Using a Punnett Using a Punnett SquareSquare

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 and1/2 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

gives up a tall gene?

What letter represents the tall gene?

So the offspring?

What if Mom gives up a short gene and Dad

gives up a short gene?

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 homozygousThe 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

homozygous or heterozygous

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%)