Punnett Square Video Lecture - NEET

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 namesake 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 as Mendel and
it's through the use of the punnett
square now the punnett square is often
overused as just a quick way to solve
genetics problems without really
understanding what's going on with the
genetics and so I want to get to that
root 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 this case
is just a monohybrid cross are going to
be on either side of this just like a
flip of the 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'll help so
we're going to start with a monohybrid
cross the 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 B 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 peas
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 goes well this
one's automatically going to give it big
P the other one's going to give a little
P and so this is the only possible
outcome we could get between a cross of
a homozygous dominant and a 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
one-to-one ratio in other words a
hundred percent 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 problems
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 the time it's going
to get a little P if we 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 so we get the big P
here in the little P here and then
finally we're going to get little P over
here 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 one to two since these are
exactly the same to one genotypic ratio
because the genotype is the letter so
there's 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 one two two two one 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
three to one ratio we're going to have
three purple to everyone white that we
have okay
let's try another one let's say we're
looking at incomplete dominance so
incomplete dominance a Snapdragon and 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 a 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 2
2 2 1
genotypic ratio but we also have a 1 2 2
2 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 selection they're going
to express both of those genes both of
those proteins now let's try one that's
a sex-linked chromosome are at x-linked
chromosome and so in this one we've got
a parent so this is a mom because she's
xx chromosome and she's a carrier of
let's say colorblindness the gene and
this is a dad that's normal so I'm going
to put dad 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 XC x 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 be XY so
that'd be a normal male and then if we
look at this one right here this would
be a male who's colorblind the reason
colorblind is that he doesn't have an X
chromosome or another gene as a backup
copy to that so those are monohybrid
crosses and usually students do fairly
well on those next are dihybrid crosses
and this is where the mistake really
start and so if we look at this pen this
is a typical dihybrid cross let me tell
you what the letters stand for the R
stands for round pea seeds and they Y
stands for yellow seeds if it's the 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 means two traits we're looking at
seed shape round or wrinkled and seek
color yellow or green so now we have to
do a dihybrid cross and so the tendency
if 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'd 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 a letter excuse me
there's only four possibilities that
they can give so it to be kind of a mess
but those are the four right here and so
those four are going to go across the
top so we've got big R big y they got
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
nations of each of the two letters same
thing down this side so it's going to be
same thing on 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 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 it's going to be
round and yellow and round and yellow
and around yellow then 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 and shape and the
only reason why is that the R is
dominant and so you could have one like
this where they're hybrid for both and
they're still going to be rounded 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 in
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 let's add the next ones
so these ones are going to be wrinkled
and yellow so again I got to get a
different pen so these ones are going to
be wrinkled in yellow
and then if we look at the last one it's
going to get all of the recessive
alleles and so that one's going to be
getting creamed it's going to be green
and wrinkled okay
and so when we say there's a 9 2 3 2 3 2
1 ratio that's just a typical dihybrid
cross we're going to have 9 of the
phenotypes that are around and yellow 3
of the phenotypes that are round and
green 3 of the phenotypes that are
yellow and wrinkled and then only one of
the ones it's not and that's only going
to work if you are able to set up your
gametes correctly on this 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 caused 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 of that
I'll leave you with it 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 squares and I hope
that's helpful