11-2 Probability and Punnett Squares photo credit: W. Perry Conway/CORBIS Copyright Pearson Prentice Hall

Genetics and Probability The principles of probability can be used to predict the outcomes of genetic crosses. Copyright Pearson Prentice Hall 2

Copyright Pearson Prentice Hall 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. The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall 3

Copyright Pearson Prentice Hall Probability Probability = number of ways a specific event can occur number of total possible outcomes The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall 4

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. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Punnett Squares Punnett Squares Punnett squares can be used to predict and compare the genetic variations that will result from a cross. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 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. The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 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 The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall 8

Copyright Pearson Prentice Hall 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. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 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. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Punnett Squares The plants have different genotypes (TT and Tt), but they have the same phenotype (tall). Although these plants have different genotypes (TT and Tt), they have the same phenotype (tall). TT Homozygous Tt Heterozygous Copyright Pearson Prentice Hall

Probability and Segregation One fourth (1/4) of the F2 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 F2 have two alleles for short (tt). The genotypic ratio is 1:2:1 The phenotypic ratios is 3:1 The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 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 The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall 13

Copyright Pearson Prentice Hall Punnett Squares A testcross is a cross between an organism with an unknown genotype and an organism with the recessive phenotype. The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall 14

Copyright Pearson Prentice Hall Independent Assortment 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. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Independent Assortment The Two-Factor Cross: F1   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 F1 offspring produced round yellow peas (RrYy). Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Independent Assortment The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y). Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Independent Assortment The Two-Factor Cross: F2  Mendel crossed the heterozygous F1 plants (RrYy) with each other to determine if the alleles would segregate from each other in the F2 generation. RrYy × RrYy Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 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. The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall 19

Copyright Pearson Prentice Hall 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. The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall 20

Copyright Pearson Prentice Hall 11-2 Copyright Pearson Prentice Hall

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

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

Copyright Pearson Prentice Hall 11-2 Organisms that have two different alleles for a particular trait are said to be hybrid. heterozygous. homozygous. recessive. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 11-2 Two F1 plants that are homozygous for shortness are crossed. What percentage of the offspring will be tall? 100% 50% 0% 25% Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 11-2 The Punnett square allows you to predict only the phenotypes of the offspring from a cross. only the genotypes of the offspring from a cross. both the genotypes and the phenotypes from a cross. neither the genotypes nor the phenotypes from a cross. Copyright Pearson Prentice Hall

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