PROBABILITY AND PUNNETT SQUARES Probability How is probability

PROBABILITY AND PUNNETT SQUARES

Probability ■ How is probability related to genetics? ? ■ The likelihood that a particular event will occur is called probability – For example: flipping a coin has a 50% probability to land as heads or tails ■ The way in which alleles segregate is completely random – like a coin flip ■ The principles of probability can be used to predict the outcomes of genetic crosses

Punnett Squares ■ The gene combinations ( genotype) that might result from a genetic cross can be determined by drawing a diagram known as a Punnett Square ■ To set up a Punnett Square, draw a grid of four boxes and add the parental genotypes along the outside of the top and left-hand side R r – Mother’s genotype: Rr – Father’s genotype: Rr Are the parents homozygous or heterozygous? R r

Punnett Squares ■ Remember that the letters outside the box represent the alleles for each gene that a sex cell from the parents can have ■ Step 1: Put the letter that is at the top of a column into each box in that column – Always put the capital letter first! ■ Step 2: Do the same for the letters on the side across each row R r R RR R r r Rr r r

Punnett Squares ■ Now we know the potential genotypes of offspring that would result from that cross ■ Each box represents a 25% chance of that trait being produced ■ The genotypes of offspring from this cross will be: – 25% homozygous RR – 50% heterozygous Rr – 25% homozygous rr ■ Say R was an allele for being right-handed and is dominant over r, which is a recessive trait for being left-handed R ■ The phenotypes of offspring from this cross will be: – 75% right-handed – 25% left-handed r R R Rr Rr rr

PRACTICE!