A Beginner’s Guide to Punnett Squares
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A Beginner’s Guide to Punnett Squares

Hi. It’s Mr. Andersen and today
I’m going to give you a beginner’s guide to punnett squares. There are a few mistakes
that students always make when they’re doing punnett squares, so I hope to kind of clear
those up. First of all we should talk about the name sake. This is Reginald Punnett. He
didn’t really work with punnett squares however he did work with genetics quite a bit. He
did work with mimicry in the butterflies. And so his name is probably associated with
genetics just as much as Mendel. And it’s through the use of the punnett square. Now
the punnett square is often over used as just a quick way to solve genetics problems without
really understanding what’s going on with the genetics. And so I want to kind of get
to that route. And if you could remember one thing from this whole video podcast, it’s
this right here. The two sides of a punnett square represent the alternatives after meiosis.
In other words, you have a bunch of genes and you give half of those genes to a sperm
or an egg. And that happens through meiosis. And so the organization of those gametes,
in this case it’s just a monohybrid cross, are going to be on either side of this. Just
like a flip of a coin. This would be for one parent. And then this would be the other parent
on the other side. And so what are the boxes on a punnett square stand for? They simply
stand for all the alternatives that could occur if we had mating between each of these
different gametes. And so let’s get to some examples and hopefully that will help. So
we’re going to start with a monohybrid cross. A monohybrid cross is simply a cross that
is looking at one trait. And so let’s do one that’s really, really simple. And so let’s
say we’re crossing purple flowers that are homozygous purple with those that are homozygous
white flowers. In other words this is the dominant trait. This is the recessive trait.
And so if you look at the parents what you want to do first of all is figure out what
are the possible gametes that could be produced in meiosis. In other words, this one could
either give a big P or it could give a big P. What does that mean? It can only give one
thing. It can only give a big P or that dominant allele. And so if you’re doing a problem like
this you don’t even need a big punnett square. One parent can only contribute big P. So let’s
look at the other parent. The other parent can either give a little p, and I try to make
them really small or a little p. Because p’s look the same. And so the other parent can
only give a little p. And so we could put that on the other side. And so what are the
opportunities that we could have as far as fertilization could goes? Well this one’s
automatically going to give big P. The other one’s going to give little p. And so this
is the only possible outcome we could get between a cross of a homozygous dominant and
homozygous recessive. And so you don’t really need a big punnett square. Now you could do
that. You could fill it in, big P over here, little p over here. But if you do that, you’re
going to get the same thing in all of the boxes. And so it’s still a 1 to 1 ratio. In
other words 100% of the time you’re going to have that. Okay. So let’s get to one that’s
a little more complicated. Let’s say we have a heterozygous cross. And so this is for purple
flowers as well. Well if you look at this one, now the problem has changed a little
bit. This one could give a big P. But it also could give a little p. And so we have to show
both of those possible gametes of meiosis. And so this would be the big P. And then this
would be the little p over here. So half of the time it’s going to give the big P. Half
of the time it’s going to give the little p. But if you look at the other parent, same
thing, it’s going to give the big P half of the time. And it’s going to give the little
p half of the time as well. So we’re going to put that here. Now we simply fill in the
boxes. And so this would be a big P with a big P. Because I’m taking this here and that
there. This is going to go over to here to give us a big P and a little p. By convention
we usually write the dominant allele first. So that would be one alternative here. Here
would be a big P little p as well. We get the big P here and the little p here. And
then finally we’re going to get little p over and a little p over here. Since they’re each
contributing a little p. And so what do we get from this cross? Well we get a 1 to 2,
since these are exactly the same, to 1 genotypic ratio. Because the genotype is the letter.
So there is one that’s like this. There’s two that look like this. And there’s one that
looks like that. So that’s going to be our 1 to 2 to 1 genotypic ratio. What about phenotypic?
Well this one’s going to be a purple flower. And so are these other two. And so if we’re
looking at the phenotypic ratio, the phenotypic ratio now is going to be a 3 to 1 ratio. We’re
going to have three purple to every one white that we have. Okay. Let’s try another one.
Let’s say we’re looking at incomplete dominance. So incomplete dominance, a snapdragon would
be an example of that. A snapdragon has two genes. If it has a red gene and a white gene,
then it’s going to be pink. And so this one actually has two alleles that it can contribute
and the same on the other side. And so we just write those out. So this would be the
parent. This would be 50% chance of giving the red, 50% chance of the white. And then
the same thing on this side over here. So now if we fill in our punnett square like
that, what do we get for all the different choices? Well now we have a 1 to 2 to 1 genotypic
ratio. But we also have a 1 to 2 to 1 phenotypic ratio. So if you’re doing a question where
it’s incomplete dominance, you use a punnett square the same way. Codominance would be
the same way. The difference is in incomplete dominance the heterozygous or the hybrids
are going to be somewhere between the two. If it’s codominance it will actually, they’re
going to express both of those genes. Both of those proteins. And now let’s try one that’s
sex-linked chromosome or a X linked chromosome. And so in this one we’ve got a parent, so
this is a mom, because she’s X X chromosome. And she’s a carrier of let’s say colorblindness,
the gene. And this is a dad that is normal. And so I’m going to put that up here. X Y
because half of the time he’s going to give the X. And half of the time he’s going to
give the Y. If we put mom over here, I’m going to put that carrier up there, she is not colorblind
because she has one deficient gene, colorblind gene. But she has another gene that works
well on her other X chromosome. So if I fill in this one here it’s going to be XcX. So
this would be a female, because two X chromosomes. But they’re going to be a carrier of that
gene. If we look down here, this would just be a normal female. If we look down here,
I’m grabbing the X from here and the Y from up there, so that would be X Y. So that would
be a normal male. And then if we look at this one right here, this would be a male whose
colorblind. The reason he’s colorblind is that he doesn’t have an X chromosome or another
gene as a back-up copy to that. So those are monohybrid crosses. And usually students do
fairly well on those. Next are dihybrid crosses. And this is where mistakes really start. And
so if we look at this parent. This is a typical dihybrid cross. Let me tell you what the letters
stand for. The R stands for round pea seeds. And the Y stands for yellow seeds. If it’s
the recessive that stands for wrinkled and if it’s a little y that stands for green.
And so we’re looking at a dihybrid, so that mean two traits. We’re looking at seed shape,
round or wrinkled. And seed color, yellow or green. So now we have to do a dihybrid
cross. And so the tendency is we look at this parent, the tendency is to see that there
are four letters over here. There’s four boxes over here and then you just simply write them
out. Big R little r big Y little y. And then you get a weird answer and you don’t know
what to do with it. Okay? That’s wrong. That’s a mistake. Okay? Whenever you’re figuring
out the gametes, remember that you have to give one of each letter. In other words, each
of the gametes is going to have one of each of the alleles. And so let me clear this mess
out of the way. So what do we do? Let’s say this parent right here, and I’m going to write
it up here so it makes a little more sense. Big R little r big Y little y. What possibilities
could they produce? They’re giving one of each letter remember. Well they could give
the big R and the big Y. So big R big Y. That would be one possibility. They could also
give the big R and the little y. So they could give the big R little y. They could also give
the little r big Y or the little r little y. Since they’re giving one of each color,
there’s only four, one of each letter excuse me, there’s only four possibilities that they
can give. So it’s going to be kind of a mess but those are the four right here. And so
those four and going to go across the top. So we’ve got big R big Y, big R little y,
little r big Y, little r little y. So those are the four gametes that you could produce.
In other words with this parent you can only get four combinations of each of the two letters.
Same thing down this side. And so it’s going to be the same thing written down this side
because this other parent is going to be exactly the same. Okay. So it would take me a long
time to write all of those out, so let me throw those in here. So these would be the
parents. All the possible gametes you could get. And if I fill those in, these first nine,
if you look at them, let me get a color that’s different. Let me grab a yellow color. And
so this one right here is going to be round and it’s going to be yellow. So it’s going
to be round and yellow. And this one you can see is going to be round and yellow. And round
and yellow. And round and yellow. Round and yellow. And round and yellow. And round and
yellow. And round and yellow. And round and yellow. In other words nine of them are going
to be round and yellow in shape. And the only reason why is that the R is dominant. And
so you could have on like this where they’re hybrid for both and they’re still going to
be round and yellow. And so let me try to add the next ones. So what about these next
ones? Well the next ones are going to be round and green. So let me get a different color.
So these ones are going to be round and green. Because the round is dominant. But they don’t
have the dominant for the yellow. So they’re not going to be. Let me get rid of that. So
the next ones, these ones are going to be wrinkled and yellow. So again I’ve got to
get a different pen. So these ones are going to be wrinkled and yellow. And then if we
look at the last one, it’s going to get all of the recessive alleles. And so that one
is going to be, getting green, it’s going to be green and wrinkled. Okay. And so when
we say there’s a 9 to 3 to 3 to 1 ration, that’s just a typical dihybrid cross. We’re
going to have 9 of the phenotypes that are round and yellow. Three of the phenotypes
that are round and green. Three of the phenotypes that are yellow and wrinkled. And then only
one of one’s that’s not. And that’s only going to work if you are able to set up your gametes
correctly on the side. Now it’s super hard for you to answer a question like this on
a test. You’re rarely going to have to draw a dihybrid cross. But it’s important that
you understand the concept of it. Because most of the genes inside your body are not
caused by one gene, they’re cause by multiple genes. So how tall you are is caused by probably
a dozen different genes inside your body. So you can imagine how big the punnett square
is going to be for that. An example that I’ll leave you with would be this one. So let’s
say we have a parent here. And the parent is big R little r big Y little y, little r
little r little y little y. The question I’m asking you is how big would your punnett square
have to be? And so you’re going to have to figure out what are all the possible gametes
that you could get from both of those and then, then draw it out. Figure out all the
possibilities you can get. If you’re thinking it’s going to be a 4 by 4 you’re doing way
too much work. And so those are punnett square squares and I hope that’s helpful.

100 thoughts on “A Beginner’s Guide to Punnett Squares

  1. This was alot easier to understand than some of the other videos I've watched!! I'm a 44yr old upgrading my Nursing through distance so your videos are Great to understand!!

  2. Mr. Anderson, Thank you for your clear lectures. I really have appreciated both your chemistry and biology video!

  3. Good presentation. But if you want to see the basis of the next generation of genetic technology you should check out Viogenesis.com if you are interested.

  4. This is really helpful! I love your videos. One thing though: I though RW x RW is co-dominance, not incomplete dominance.

  5. In my mom's generation they never had the Internet and no YouTube explanations. All she had was useless teachers and long winded books . We are SO fucken lucky today .

  6. Atleast i understand it better unlike the way my teacher was teaching i was so confused thankyou Youtube u made Genetics crossing easier for me

  7. I’m sorry but how is the last punnet square not going to be 4×4? Shouldn’t it be “RY Ry rY ry” x “ry ry ry ry”?

    Edit: Just found out phenotype ratio is 1:1:1:1

  8. i’ve been learning this in school for the past 2 months and couldn’t understand it and now i’ve clocked it in about 2 minutes

  9. My fucking asian teacher can barely speak English and teachers university level biology like fuck man. This man is amazing.

  10. Fell asleep while my bio teacher was teaching this. Almost failed the test if it weren't for my bf giving me answers.Finally learned after 3 weeks, Thank you so much for this.

  11. For the dihybrid cross, in order to get your letters in position, think of FOIL (the way my teacher tought us)

    F: First
    O: Outter
    I: Inner
    L: Last

    (Ex for this vid)
    First "R", First "Y"
    Outter "R", Outter "y"
    Inner "r", inner "Y"
    Last "r", Last "y"

  12. You know what it is
    Round and yellow round and yellow round and yellow round and yellow round and yellow round and yellow round and yellow round and yellow round and yellow

  13. How do you draw a punnet square when one parent is heterozygous (AbBb) for two different alleles, and the other parent is heterozygous for one, but homozygous for the other (aaBb)?
    For the parent that has (aaBb) I still used four columns, but just repeated "aB" and "ab" twice, so as not to affect the final percentage of outcomes. Is this correct, or should I have just used two columns for that parent? This is my punnet square: http://oi66.tinypic.com/345jxpx.jpg

  14. So if punnett didnt use the punnett square why is it names after HIM instead of whoever made the punnet square?

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