1. Probability and Punnett Squares

2. 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

3. 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?

4. 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

5. 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

6. Practice!