Punnett Squares


Punnett Squares are diagrams used by scientists to help them to figure out how inherited traits (characteristics) will be distributed.
 

     The yellow rectangles show the genes that the parents have.

The genes of one parent are in the two boxes at the top: the A and the A.

This parent has one copy of this gene on each of two chrosomes.

     The genes of the second parent are in the two boxes at the left side of the diagram: a and a.

    This parent also has a pair of chromosomes with one of these genes on each one of the pair.

The green boxes represent what each of four children will inherit.

Each child inherits one gene on the chromosome it gets from each parent.

One gene plus one gene = 2 genes: in this case, Aa.

You may want to use Punnett squares to explain how your life forms pass on characteristics, or to help you to figure out what is going on in the Chromosome Kindergarten!

     Let's try some samples and see how these squares can help us.

   

 

     A is a dominant characteristic.

 

 

     a is a recessive characteristic.

This bird has two genes for orange feathers.

  Its genotype is AA.

This bird has two genes for blue feathers.

  Its genotype is aa.

This Punnett Square shows how we can diagram the genes.

The orange bird has two dominant Agenes.

We put two A s along the top of the square.

 

The blue bird has two recessive a genes.

  We put two a s down along the left side of the square.


All the offspring have the genes Aa.
They will all have orange feathers, but will carry a recessive gene for blue feathers.
This is called the F1 generation.

Now suppose that two individuals from the F1 generation become parents.

 

  Here they are!
  The baby birds are called the F2 generation.

  You can see how their genes work out.

  The offspring are coded in the squares.

  One bird will be orange with two AA genes.
  Two birds will be orange with genes coded Aa.
 One bird will be blue and will have two recessive aa genes.

 Individual nests of birds may not turn out exactly like this, but if there are many baby birds, they will work out genetically with the ratios 1:2:1.

What if we are studying more than one characteristic?

 

two characteristic punnet square

Here we are dealing with two characteristics:

   feather color -- ORANGE or blue

  CREST or no crest

We are dealing with two sets of chromosomes:

   The yellow chromosomes carry the feather color.

   The brown chromosomes carry the crest gene.

Remember, each parent has a pair of each chromosome.

Each child inherits one chromosome from each pair.


  In the Punnett Square we have put a symbol for each of the chromosomes that the parent has. 

Here we are only counting one gene on each of the two chromosomes. In real life an organism has thousands of genes on each of a number of chromosomes. For example, human beings have 46 chromosomes, and inherit 23 from each parent.

Living things are very complex.


© Elizabeth Anne Viau, 1999. This material may be used freely for instructional purposes but not sold for a price beyond the cost of reproduction. Please inform the author if you use it at eviau@earthlink.net.