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.