What is a Punnett Square and how do we use it?

A Punnett square is a chart which shows all likely gene combinations in a cross of parents (whose genes are known), a way to show phenotype & genotype. Punnett squares are named for an English geneticist, Reginald Punnett. He discovered some basic principles of genetics, including sex linkage and sex determination.
All you have to do is know the genotypes of the parents-- that tells you which alleles (
form of a gene) each parent could offer to the next generation. The alleles they could give are what you write along the side and top of the square; once you figure out what letters to write along the side and the top of your square, it is easy to fill in. Letters stand for dominant and recessive alleles; an uppercase letter stands for a dominant allele a lowercase letter stands for a recessive allele

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See if you remember how to do this…

Did yours turn out like this?


So lets run through it together
*In pea plants (which Gregor Mendel studied), tall pea plants are dominant over short pea plants. Using Punnett Squares, you can predict the genotypes and phenotypes of the offspring of a cross between a homozygous tall pea plant and a homozygous short pea plant. (homozygous means the same, so either TT or tt)(heterozygous means different so Tt).

v Step 1.
Ø Assign letters to represent the traits. Capital letters represent dominant traits T, and lowercase letters represent recessive traits, t.
v Step 2.
Ø Write down the genotypes (genes) of each parent. These are often given to you or are possible to determine.
Parent 1 =TT parent 2= tt
v Step 3.
Ø Draw a Punnett square - 4 small squares in the shape of a window. Write the possible gene(s) of one parent across the top and the gene(s) of the other parent along the side of the Punnett square.

v Step 5.
Ø Fill in each box of the Punnett square by sliding the letter above and in front of each box into each correct box (so the top letter will fill in both squares below, and the letter on the side will fill in both letters to the left). Generally, the capital letter goes first and a lowercase letter follows.
v Step 6.
Ø List the possible genotypes and phenotypes of the offspring for this cross.
Ø There are 4 boxes, and the genotypic results can be written either as fractions or percents. In this case, all 4 boxes out of the 4 are showing the Tt genotype. Therefore, each of the offspring has a 4/4 or 100% chance of showing the Tt genotype.

We have also written the phenotype (physical appearance so what your able to see) in each box under the genotype. Remember, T = tall and t = short, Since a capital letter indicates a dominant gene, T (tall) if the traits there that’s what you can expect to see not the recessive gene. So each of the offspring has a 4/4 or 100% chance of being tall.

What are the possible genotype(s) of a tall plant?

What are the possible genotype(s) of a short plant?

What would be the phenotype of TT?

What would be the phenotype of tt?

Why is the phenotype of Tt tall and not medium/average?

Now that we remember how to do this, why is it important?
You might not care what the chances are of getting white flowers from the next generation when a purple flowering plant crosses with a white flowering plant. But if you were a gardener breeding roses and the question had to do with rose flower color (when one flower color is more valuable than another), you might care. You might also care about chances in inheritance if you thought that a certain disease ran in your family—say the Tay Sachs disease gene, for example. It’s a way to help predict the possibility of having a child or breeding a rose with certain qualities.

Try this!!!
1) A green pea plant (Gg) is crossed with a yellow pea plant (gg).


2) A tall plant (TT) is crossed with a tall plant (Tt).


3) A tall plant (Tt) is crossed with a short plant (tt).


4) A red flower (Rr) is crossed with a white flower (rr).


5) A white flower (rr) is crossed with a white flower (rr).

6) A black chicken (BB) is crossed with a black chicken (BB).

Complete the following problems. List the parent genotypes, draw and fill in a Punnett square, and then list the offspring genotypes and phenotypes.

  1. A homozygous dominant brown mouse is crossed with a heterozygous brown mouse (tan is the recessive color).

  1. Two heterozygous white (brown fur is recessive) rabbits are crossed.

  1. Two heterozygous red flowers (white flowers are recessive) are crossed.

4. A homozygous tall plant is crossed with a heterozygous tall plant (short is the recessive size).

  1. A heterozygous white rabbit is crossed with a homozygous black rabbit.

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