Punnett Squares & Probability

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

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

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

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

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

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

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

D. Example 1 4. Fill in the squares

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

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

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

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

Punnett Squares & Probability