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Algernon Offline
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Question Punnett Squares? - January 26th 2010, 12:41 AM

Can someone give me a really good, but non-confusing explanation of Punnett Squares? Thanks.


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Re: Punnett Squares? - January 26th 2010, 12:52 AM

A Punnett Square is a chart used for genetics.

It has (usually for a simple one) four squares and it has one genetic combo in each square.

These two genetics come from two parents.

Each Punnett Square has to have 2 kinds of genetics. Tall & Short, Blonde & Brown, etc.

You mix these two genetics together to get the chances of this combo happening.

For example, tt= short TT=tall tT/Tt=medium

Here are the chances in my square:

tt l tT
Tt l TT

So there is a 25% chance it will be short, a 25% chance it will be tall, and a 50% chance of medium.

I don't know if Im EXACTLY correct, but if you need help with a problem go here: http://www.changbioscience.com/genetics/punnett.html




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Re: Punnett Squares? - January 26th 2010, 01:36 AM

Ok. The best way for me to look at the boxes like rows. If the letters are (a,b) and (a,b) then you would write an a all on the top two boxes, because it is across. The row under it you would write a b on the boxes acrosses. Then you look at it from the columns on the top. The first would have an a going down in both and a b going down in the second. So the pattern would be aa, ab, ab,bb.
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Re: Punnett Squares? - January 26th 2010, 03:25 AM

Punnett Squares are a statistical way of determining the likely outcomes of the mating between two individuals. The simplest one is when only 1 gene is being analyzed. Before I mention what it is, I just want to say, you don't need to use Punnett Squares for such analysis. You can do it through a more mathematical way but it's a bit longer.

So, let's go for gene G. Allele G is dominant, g is recessive. Instead of inserting an image and you seeing my absolutely horrid artistic skills, ignore the white "g"s if your screen displays them.
ggggGgggggg
G
g

The Gg x Gg are the parents and as you can see, they're heterozygous.
ggggGgggggg
GggGGgggGg
gggGggggggg

The bolded letters are the likely outcomes for the offspring. To obtain these outcomes, you can think of each row as R1 and R2, and each column as C1 and C2. For GG, you went across R1 and then down C1. For gg, you went across R2 and down C2. If you have difficulty with this, you can draw pencil lines down each column and across each row. Where they intersect is where you obtain the outcomes.

What these results indicate is that 1/2 the offspring will be homozygous, 1/2 will be heterozygous. Of the homozygous, 1/2 will be homozygous recessive and the other 1/2 will be homozygous dominant. Said in another way, 1/2 will be heterozygous, 1/4 will be homozygous dominant and 1/4 will be homozygous recessive.
   
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