Punnett square fun | Biomolecules | MCAT | Khan Academy


In the last video, I drew this
grid in order to understand better the different
combinations of alleles I could get from my
mom or my dad. And this grid that I drew is
called a Punnett square. And I looked up what Punnett
means, and it turns out, and this might be the biggest
takeaway from this video, that when you go to the farmers’
market or you go to the produce and you see those little
baskets, you see those little baskets that often you’ll
see maybe strawberries or blueberries sitting in, they
have this little grid here, right there. Sometimes grapes are in them,
and you have a bunch of strawberries in them
like that. That green basket
is a punnett. That’s a punnett. Apparently, in some countries,
they call it a punnett. I think England’s one of them,
and you UK viewers can correct me if I’m wrong. And so I guess that’s where
the inspiration comes for calling these Punnett squares,
that these are kind of these little green baskets that
you can throw different combinations of genotypes in. And these Punnett squares
aren’t just useful. If you’re talking about crossing
two hybrids, this is called a monohybrid cross
because you are crossing two hybrids for only one trait. It could be useful for a whole
set of different types of crosses between two reproducing
organisms. It doesn’t even have to be a
situation where one thing is dominating another. Let’s do a bunch of these,
just to make you familiar with the idea. So let’s say you have a mom. So instead of doing two hybrids,
let’s say the mom– I’ll keep using the blue-eyed,
brown-eyed analogy just because we’re already reasonably
useful to it. Let’s say that she’s homozygous
dominant. And let’s say that the dad is a
heterozygote, so he’s got a brown and he’s got a blue. And we want to know the
different combinations of genotypes that one of their
children might have. So what we do is we draw a Punnett
square again. Let me draw a grid here and
draw a grid right there. And up here, we’ll write the
different genes that mom can contribute, and here, we’ll
write the different genes that dad can contribute, or the
different alleles. I didn’t want to write gene. I wanted to write dad. So the mom in either case is
either going to contribute this big B brown allele from
one of the homologous chromosomes, or on the other
homologous, well, they have the same allele so she’s
going to contribute that one to her child. The dad could contribute this
one, that big brown-eyed– the capital B allele for brown eyes
or the lowercase b for blue eyes, either one. So the different combinations
that might happen, an offspring could get both of
these brown alleles from one copy from both parents. This could also happen where you
get this brown allele from the dad and then the other brown
allele from the mom, or you could get a brown allele
from the mom and a blue-eyed allele from the dad, or you
could get the other brown-eyed allele from the mom, right? When the mom has this, she has
two chromosomes, homologous chromosomes. Each of them have the same
brown allele on them. They both have that same brown
allele, so I could get the other one from my mom and
still get this blue-eyed allele from my dad. So if you said what’s the
probability of having a blue-eyed child, assuming that
blue eyes are recessive? And remember, this
is a phenotype. These particular combinations
are genotypes. Well, in order to have blue
eyes, you have to be homozygous recessive. You have to have two
lowercase b’s. So what’s the probability
of having this? Well, there are no combinations
that result in that, so there’s a 0%
probability of having two blue-eyed children. What’s the probability
of having a homozygous dominant child? Let me write that. A homozygous dominant. And now we’re looking
at the genotype. We care about the
specific alleles that that child inherits. Well, which of these are
homozygous dominant? Well, you have this one right
here and you have that one right there, and so two of
the four equally likely combinations are homozygous
dominant, so you have a 50% shot. And we can do these
Punnett squares. They don’t even have to be for
situations where one trait is necessarily dominant
on the other. For example, you could have the
situation– it’s called incomplete dominance. Let’s say you have two traits
for color in a flower. You could have red flowers or
you could have white flowers. And let’s say I were to cross
a parent flower that has the genotype capital R– I’ll just
make it in a capital W. So that could be the mom or the
dad, although the analogy breaks down a little bit with
parents, although there is a male and female, although
sometimes on the same plant. And let’s say the other plant
is also a red and white. The other plant has
a red allele and also has a white allele. So what are the different
possibilities? Well, we just draw our
Punnett square again. Let me draw our little grid. So the child could inherit both
of these red alleles. He could inherit this white
allele and then this red allele, so this red one and then
this white one, right? That’s that right there
and that red one is that right there. Or it could inherit this red one
from– let’s say this is the mom plant and then the
white allele from the dad plant, so that’s that
one right there. Or you could inherit
both white alleles. What I said when I went into
this, and I wrote it at the top right here, is we’re
studying a situation dealing with incomplete dominance. So what does that mean? Well, that means you might
actually have mixing or blending of the traits when
you actually look at them. So if this was complete
dominance, if red was dominant to white, then you’d say, OK,
all of these guys are going to be red and only this guy right
here is going to be white, so you have a one in four
probability to being white. But let’s say that a
heterozygous genotype– so let me write that down. Let’s say when you have one R
allele and one white allele, that this doesn’t
result in red. This results in pink. So this is what blending is. It’s kind of a mixture
of the two. So if I said if these these two
plants were to reproduce, and the traits for red and white
petals, I guess we could say, are incomplete dominant,
or incompletely dominant, or they blend, and if I were to say
what’s the probability of having a pink plant? And now when I’m talking
about pink, this, of course, is a phenotype. So the probability of pink,
well, let’s look at the different combinations. How many of these are pink? This one is pink and
this is pink. So two are pink of a total
of four equally likely combinations, so it’s a 50%
chance that we’re pink. And we could keep doing this
over multiple generations, and say, oh, what happens in the
second and third and the fourth generation? Actually, we could even have
a situation where we have multiple different alleles, and
I’ll use almost a kind of a more realistic example. I’ll use blood types
as an example. So there’s three potential
alleles for blood type. You can have a blood type A, you
could have a blood type B, or you could have
a blood type O. What happens is you have a
combination here between codominance and recessive
genes. And I’m going to show you what
I talk about when we do the Punnett squares. Maybe I’ll stick to one color
here because I think you’re getting the idea. So let’s say I have a
parent who is AB. So that means that they have
on one of their homologous chromosomes, they have the A
allele, and on the other one, they have the B allele. That’s what AB means. So the phenotype is
the genotype. They’re codominant. They both express themselves. They don’t necessarily blend. They both express. That’s an AB blood type. Let me write this right here. This is AB blood type. And then the other parent is–
let’s say that they are fully an A blood type. Let’s say they’re
an A blood type. Let’s say their phenotype is an
A blood type– I hope I’m not confusing you– but their
genotype is that they have one allele that’s an A and their
other allele that’s an O. So this is what’s interesting
about blood types. It’s a mixture. O is recessive. O is recessive, while these
guys are codominant. So if you have either of
these guys with an O, these guys dominate. If you have them together, then
your blood type is AB. So what are all the different
combinations for these for this couple here? Well, you could get this A and
that A, so you get an A from your mom and you get an A from
your dad right there. And clearly in this case, your
phenotype, you will have an A blood type in this situation. You could get the A from your
dad and you could get the B from your mom, in which case
you have an AB blood type. You could get the A from your
mom and the O from your dad, in which case you have an A
blood type because this dominates that. Or you could get the B from
your– I dont want to introduce arbitrary colors. You could get the B from your
mom, that’s this one, or the O from your dad. No, once again, I introduced
a different color. And this is a B blood type. So if I said what’s the
probability of having an AA blood type? And once again, we’re talking
about a phenotype here. So which of these are
an A blood type? This one definitely is,
because it’s AA. If you have two A alleles,
you’ll definitely have an A blood type, but you also have
an A blood type phenotype if you have an A and then an O. O is recessive. So this is also going to
be an A blood type. So these are both A blood, so
there’s a 50% chance, because two of the four combinations
show us an A blood type. And you could do all of the
different combinations. You say, well, how do you
have an O blood type? Well, both of your parents
will have to carry at least one O. So, for example, to have a–
that would’ve been possible if maybe instead of an AB, this
right here was an O, then this combination would’ve been
two O’s right there. So hopefully, that gives you
an idea of how a Punnett square can be useful, and it can
even be useful when we’re talking about more
than one trait. So let’s go to our situation
that I talked about before where I said you have little b
is equal to blue eyes, and we’re assuming that that’s
recessive, and you have big B is equal to brown eyes,
and we’re assuming that this is dominant. And let’s say we have
another trait. I introduced that tooth
trait before. So let’s say little t is
equal to small teeth. I don’t know what type of
bizarre organism I’m talking about, although I think
I would fall into the big tooth camp. Let’s say big T is equal
to big teeth. So an individual can have–
for example, I might be heterozygous brown eyes, so my
genotype might be heterozygous for brown eyes and then
homozygous dominant for teeth. So this might be my genotype. And the phenotype for this one
would be a big-toothed, brown-eyed person, right? Let me make that clear. This is big tooth phenotype. And this is the phenotype. What you see is brown eyes. A big-toothed, brown-eyed
person. Now if we assume that the genes
that code for teeth or eye color are on different
chromosomes, and this is a key assumption, we can say that
they assort independently. Let me write that down:
independent assortment. So this is a case where if I
were look at my chromosomes, let’s say this is one homologous
pair, maybe we call that homologous pair 1, and
let’s say I have another homologous pair, and obviously
we have 23 of these, but let’s say this is homologous pair 2
right here, if the eye color gene is here and here, remember
both homologous chromosomes code for
the same genes. They might have different
versions. Those are alleles. And if teeth are over here,
they will assort independently. So after meiosis occurs to
produce the gametes, the offspring might get this
chromosome or a copy of that chromosome for eye color and
might get a copy of this chromosome for teeth
size or tooth size. Or it could go the other way. Maybe another offspring gets
this one, this chromosome for eye color, and then this
chromosome for teeth color and gets the other version
of the allele. So because they’re on different
chromosomes, there’s no linkage between if you
inherit this one, whether you inherit big teeth, whether
you’re going to inherit small brown eyes or blue eyes. Now, if they were on the same
chromosomee– let’s say the situation where they are
on the same chromosome. So let me pick another
trait: hair color. Let’s say the gene for hair
color is on chromosome 1, so let’s say hair color, the
gene is there and there. These might be different
versions of hair color, different alleles, but the
genes are on that same chromosome. In this situation, if someone
gets– let’s say if this is blue eyes here and this is blond
hair, then these are going always travel together. You’re not going to have these
assort independently. And these are called
linked traits. Let me highlight that. So these right there, those
are linked traits. But for a second, and we’ll talk
more about linked traits, and especially sex-linked traits
in probably the next video or a few videos from now,
but let’s assume that we’re talking about traits that
assort independently, and we cross two hybrids. So this is called a
dihybrid cross. Very fancy word, but it just
gives you an idea of the power of the Punnett square. So let’s say both parents are–
so they’re both hybrids, which means that they both have
the dominant brown-eye allele and they have the
recessive blue-eye allele, and they both have the dominant
big-tooth gene and they both have the recessive little
tooth gene. So this is the genotype
for both parents. Both parents are dihybrid. They’re hybrids for both
genes, both parents. What are all the different combinations for their children? And I could have done this
without dihybrids. I could have made one of them
homozygous for one of the traits and a hybrid for the
other, and I could have done every different combination,
but I’ll do the dihybrid, because it leads to a lot of our
variety, and you’ll often see this in classes. So if I’m talking about the mom,
what are the different combinations of genes that
the mom can contribute? Well, the mom could contribute
the brown– so for each of these traits, she can only
contribute one of the alleles. So she could contribute this
brown right here and then the big yellow T, so this is one
combination, or she could contribute the big brown and
then the little yellow t, or she can contribute
the blue-eyed allele and the big T. So these are all the different
combinations that she could contribute. And then the final combination
is this allele and that allele, so the blue eyes
and the small teeth. So that’s from mom. And, of course, dad could
contribute the same different combinations because dad
has the same genotype. Let me write that down. Let me just write it like this
so I don’t have to keep switching colors. Actually, I want to make them
a little closer together because I’m going to run
out of space otherwise. Nope. Let me do it like that. OK, brown eyes, so the dad could
contribute the big teeth or the little teeth, z along
with the brown-eyed gene, or he could contribute the
blue-eyed gene, the blue-eyed allele in combination with the
big teeth or the yellow teeth. Not the yellow teeth,
the little teeth. That would be a different gene
for yellow teeth or maybe that’s an environmental
factor. So these are all the different
combinations that can occur for their offspring. So let’s draw– call this maybe
a super Punnett square, because we’re now dealing
with, instead of four combinations, we have
16 combinations. It looks like I ran out
of ink right there. It’s strange why–
16 combinations. Let me write that out. Something’s wrong
with my tablet. Maybe there’s something weird. OK, so there’s 16 different
combinations, and let’s write them all out, and I’ll just
stay in one maybe neutral color so I don’t have
to keep switching. I could get this combination,
so this brown eyes from my mom, brown eyes from my dad
allele, so its brown-brown, and then big teeth from both. I could have this combination,
so I have capital B and a capital B. And then I have a capital
T and a lowercase t. And then let’s just keep
moving forward. So I could get a capital B and a
lowercase B with a capital T and a capital T, a big B,
lowercase B, capital T lowercase t. And I’m just going to go through
these super-fast because it’s going to take
forever, so capital B from here, capital B from there;
capital T, lowercase t from here; capital B from each and
then lowercase t from each. You have a capital B and then
a lowercase b from that one, and then a capital T from the
mom, lowercase t from the dad. Hopefully, you’re not getting
too tired here. And so then you have the capital
B from your dad and then lowercase b
from your mom. Two lowercase t’s– actually
let me just pause and fill these in because I don’t want
to waste your time. There I have saved you some time
and I’ve filled in every combination similar to what
happens on many cooking shows. But now that I’ve filled
in all the different combinations, we can talk a
little bit about the different phenotypes that might
be expressed from this dihybrid cross. For example, how many of these
are going to exhibit brown eyes and big teeth? So big teeth, brown-eyed kids. Let me write this down here. So if I want big teeth
and brown eyes. All of a sudden, my pen
doesn’t– brown eyes. So how many are there? Big teeth and brown eyes. So they’re both dominant, so if
you have either a capital B or a capital T in any of them,
you’re going to have big teeth and brown eyes, so this is
big teeth and brown eyes. Big teeth right here,
brown eyes there. Or maybe I should just say
brown eyes and big teeth because that’s the order that
I wrote it right here. Brown eyes and big teeth, brown
eyes and big teeth. Even though I have a recessive
trait here, the brown eyes dominate. I had a small teeth here, but
the big teeth dominate. This is brown eyes
and big teeth. This is brown eyes
and big teeth. Let’s see, this is brown eyes
and big teeth, brown eyes and big teeth, and let me see,
is that all of them? Well, no. This is brown eyes
and little teeth. This is brown eyes and big teeth
right there, and this is also brown eyes and big teeth. They’re heterozygous for each
trait, but both brown eyes and big teeth are dominant, so these
are all phenotypes of brown eyes and big teeth. So how many of those
do we have? We have one, two, three,
four, five, six, seven, eight, nine of those. So we have nine. Nine brown eyes and big teeth. Now, how many do we
have of big teeth? Let me write in a different
color, so let me write brown eyes and little teeth. Something on my pen tablet
doesn’t work quite right over there. So brown eyes and
little teeth. So let’s see, this
is brown eyes and little teeth right there. This is brown eyes and little
teeth right there. This is brown eyes and little
teeth right there. So there’s three combinations
of brown eyes and little teeth. And if I were to say blue eyes,
blue and big teeth, what are the combinations there? Well, this is blue eyes and big
teeth, blue eyes and big teeth, blue eyes and big
teeth, so there’s three combinations there. And if I want to be recessive on
both traits, so if I want– let me do this. I want blue eyes, blue
and little teeth. There’s only one. Out of the 16, there’s only one
situation where I inherit the recessive trait from both
parents for both traits. So if you look at this, and
you say, hey, what’s the probability– there’s only
one of that– what’s the probability of having a big
teeth, brown-eyed child? And these are all
the phenotypes. There were 16 different
possibilities here, right? There are 16 squares here, and
9 of them describe the phenotype of big teeth
and brown eyes, so there’s a 9/16 chance. So it’s 9 out of 16 chance
of having a big teeth, brown-eyed child. What’s the probability
of a blue-eyed child with little teeth? 1 in 16. So hopefully, in this video,
you’ve appreciated the power of the Punnett square, that it’s
a useful way to explore every different combination of
all the genes, and it doesn’t have to be only one trait. It can be in this case where
you’re doing two traits that show dominance, but they assort
independently because they’re on different
chromosomes. You could use it– where’d
I do it over here? You could use it to explore
incomplete dominance when there’s blending, where red and
white made pink genes, or you can even use it when there’s
codominance and when you have multiple alleles,
where it’s not just two different versions of the genes,
there’s actually three different versions. So hopefully, you’ve
enjoyed that.

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Comments

  1. Im doing all of this high school and college stuff in middle school and bro you have really helped me and i am amazed by your skill i wish you were my teacher

  2. FYI the term Punnett was used in Punnett square becuase off Reginald C. Punnett who worked with genes. Its not becuase of little green baskets. Just thought I could help clear that up. Enjoed the video.

  3. FML! I've been lost for the past two weeks in class, and now you saved me just before my quiz tomorrow!! God Bless you Sal! <33

  4. how do you know that the family of Reginald C. Punnett did not invent this method of vegetation baskets

  5. Reginald C. Punnet invented the Punnet Square and why the baskets are called punnets is because he was a farmer and bred Chicken famously the Crème Legbar. Punnet for a basket is slang in the farmers market scene

  6. I mean foil, like First, Outer, Inner, Last. Remember from algibra? Sorry if some people took it the wrong way, I wasn't trying to be mean or anything 🙁

  7. In A type blood we have A antigen. Antigen makes our blood produce antibodies against that antigen. Then why A antigen exist in A type blood? Doesn't blood produce anything against A antigen in A type blood? It should do according to antigen's definition.

  8. Seriously, he doesn't know that these are named for the guy, Punnett, who came up with this system?  Baskets at the market?  What?

  9. These videos along with others are part of my daily morning ritual. I love YouTube for all of the amazing minds sharing knowledge in their unique ways.

  10. Sal, you're awesome and I appreciate all the work you put into these videos. I just wanted to point out the error in your incomplete dominance example. The parental generation couldn't be red and white with the genotype RW, because their phenotype would be pink!

  11. Okay, so a quick Google search reveals that yes, Reginald Punnett developed the square, and was a famous biologist…but the word Punnet (with one t) is indeed a wire basket. My question is: these two things seem to be related, grids within a box, so do they call the baskets after the scientist, did the scientist see the similarity in his name and apply it to his work, or (And this is the one I'm leaning to) were aliens involved?

  12. they are called punnet squares because of the guy who invented them… Reginald C. Punnet. Just thought i would let you know.

  13. To everyone out there complaining about Sal not knowing the origin of the term Punnet. You have to understand that this is not the only area of knowledge Sal teaches about. If he was exclusively a biology teacher than it would be strange but he teaches a multitude of subjects, such as Math, physics, chemistry, economy, and so on. Give the man a break. Also, it's not like it's something that is critical for you to know since pretty much nothing is dependent on its knowledge.

  14. I'm very happy that people like him does exist. I'm not English native speaker and those videos really help me a lot. I can learn another language at the same time I'm learning Biology, History, Mathematics, etc. So awesome! Thanks internet. I'm really living a gold era.

  15. Good video, very helpful to my studies

    Funny Moment: Get this chromosome for this eye color, and this chromosome for this teeth color.

  16. At time it was kinda confusing as he did not use upper case and lower case to indicate dominant and recessive alleles. However, it was really helpful and thats just the way my teacher taught me.

  17. Actually the word Punnett comes from the scientist Reginald C. Punnett who created the punnett square to learn more about cross genetics.
    (but yes those baskets are sometimes called punnett baskets depending on where you live in the world)

  18. It's not because of those strawberry boxes , it's called a Punnett square
    Punnett was a scientist who contributed to genetics and talked about mimicry in butterflies. That's the reason for it to be called a Punnett square..

  19. This is where the name Punnett came from. LOL
    Reginald Punnett British geneticist
    Reginald Crundall Punnett FRS was a British geneticist who co-founded, with William Bateson, the Journal of Genetics in 1910

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