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Author Topic: Punnet Squares  (Read 2141 times)  Share 

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a212345

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Punnet Squares
« on: November 12, 2017, 10:42:32 pm »
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I don't really understand punnet squares and
genotypes and phenotypes
and how to find genotypes to start a punnet square

if that makes sense
would love some help
thankyou so much  :) 

PopcornTime

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Re: Punnet Squares
« Reply #1 on: November 13, 2017, 02:09:45 am »
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Hopefully the definitions below help.

Genotypes are the genetic makeup/identity of an organism (sets of alleles)

Phenotypes are the physical characteristics of an organism. Examples = eye colour, hair colour, etc.

Genes are units which determines characteristics

Alleles are alternative forms of genes (represented by letters, e.g A, a, B, b).

Here are a couple of videos to explain punnet squares:
https://www.youtube.com/watch?v=i-0rSv6oxSY
https://www.youtube.com/watch?v=prkHKjfUmMs
https://www.youtube.com/watch?v=9Bkc7SGVwqI

AngelWings

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Re: Punnet Squares
« Reply #2 on: November 13, 2017, 02:11:44 am »
+4
Hey there A212345! :)

Let's start with some definitions.
Definitions
Phenotypes are essentially what would you see in the organism (What type of traits or appearance does it have and, more importantly, what's being changed?) e.g. blue eyes.
Genotypes are the alleles* that an organism has that actually affect a certain trait. (What alleles* do they have?)
Phenotypes are usually dictated by an individual organism's genotypes e.g. bb**.

Also, you'll likely come across these two terms: "homozygous/ homozygote" and "heterozygous/ heterozygote"...
Homozygous: having matching alleles* in one genotype; having two of the same allele* for the one genotype (homozygote: an individual who is homozygous for a trait) e.g. BB**
Heterozygous: having two different alleles* for the one genotype (heterozygote: an individual who is heterzygous for a trait) e.g. Bb**

... and these two: "dominant alleles* " vs. "recessive alleles* " (also defined here and explained here).
Dominant alleles* : the form of the gene that will show up regardless of what the other form is
Recessive alleles* : the form of the gene that will only show up if the individual has two of the same allele*
If a gene is dominant, it will show up in preference over the other allele*, regardless of what the other allele* is e.g. BB** and Bb** have the same phenotype. (It may be easier to think of recessive as the opposite of dominant.)

These definitions are me trying to use as little scientific language as possible, so it's likely that your textbook or sheets have much more technical definitions than these. (I have also included blue links to resources, so hopefully one of those make sense to you. Just click away!)

* alleles = a form of a gene; different forms of alleles gives different traits e.g. B vs. b
** This will make more sense when you see the example below. Punnett squares tend to take on capital and lower case letters, so be careful.

Punnett squares assume a few things:
Punnett square assumptions
1. Each organism gets one copy from each of its biological parents; they are inherited from parent (i.e. biological dad and biological mum) to child/ offspring - "progeny".
2. The alleles come in pairs as a result***. (See Assumption #3 for more detail)
3. Each Punnett square gives traits from those parents through the genes assuming that they're of equal value, unless stated as dominant or recessive. To keep things simple, each parent contributes equally to the progeny, each of these genes are inherited via the nucleus of the cell and each trait is discrete (basically, one or a small number of genes*** control the trait entirely, so they'll either have the alleles which gives a trait or they don't and don't have that trait; think of it as an on/ off switch when there are just two alleles).
*** Note: I'll only be using one gene here.

Punnett squares also need a bit of information, so let's run through an example:
Example
The most classic question I've seen this being taught with is actually more complex in real life (i.e. > 1 gene controls it), but I'll pretend it's controlled by one gene here so you understand how to use Punnett squares. (I usually would prefer to use the original experiment done to prove this, but it's on pea plants and pea plants just aren't as relatable.)
A heterozygous brown eyed woman has a blue eyed husband and has discovered she is pregnant. What is the chance her child has blue eyes, knowing that the brown eye allele is dominant against the blue eye allele? Draw a Punnett square to show your answer.

Step 1: Write down what each allele means.
Start off by giving our alleles some names - let's go with 'B' in this case. Write down "Let B = brown eyes and b = blue eyes".
It doesn't necessarily have to be 'B' per se and you can use whatever letter you like, but it is the most logical letter in this instance and will save you some hassle later if you choose a letter in which has very distinctly different capital and lower case letters. (Otherwise, ensure your capital and lower case letters are distinctly different sizes when written.)
Punnett squares usually take on capital and lower case letters, so be careful with this. (The reason they do this is because genetics isn't always simple and is often dictated by many genes for the one trait. Dominant alleles or the 'wild type' [gene seen most often in nature] allele is usually denoted as capital letters, while lowercase letters are usually used for recessive alleles. This is not always the case, but for the majority of cases you'll encounter, this generalisation will suffice.)

Step 2: Draw a grid.
In this case, we're only dealing with one gene. Draw out a 3 x 3 grid. All will be explained in a second. (Note: 2 genes = 4 x 4 grid. You will unlikely work with any more than that, but if you're curious, here's a nice website.)

Step 3: Write down the genotypes of the parents
We already know that one gene gives two alleles per parent; each kid has one biological 'mother' and 'father', both of which have two alleles each. This gives us two letters per gene per parent, so all of our organisms will have two letters for their genotype in the end.
(a) We know from our question that brown is dominant. In genotype, you can get 4 combinations with 1 gene:
- BB (homozygous dominant - brown eyes phenotype)
- Bb (heterozygous - still brown eyes phenotype, because it has one copy of the brown eye phenotype, which is shown against the blue)
- bB (which we usually denote the capitals first anyway; heterozygous - brown eyes phenotype, refer to Bb above)
- bb (homozygous recessive - blue eyes phenotype)
(b) Thus, Mum can't be 'bb' because she's known to have brown eyes and 'bb' has no brown eye genes (Capital B = brown eyes, remember?). We also know from the question that she's heterozygous - meaning she must be 'Bb'. Since Mum only gives one copy to each of her progeny, write a letter 'B' in the top centre box of the 3 x 3 grid and a 'b' in the top right. If you wish, you can write "mother" above the grid for reference.
(c) We also know that Dad has blue eyes. Blue eyes can only be given by the genotype 'bb'. Any big 'B's here and the brown eye phenotype is given to the progeny, so they have to have two small 'b's instead. Write a small 'b' in the first column, second row (columns go vertically/ downwards, rows go horizontally/ left and right) and another 'b' in the first column, third row. If you wish, you can write down "father" on the left of the grid for reference.

Your incomplete Punnett square should look like this:
incomplete Punnett square
                B     b
b               
b                         
       
Step 4: Write down the progeny formed from these combinations
Except the top left box of the grid, run down each column and row, write in what you get when they meet to get a new genotype with two alleles. (Except for the top left box, there should be four boxes that receive a total of two letters.) For example, when you run down the second column and second row, you'll find that you get B and b, so you'll pencil in "Bb" for the box in the second column, second row. The third column, second row will give "bb". Keep doing this until all of the boxes are filled out in the grid, except that top left box, which is left empty.
Finished Punnett square
The complete Punnett square should look like this:
                                  mother
                  [blank]     B     b
father         b              Bb   bb
                  b              Bb   bb
The bottom right 4 boxes are the types of progeny you could get from parents with the given genotypes.

Step 5: Answer the question
Yay! You've done the Punnett square, now you just need to answer the actual question. What chance is there that the progeny (aka the child) will be blue-eyed?
Look at your four new combinations in your completed Punnett square. Two of those four are now filled with 'bb' - the blue eyes phenotype. Using a little math, 2/4 simplifies to 1/2 so 50% chance is the answer you need to write down. 
All Punnett squares should follow the first four steps, but with different letters for the genotypes as a result of changes in the parents'. 

Hopefully that all makes sense. Please tell me if any of that didn't make sense.
EDIT: PopcornTime beat me to it, but I thought my perspective may be helpful.
« Last Edit: November 13, 2017, 01:53:22 pm by AngelWings »
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a212345

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Re: Punnet Squares
« Reply #3 on: November 13, 2017, 12:56:00 pm »
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Thankyou Both so much

emmawatsonlover990

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Re: Punnet Squares
« Reply #4 on: November 16, 2017, 08:00:54 pm »
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Punnet squares are also the result of both Parents. They show what the offspring will most likely look like and whether any of the offspring will have any mutations of any sort.

Hope this helps as well.  ;D

The dominant traits are shown with capital letters and the recessive traits are shown by lower case letters.  ;) :-*

Hope these two answers help.

Post Merge: You can use the 'Modify posts' button to add to your last post instead of posting three times in a row :)
« Last Edit: November 16, 2017, 08:08:39 pm by jamonwindeyer »
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