Using a Punnett Square

What is a PUNNETT SQUARE? A tool to predict the probability of certain traits in offspring that shows the different ways alleles can combine A way to show phenotype & genotype A chart that shows all the possible combinations of alleles that can result when genes are crossed

What is a PUNNETT SQUARE? Letters stand for dominant and recessive alleles An uppercase letter stands for a dominant allele Lowercase letters stand for recessive alleles

Example: Tall is the dominant trait in pea plants Therefore T = tall and t = short

We also know that an organism gets 1/2 its genes from its mother and 1/2 from its father. So, lets look at some possibilities: Remember: T = tall t = short

What if Mom gives up a tall gene and Dad gives up a short gene? What letter represents the tall gene? What letter represents the short gene? So the offspring?

What if Mom gives up a tall gene and Dad What letter represents the tall gene? So the offspring?

What if Mom gives up a short gene and Dad What letter represents the short gene? So the offspring?

Now for the possibilities Tt is called hybrid or heterozygous. In a hybrid the dominant trait always shows. The offspring would be tall (phenotype). Phenotype = physical makeup - what it looks like TT is called pure or homozygous. The offspring would also be tall. tt is called pure or homozygous The offspring would be short. This is the ONLY way a recessive trait shows.

Using a PUNNETT SQUARE To set up a Punnett square, draw a large square, and then divide it into 4 equal sections (also squares). It should look something like this:

Using a PUNNETT SQUARE Now you need two parents, ones with a known genotype (genetic makeup) For example, a red flower (genotype Rr) and a white flower (genotype rr). Rr x rr

Rr pure or hybrid? homozygous or heterozygous rr pure or hybrid

Using a PUNNETT SQUARE Place one of the parents on top, and one on the left. You should get a something similar to this:

Using a PUNNETT SQUARE Finally, take each letter in each column and combine it with each letter from each row in the corresponding square. You should now have a picture close to this:

Using a PUNNETT SQUARE The two-letter combinations are the possible genotypes of offspring They are: Rr, Rr, rr, and rr genotypes From this it is possible to determine the probability (chance) that a flower will have a red phenotype (2/4 or 50%) or a white phenotype (2/4 or 50%)