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**11-2 Probability and Punnett Squares**

photo credit: W. Perry Conway/CORBIS Copyright Pearson Prentice Hall

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**Genetics and Probability**

The likelihood that a particular event will occur is called probability. Let’s consider a coin toss. Copyright Pearson Prentice Hall

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**probability can be used to predict the outcomes of genetic crosses.**

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Punnett Squares Punnett Squares Offspring can be determined by a Punnett square. Copyright Pearson Prentice Hall

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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

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Punnett Squares Gametes produced by each F1 parent are shown along the top and left side. 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

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Punnett Squares Organisms that have two identical alleles for a particular trait are said to be homozygous (TT or tt). Organisms that have two different alleles for the same trait are heterozygous (Tt). Homozygous organisms are true-breeding for a particular trait. Heterozygous organisms are hybrid for a particular trait. Copyright Pearson Prentice Hall

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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. Copyright Pearson Prentice Hall

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**The genotype of one third of the tall plants is TT, **

While the genotype of two thirds of the tall plants is Tt. Copyright Pearson Prentice Hall

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Punnett Squares These 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

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11-2 Copyright Pearson Prentice Hall

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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

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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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