Punnett Squares

Predicting the Probability of Inherited Traits

Probability: the likelihood that a particular event will occur

Consider a coin flip: There are two possible outcomes (head

or tails) The chance (or probability) of either

outcome (heads or tails) is equal The probability that a single coin flip will

be heads is 1 chance in 2 possibilities. 1/2 = 50% of flipping a heads or tails

Punnett Square: a tool to predict the probability of inheriting a specific trait.

Mendel’s first generation cross: homozygous purple (PP) X purebred white (pp)1. Draw a box:2. Mom’s alleles --> P P3. Dad’s alleles --> p

p

4. Fill in the boxes5. Offspring genotype- 100% Pp6. Offspring phenotype - 100% purple

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Pp Pp

Pp Pp

Heterozygous (Pp) X Heterozygous (Pp) P p

P

p

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X

PP Pp

Pp pp

Offspring Genotypes:1 PP: 2Pp: 1pp

Offspring Phenotypes:3 Purple: 1 White

Genotypic Ratio: The probability of having each Genotype. 1:2:1 = 25% chance of PP, 50% chance of Pp, 25% pp

Phenotypic Ratio: The probability of having each phenotype.3:1= 75% chance of being purple, 25% chance of being white