How do I make and use a Punnett square?

2 Answers
Apr 17, 2018

A Punnett Square shows the genotypes two individuals can produce when crossed. To draw a square, write all possible allele* combinations one parent can contribute to its gametes across the top of a box and all possible allele combinations from the other parent down the left side.

Explanation:

Diagrams and online calculator here:
http://scienceprimer.com/punnett-square-calculator

Apr 17, 2018

See below:

Explanation:

A Punnett square is a grid diagram that is used to predict the outcome of breeding.

Let's pretend that we have a type of plant that has two alleles for height:

#"T"# which means #"tall"# and is #"dominant"#
#"t"# which means #"short"# and is #"recessive"#

A #"Tt"# tall plant breeds with another #"Tt"# tall plant. What types of offspring can they produce?

We will make a 4-square grid and put the father's alleles on the top, and the mother's alleles on the sides:
enter image source here

Now we will move each letter down on it's column or to the right in it's row.

enter image source here

The red #color(red)"T"# and #color(red)"t"# are from the father and the blue #color(blue)"T"# and #color(blue)"t"# are from the mother.

So the possibilities of their offspring's allele combinations would be in each of the four squares:

#"TT, Tt, Tt, and tt"#

What are the percent chances of each of the allele combinations?

#"TT"#: #1/4# plants or #25%#

#"Tt"#: #2/4# plants or #50%#

#"tt"#: #1/4# plants or #25%#

What are the chances that the offspring will be tall, and what are the chances that they'll be short?

To be tall, they only need one capital #"T"# allele, since #"tall"# is a dominant trait.

#"Tall"#: #3/4# plants or #75%#

#"Short"#: #1/4# plants or #25%#

Now remember that these are just percentages! That doesn't mean that these two parents will definitely produce #4# offspring, #3# tall and #1# short. They're just estimations, so they can be wrong!

Punnett squares can be very complicated, but I hope this has helped you understand the basics! Please let me know if you have more questions - I can just update the answer :)