. Slide 2 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares Probability • Heredity patterns can be calculated with probability. • Probability is the likelihood that something will happen. • Probability predicts an average number of occurrences, not an exact number of occurrences. • Probability applies to random events such as meiosis and fertilization. Slide 3 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares Probability • Probability = number of ways a specific event can occur number of total possible outcomes Slide 4 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares Probabilities Predict Averages Probabilities predict the average outcome of a large number of events. Probability cannot predict the precise outcome of an individual event. In genetics, the larger the number of offspring, the closer the resulting numbers will get to expected values. Slide 5 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares Punnett squares can be used to predict and compare the genetic variations that will result from a cross. Slide 6 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares • The Punnett square is a grid system for predicting all possible genotypes resulting from a cross. – The axes represent the possible gametes of each parent. – The boxes show the possible genotypes of the offspring. Slide 7 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares A capital letter represents the dominant allele for tall. A lowercase letter represents the recessive allele for short. In this example, T = tall t = short Slide 8 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares Organisms that have two identical alleles for a particular trait are said to be homozygous. Organisms that have two different alleles for the same trait are heterozygous. Homozygous organisms are true-breeding for a particular trait. Heterozygous organisms are hybrid for a particular trait. Slide 9 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares All of the tall plants have the same phenotype, or physical characteristics. The tall plants do not have the same genotype, or genetic makeup. One third of the tall plants are TT, while two thirds of the tall plants are Tt. Slide 10 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares The plants have different genotypes (TT and Tt), but they have the same phenotype (tall). TT Homozygous Tt Heterozygous Slide 11 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares Probability and Segregation One fourth (1/4) of the F 2 plants have two alleles for tallness (TT). 2/4 or 1/2 have one allele for tall (T), and one for short (t). One fourth (1/4) of the F 2 have two alleles for short (tt). The genotypic ratio is 1: 2: 1 The phenotypic ratios is 3: 1 Slide 12 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares • A monohybrid cross involves one trait. • Monohybrid crosses examine the inheritance of only one specific trait. – homozygous dominant x homozygous recessive=all heterozygous, all dominant Slide 13 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares • A testcross is a cross between an organism with an unknown genotype and an organism with the recessive phenotype. Slide 14 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares Independent Assortment To determine if the segregation of one pair of alleles affects the segregation of another pair of alleles, Mendel performed a two-factor cross. Slide 15 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares Independent Assortment The Two-Factor Cross: F 1 Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY) with true-breeding plants that produced wrinkled green peas (genotype rryy). RRYY x rryy All of the F 1 offspring produced round yellow peas (Rr. Yy). Slide 16 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares Independent Assortment The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y). Slide 17 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares Independent Assortment The Two-Factor Cross: F 2 Mendel crossed the heterozygous F 1 plants (Rr. Yy) with each other to determine if the alleles would segregate from each other in the F 2 generation. Rr. Yy × Rr. Yy Slide 18 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares • A dihybrid cross involves two traits. • Predicted genotypic and phenotypic ratios can be determined from a Punnett square. • Specific ratios will vary based on the example Punnett Square used. • Example, Mendel’s dihybrid crosses with heterozygous plants yielded a 9: 3: 3: 1 phenotypic ratio. Slide 19 of 21 Copyright Pearson Prentice Hall End Show

11 -2 Probability and Punnett Squares • Mendel’s dihybrid crosses led to his second law, the law of independent assortment. • The law of independent assortment states that allele pairs separate independently of each other during meiosis. • Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms. Slide 20 of 21 End Show Copyright Pearson Prentice Hall

11 -2 Click to Launch: Continue to: - or - Slide 21 of 21 End Show Copyright Pearson Prentice Hall

11 -2 Probability can be used to predict a. average outcome of many events. b. precise outcome of any event. c. how many offspring a cross will produce. d. which organisms will mate with each other. Slide 22 of 21 End Show Copyright Pearson Prentice Hall

11 -2 Compared to 4 flips of a coin, 400 flips of the coin is a. more likely to produce about 50% heads and 50% tails. b. less likely to produce about 50% heads and 50% tails. c. guaranteed to produce exactly 50% heads and 50% tails. d. equally likely to produce about 50% heads and 50% tails. Slide 23 of 21 End Show Copyright Pearson Prentice Hall

11 -2 Organisms that have two different alleles for a particular trait are said to be a. hybrid. b. heterozygous. c. homozygous. d. recessive. Slide 24 of 21 End Show Copyright Pearson Prentice Hall

11 -2 Two F 1 plants that are homozygous for shortness are crossed. What percentage of the offspring will be tall? a. 100% b. 50% c. 0% d. 25% Slide 25 of 21 End Show Copyright Pearson Prentice Hall

11 -2 The Punnett square allows you to predict a. only the phenotypes of the offspring from a cross. b. only the genotypes of the offspring from a cross. c. both the genotypes and the phenotypes from a cross. d. neither the genotypes nor the phenotypes from a cross. Slide 26 of 21 End Show Copyright Pearson Prentice Hall

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