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Genetics Info & Tutorial: Punnett Squares
Genetics For Herpers

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As you can see on the previous page, determining the offspring of a mixed match can get confusing. One way to simplify this is to use what’s called a Punnett square, which is named after the person who invented the method.

The idea of a Punnett square is to determine what combination of offspring you will get, and how many of each. This is done by determining which gene(s) the mother can pass to the offspring, and what gene(s) the father can hand down. If an animal is homozygous for a trait, it is pretty easy: it can give the one gene it is homozygous for. If it is heterozygous, it can give either one or the other. Let’s take a look.

In our first example, we will mate a normal female to an amel male (just like the previous page) which is abbreviated as “Normal X Amel.” These are both homozygous animals. For the amelanism gene we will use A for the wild type (it is capital since it is the dominant gene) and a for the mutation (small letter is recessive) Since each animal has two genes for the trait, our normal could be shown as “ AA” and the amel could be shown as “aa.”

Now that we know what genes we are working with in each parent, we can create a Punnett square. This is done by listing the genes in the mother’s possible eggs in columns, and the genes of the father’s sperm in rows. Remember that each sperm or egg can only carry one copy of a gene...

Mother's egg --\/


Father's sperm --->
A
a
Start by determining the gene(s) each parent can contribute.



Now mix each of the father’s sperm with each of the mother’s eggs. In this case there’s only one outcome: Aa
A
a
Aa



The first example is as simple as it can get. Let’s try a slightly bigger one. This time let’s do an Amel to a Normal that is het for amel. What’s the genes of an amel? Correct, it’s aa. And what about a het for amel? If you said Aa then you must have been paying attention. :)



The amel will always pass down a mutant gene. --->
A
a
a
<--- the “het” can pass down either a normal or mutant copy.



A a
a
Aa
aa
The two outcomes are Aa and aa. Each has an even chance of appearing. Aa is “het” and will appear normal. aa will show the mutation.



Get more details with the book Genetics For Herpers!
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