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Name: Pat T Seeman
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Question:
I want to learn any thing and everything about the Punnett square. If any one can tell me about it I would be grateful.



Replies:
Pat:
It would be helpful to know how old you are and what you already know about the Punnet square. In short, it is a mathematical way to predict the possible offspring from two particular parents, given that you know something about their genes. Could you ask something a little more specific, so I'll know exactly what to tell you?

Ellen Mayo


The Punnet square is a tool used by geneticists and students of genetics to predict the outcome of a cross (mating) between two individuals with a known genotype (set of genes). I suppose it was invented by a person named Punnet (or perhaps his graduate student). The Punnet square is an array of cells that represent all of the possible offspring of the cross. It is made by listing all of the possible gametes (sperm or eggs) of one parent at the head of each column and all of the possible gametes of the other parent at the left of each row of the array. To determine each possible offspring, combine the genotypes of each gamete contributing to a particular offspring (that is, write in a particular cell the genotype of the column and row heading. A simple example to illustrate:
A monohybrid cross between two individuals of genotype Tt. Each parent can produce two kinds of gametes: T and t. The square will look like this:
        T          t
T     TT       Tt
t      Tt         tt
Therefore, the expected ratio of genotypes among the offspring is 1 TT : 2Tt : 1 tt. This is a simple example. To see the usefulness, try to to do a Punnet square for the trihybrid cross between two parents of genotype AaBbCc. What would be the expected frequency among the offspring of individuals with the genotype AaBbCc?

--Brian



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