Punnett SquaresPunnett SquaresCornell Notes Page 47

For what is a Punnett Square Used?

• to show all possible ways alleles from parents can combine to create offspring

• to predict the probability of offspring inheriting a particular trait.

How is a Punnett Square Used?

• Parents’ genotypes go along outside of box

• Offspring genotypes are written inside each box

• Each box represents 25% of possible offspring

• Look at each genotype to determine phenotype.

Practice 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 mM

m

Using a PUNNETT SQUARE

The cross looks like this:

M mM

m

M MM m

Mm mm

Using a PUNNETT SQUARE

Each square stands for 25% of the possible offspring. Or, ¼ of the possible offspring.

M mM

m

M MM m

Mm mm

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

M mM

m

M MM m

Mm mm

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 mM

m

M MM m

Mm mm

Expressing PROBABILITIEScan use for genotypes

25% - MM

50% - Mm

25% - mm

or phenotypes

3/4 - muggles

1/4 – wizards

3:1 muggles

to wizards

M mM

m

M MM m

Mm mm

Practice #1

Mendel crossed a pure-breeding purple flower (PP) with a pure-breeding white flower (pp). Complete a Punnett square for this cross.

A. Which is the dominant trait?

B. What are the offspring genotypes?

C. For the offspring, what fraction will have purple flowers?

D. What percentage will have genotype Pp?

Widow’s Peak No Widow’s Peak

Widow’s Peak

Practice #2A 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.

A. Which is the dominant trait?B. What are the offspring genotypes?C. What is the ratio of widow’s peak to no widow’s peak for the offspring?D.What is the ratio of Ww to ww for the offspring?

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.

A. Which is the dominant trait?

B. What are the offspring genotypes?

C. What fraction of offspring will have a cleft chin?

D. What percentage will have genotype Cc?