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 R r R r Are the parents homozygous or heterozygous? 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 Mother’s genotype: Rr Father’s genotype: Rr R r R r Are the parents homozygous or heterozygous?

Punnett Squares R r R r R R R r r r 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 r R R R r r r

Punnett Squares R r R r RR Rr rr 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 R r R r RR Rr rr Say R was an allele for being right-handed and is dominant over r, which is a recessive trait for being left-handed The phenotypes of offspring from this cross will be: 75% right-handed 25% left-handed

Practice!