Punnet Squares?

[martyb]

I dont know how to read them or use them and i keep seeing them around i just wondered if anyone could give me a crash course in them?

[Snakeman]

there are some examples on this site but im not sure exactly where they are.

to read is actually quite simple, you look at them as if you were looking at a multiplication table.

[martyb]

ok thanks

[littleindiangirl]

say an animal , I'll use rats here, is albino, thats recessive, his Locus will look like this: Rat 1 cc (albino:recessive)

The rat its getting bred to is not albino: Rat 2 Cc (non albino: carries albino gene) Cc

Put one across seperating each locus (rat 1), and one down separating each locus and multiply (rat 2)

----------c c
C--------Cc Cc
c--------cc cc

And there you go, 2 rats will be non albino but carrying the albino gene, and 2 rats will be albino.

This works for more than one locus at a time too, and those get bigger and more complicated, but the same idea to multiply like this.

[littleindiangirl]

Here is a fairly easy explanation of punnet squares for more than one locus. This doesn't include the Co-dom genes for snakes. Nerd has an explanation of that I believe on their website. I do believe there is a big difference in those.

http://www.boardmanweb.com/rattery/geneticbasics.htm

[Jay_Bunny]

I must make a point here.

When doing punnet squares and you get this

c c

C Cc Cc

c cc cc

This does not mean you will get two albinos and two normals. This means each offspring has a 50% chance of being albino and a 50% chance of being normal. Each punnet square is per individual.

[martyb]

im still having trouble with this could you do a punnet square for dummies plz?

[littleindiangirl]

I must make a point here.

When doing punnet squares and you get this

c c

C Cc Cc

c cc cc

This does not mean you will get two albinos and two normals. This means each offspring has a 50% chance of being albino and a 50% chance of being normal. Each punnet square is per individual.

Thats true, this is not a sure fire way of predicting what you will get.

[littleindiangirl]

im still having trouble with this could you do a punnet square for dummies plz?

So i can narrow it down, exactly what part are you having trouble with? Reading the punnet square or genetics in general?

A punnet square is easy.

Imagine this is in excel, with clearly defined rows (horizontal) and columns (vertical).

Recessive: A gene that is not “expressed” unless there are two copies. One comes from each parent.
Expressed: What the rat looks like, his Phenotype.
Carries: Recessive genes that the rat has that are not expressed or his Genotype.
The albino locus is defined as "C"
CC = NON albino
Cc = NON albino, but carries the albino gene, but it is not expressed.
cc = Albino, and shows the albino traits.


In the TOP row, you will put the locus of the albino (rat 1). cc is recessive. splitting it into 2 columns
____c____c
In the FIRST column, row 2. you put the locus of the second rat, splitting in half into two rows
C

c

So the top row is rat 1's locus, and column 1 rat 2's locus.
___c___c
C---Cc---Cc

c---cc---cc

In 4 offspring, 2 will be carriers of the albino gene, but not show it and 2 will be true albino.

Make sense yet?

It's difficult when you don't actually see it in a chart.

[Jay_Bunny]

Ok we'll do a simple recessive (meaning it requires both recessive alleles to show. Recessive genes only show up when there are two of the same recessive allele are there.)

So lets take the albino for instance. Albino only shows up when two albino alleles are present. If there is only one, the individual will appear normal but still carry that albino allele to possibly pass on to its offspring.

Ok, so lets say you have an albino rat. To put it into a simple punnet square you would right it out as (aa). Lower case letters mean that it is a recessive gene. Since albino is recessive, an albino rat would be written with two lower case letters, showing it carries both of the recessive alleles.

Alright now, say you have a normal rat you wish to breed with that albino rat. The normal rat does not carry the albino gene. Its alleles in the same slot are that for normal. Because normal is dominant over the recessive albino, it will show up normal even if it carries one albino allele. We'll say this normal rat doesn't carry any albino. It would be written as AA. Uppercase letters stand for dominant alleles. This normal rat carries no recessive genes so its all uppercase.

Now you want to breed the two. Each rat is going to give a copy of one of its alleles to each individual offspring. So each punnet square is showing you the chances of what an individual offspring might carry. Lets say you breed the two rats and there is only one baby but it isn't born yet. So you want to know if its going to be albino or not. You set up the punnet square for this baby.

The normal rat lends all A's.
The albino rat lends all a's.

A A - normal
a Aa Aa

a Aa Aa

^ Albino

You know that that baby would come out as normal. It can't carry two recessive alleles do to the fact the normal doesn't lend anything other than a dominant normal allele. It will however, carry an albino allele.

If that normal rat did carry one albino allele, you wouldn't know unless the pairing of the normal and albino produced an albino. The punnet square would go like this...

The albino gives only a's.
The normal can give an A or an a.

A a - normal

a Aa aa

a Aa aa

^ Albino

So that baby that you are drawing up the punnet square for, in this case of the normal carrying an albino allele, has a 50% chance of being albino since the normal can contribute the albino allele it carries. It also has a 50% chance of being normal. If it is normal, it will carry one albino allele that it can later give to future offspring.

Does that make any sense?