MadSci Network: Genetics

Re: How do you do a tri-cross punnet square? (crossing three traits)

Date: Mon Nov 27 22:36:04 2000
Posted By: Sanjida Rangwala, Graduate Student, Washington University
Area of science: Genetics
ID: 975359990.Ge

Greetings -

Check out the links on the following page:
Explain the Punett Square .

You might find some more examples on Punnett squares there.
As long as you have every combination of alleles on the top row and first column 
of your Punnett square, you should be doing it right.
An easier way, if you're looking for the probability of a certain genotype, is 
to do each cross independantly as a simple statistical probability problem. For 
example, if you're crossing TtSsAa X TtSsAa
and wondering about the chance of getting TtSSaa,
you could look at each trait separately and multiply the probabilities to get 
the combined probability.
I.e. Tt X Tt gives 1/2 Tt
     Ss X Ss gives 1/4 SS
     Aa X Aa gives 1/4 aa  (do a Punnett square if you're unconvinced of these 
So, the chance of TtSSaa together is 1/2X 1/4 X 1/4 = 1/32!

In the real world, geneticists often find the above method much much easier than 
doing Punnett squares, though I do admit, they are fun!

	Sanjida Rangwala, Admin MadSci Network

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