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Constructing Punnett Squares
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Constructing Punnett Squares

lesson introduction

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:

  1. Genetic diagrams
  2. Punnett squares
  3. Family pedigrees

In this lesson, we'll learn how to construct Punnett squares, and recap how to interpret them.

1

Here is a completed Punnett square

It shows you a cross between two homozygote parents - one dominant for a trait and one recessive.

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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?

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1

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.

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2

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

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3

Are we ready now to add in the offspring genotype combinations? Answer yes or no.

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4

With the parental alleles (Tt and Tt), which genotype combinations can be produced?

A) TT, Tt, Tt, tt B) TT, TT, Tt, tt

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5

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.

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What's the phenotype ratio of the completed Punnett square for tall plants to short plants from two heterozygote parents?

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What's the percent probability of having a heterozygote offspring from two heterozygote parents?

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What is the direct proportion of the short phenotype?

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