Using a Punnett Square

What is a PUNNETT SQUARE? If you know the genes of the parents, a Punnett square can be used to… show the different ways alleles in the sex cells can combine predict the probability of certain traits in offspring

Using a PUNNETT SQUARE One of Harry Potter’s best friends is Hermione Granger. Hermione is a wizard, but her parents are not. So, how did Hermione get the wizarding trait? We can use a Punnett square to figure it out…

Using a PUNNETT SQUARE A letter is chosen to represent a characteristic, such as magical powers. Having magical powers is a recessive trait, so we’ll use: M for muggle (non-wizard) m for wizard

Using a PUNNETT SQUARE Both of Hermione’s parents are muggles and both have a genotype of Mm. Genotypes: Mm x Mm (mom x dad) Phenotypes: both muggles (non-wizards)

Using a PUNNETT SQUARE Cross Hermione’s parents: M m

Using a PUNNETT SQUARE The cross looks like this: M m

M m Using a PUNNETT SQUARE Each square stands for 25% of the possible offspring. Or, ¼ of the possible offspring. M m

M m Using a PUNNETT SQUARE Remember, the recessive trait is having magical powers. What’s the probability of being a wizard? M m

Expressing PROBABILITIES Can express probability several ways: 3:1 ratio of muggles to wizards 1/4 chance of being wizard 25% chance of being wizard M m

Expressing PROBABILITIES can use for genotypes 25% - MM 50% - Mm 25% - mm or phenotypes 3/4 - muggles 1/4 – wizards 3:1 muggles to wizards M m M M M M m m M m m m

Practice #1 Mendel crossed a pure-breeding purple flower (PP) with a pure-breeding white flower (pp). Complete a Punnett square for this cross. Which is the dominant trait? What are the offspring genotypes? For the offspring, what fraction will have purple flowers? What percentage will have genotype Pp?

Widow’s Peak Widow’s Peak No Widow’s Peak

Practice #2 A mom with a widow’s peak (Ww) is crossed (mates) with a dad who does not have a widow’s peak (ww). Complete a Punnett square for this cross. Which is the dominant trait? What are the offspring genotypes? What is the ratio of widow’s peak to no widow’s peak for the offspring? What is the ratio of Ww to ww for the offspring?

Cleft Chin

Practice #3 A man and a woman both have a cleft chin. Both have genotypes of Cc. Complete a Punnett square for a cross between them. Which is the dominant trait? What are the offspring genotypes? What fraction of offspring will have a cleft chin? What percentage will have genotype Cc?