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Punnett Squares & Probability

This guide provides an overview of Punnett squares, a vital tool in genetics for predicting the outcomes of genetic crosses. It covers essential vocabulary, such as genotype and phenotype, and offers a step-by-step procedure for constructing and analyzing Punnett squares. The document discusses various genetic ratios, distinguishing between homozygous and heterozygous traits. With clear examples and conventions, students will learn how to effectively use Punnett squares to determine genotypic and phenotypic ratios, enhancing their understanding of genetic inheritance.

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Punnett Squares & Probability

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  1. Punnett Squares & Probability How to figure out what the probable outcome of a genetic cross will be

  2. I. Punnett Squares • Vocabulary 1. Genotype= genetic makeup, what the genes say (EX: BB) 2. Phenotype= physical appearance, how it looks (Color) 3. Genotypic ratio= (3 #s) homozygous dominant: heterozygous: homozygous recessive 4. Phenotypic ratio= (2 #s) dominant appearance : recessive appearance

  3. More Vocabulary • Homozygous – Both alleles are the same • Homozygous dominant – AA - 2 dominant alleles • Homozygous recessive – aa – 2 recessive alleles • Heterozygous – Two different alleles one dominant and one recessive - Aa

  4. B. Punnett Square Procedure • Figure out the cross • Write down what the cross is • Draw & label the Punnett Square • Fill in the squares • Write down the genotypic & phenotypic ratios

  5. C. Punnett Square Conventions 1. A capital letter stands for the dominant trait (lower case = recessive) 2. If there is a capital letter, write it first

  6. D. Example 1 • Let T = tall pea plant • Let t = short pea plant • Cross a heterozygous pea plant with a short pea plant • What is the cross? • Tt x tt

  7. D. Example 1 3. Draw and label square http://users.adelphia.net/~lubehawk/BioHELP!/psquare.htm

  8. D. Example 1 4. Fill in the squares

  9. D. Example 1 4. Fill in the squares

  10. D. Example 1 4. Fill in the squares

  11. D. Example 1 5. Figure out the genotypic ratio Homozygous dominant = none Heterozygous = 2 Homozygous recessive = 2 genotypic ratio = none : 2 : 2

  12. D. Example 1 Figure out the phenotypic ratio Dominant appearance: recessive appearance 2 : 2 reduce to 1 : 1

  13. E. Example 2 Cross two pea plants heterozygous for tallness • Tt x Tt • Draw and label square

  14. 3. Fill in squares 4. Genotypic & Phenotypic ratios gr = 1 : 2 : 1 pr = 3 : 1 E. Example 2

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