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Genetics Punnett Squares and Mendel S Genes and Dominance Mendel’s F1 Crosses on Pea Plants Copyright Pearson Prentice Hall Genes and Dominance Mendel’s Seven F1 Crosses on Pea Plants Mendel’s F1 Crosses on Pea Plants Copyright Pearson Prentice Hall Genes and Dominance S What is the principle of dominance? Copyright Pearson Prentice Hall Genes and Dominance S The principle of dominance states that some alleles are dominant and others are recessive. Copyright Pearson Prentice Hall Segregation P Generation F2 Generation F1 Generation S Mendel's F2 Generation Tall Short Tall Tall Tall Tall Tall Short Copyright Pearson Prentice Hall Segregation S The reappearance of the trait controlled by the recessive allele indicated that at some point the allele for shortness had been separated, or segregated, from the allele for tallness. Copyright Pearson Prentice Hall S Segregation Alleles separate during gamete formation. Copyright Pearson Prentice Hall Punnett Squares S How do geneticists use Punnett squares? Copyright Pearson Prentice Hall Punnett Squares S A capital letter represents the dominant allele for tall. S A lowercase letter represents the recessive allele for short. S In this example, S T = tall S t = short Copyright Pearson Prentice Hall Punnett Squares S Possible gene combinations for the F2 offspring appear in the four boxes. Copyright Pearson Prentice Hall Punnett Squares S Organisms that have two identical alleles for a particular trait are said to be homozygous. S Organisms that have two different alleles for the same trait are heterozygous. S Homozygous organisms are true-breeding for a particular trait. S Heterozygous organisms are hybrid for a particular trait. Copyright Pearson Prentice Hall Punnett Squares S All of the tall plants have the same phenotype, or physical characteristics. (What it LOOKS like) S The tall plants do not have the same genotype, or genetic makeup.(The alleles that make up the characteristics) Copyright Pearson Prentice Hall Punnett Squares S The plants have different genotypes (TT and Tt), but they have the same phenotype (tall). TT Homozygous Tt Heterozygous Copyright Pearson Prentice Hall Probability and Segregation S Probability and Segregation S One fourth (1/4) of the F2 plants have two alleles for tallness (TT). S 2/4 or 1/2 have one allele for tall (T), and one for short (t). S One fourth (1/4) of the F2 have two alleles for short (tt). Copyright Pearson Prentice Hall
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