What is a Punnett square and how is it used?

Expert Answers

An illustration of the letter 'A' in a speech bubbles

A Punnett square is a tool used in Mendelian inheritance to show the possible genotypes that are formed when a male and female gamate unite.  It can also be used to predict the most likely phenotype of how the trait will be expressed.

A Punnett square is made by drawing a square divided into four smaller squares (2x2).  The male's allele is written across the top.  The female's allele is written down the side.  In each of the four smaller squares is written the combination of one male allele and one female allele.

I have attached a link to a picture of a Punnett square.

 

Approved by eNotes Editorial Team
An illustration of the letter 'A' in a speech bubbles

A Punnett square is something used to find out how traits are passed down.  Punnett squares were the result of the work by Mendel on pea plants and he realized many different things about how traits pass down.  An example:  Say you want to cross a plant that is tall to one that is short.  The short trait is recessive in pea plants and the tall trait is dominant.  (Remember that the dominant trait will show in the plant over the recessive, it only needs one allele from a parent, while the recessive can only show when it's totally recessive.)  Since the short trait is recessive, we know that the short plant must have two short alleles from its "parents".  The tall plant could have two tall, or one tall and one short allele, since the tall trait is dominant.  I'll show both examples.

First, let's look at a short plant, which would have two short alleles, this is also called homozygous recessive.  We'll use lower case t to mean the recessive trait.  And we'll cross it with a tall plant that is homozygous dominant, or it has two tall alleles.  We'll use upper case T to mean the dominant trait.  So we make a table separating the two alleles from both plants, one across the top and one down the left side.  We fill in the table by using the allele at the top of each column in each spot below it and using the allele at the left of the row in each spot to the right.

 

T T

t Tt Tt

t Tt Tt

 

Sorry, you'll have to imagine lines in between the columns and rows. What you end up with is what would be the genotypes (the two alleles) of the progeny (the children).  When you cross this example, each progeny ends up with Tt, which means it has one tall and one short allele (this is called heterozygous).  Again, since tall is dominant, the plants will look tall (their phenotypes).

 

#2  Now let's cross the short plant (homozygous recessive tt) with the tall plant that has one tall and one short allele (heterozygous Tt).

 

T t

t Tt tt

t Tt tt

 

This time, half of the progeny (children) will have the Tt genotype (heterozygous), while the other half have the tt genotype (homozygous recessive).  The Tt half will look tall since tall is dominant, while the tt half will look short.

 

Just like all probability, these numbers aren't exact in real life, but the more you experiment with, the closer it gets to those numbers.

 

Hopefully this gives you a good start.

 

Approved by eNotes Editorial Team