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Constructing Punnett Squares
Constructing Punnett Squares
We can make Punnett squares to help us understand a genetic cross. This lesson will walk through the stages of constructing Punnett squares and understanding their outcome.
There are three main ways to display genetic inheritance information:
- Genetic diagrams
- Punnett squares
- Family pedigrees
In this lesson, we'll learn how to construct Punnett squares, and recap how to interpret them.
Here is a completed Punnett square
It shows you a cross between two homozygote parents - one dominant for a trait and one recessive.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/fanj8FUQW6gOc8S83tWP.png)
We are going to draw a Punnett square to show the cross between two heterozygote parent plants, with the alleles Tt and Tt, where T is for tall plants and t is for short plants.
In how many genotype combinations do you get when you cross the Tt from parent 1 with the Tt from parent 2?
So if we want to cross Tt from parent 1 with Tt from parent 2, we will get 4 offspring genotype combinations in total.
So the first thing we do is to draw a square or hashtag with 4 spaces.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/XIXwuaqBQwao8KaHbHqe.png)
Now we've filled in some letters. Which letters are these?
A) The alleles from the offspring genotypes B) The alleles from the parental genotypes
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/XIXwuaqBQwao8KaHbHqe.png)
Are we ready now to add in the offspring genotype combinations? Answer yes or no.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/XIXwuaqBQwao8KaHbHqe.png)
With the parental alleles (Tt and Tt), which genotype combinations can be produced?
A) TT, Tt, Tt, tt B) TT, TT, Tt, tt
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/XIXwuaqBQwao8KaHbHqe.png)
So you fill in the offspring genotype combinations by...
taking the parental allele next to the square and the parental allele above the square and combining them.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/LXHPyzMJQxG0wypgSHUL.png)
What's the phenotype ratio of the completed Punnett square for tall plants to short plants from two heterozygote parents?
What's the percent probability of having a heterozygote offspring from two heterozygote parents?
What is the direct proportion of the short phenotype?