Hardy Weinberg Principle & Algebraic Equations Video Lecture - Biology Class 12 - NEET

hi it's mr. Andersen and in this video
I'm going to talk about the
hardy-weinberg equation which is an
equation that's used to determine the
frequency of genes in a population
before we get to the equation let's
define a few terms the first one is a
phenotype a phenotype is going to be the
physical characteristics that an
organism has and so the only difference
between these two individuals here is in
the color of their hair and so the
difference is going to be in their
phenotypes this person has red hair and
this person has non red hair now when
you're looking at me you're looking at
my phenotypes
now the phenotypes however are caused by
genes that we possess and so we're going
to possess two genes and a gene is
simply a section of DNA that codes for a
specific phenotype and so the types of
genes are called alleles and so in this
case this individual has two genes but
both of them are going to be non red
jeans or non red alleles this person
right here has red hair phenotype but
its alleles are going to be two red
genes and when we're solving genetic
problems we'll use letters to represent
those big R big R stands for non-red
genes and little R little R stands for
red hair but what happens if you were to
get one of each what happens if an
individual has one of the non-red genes
and one of the red genes what color is
their hair going to be well if you know
anything about genetics you know that
they're going to have non red hair and
that's because the red gene is recessive
to the non red gene and so these two
things pictured here are the phenotype
or the physical characteristics of this
individual and the genotype or the genes
that they possess and so let's check our
understanding I'm going to show you a
few things and what I want you to do is
tell me if this is a phenotype or a
genotype I'll pause for a second to let
you answer and so let's start with the
first one let's say an individual has
Huntington's disease
that's right that should be a phenotype
because it's going to be the physical
manifestation of their genes let's try
another one let's say an individual can
taste PTC or a chemical
that would also be a phenotype because
it's our physical manifestation of their
genes let's try another one let's say an
individual is a carrier of cystic
fibrosis
that would be their genotype now what
that means is that they're carrying one
cystic fibrosis gene but we couldn't see
that by looking at them in other words
they're not going to have cystic
fibrosis and so this would be a genotype
and when you're solving problems related
to hardy-weinberg it's important that
you figure out right away are we talking
about a phenotype or are we talking
about a genotype now the way I like to
explain alleles and how genes are
interact is using socks and so basically
you might think of it like this if you
have a red sock that's like having an
allele for red hair and if you have a
non red sock that's like having an
allele for non red hair and so basically
an individual is going to have two
copies of a gene and so if you have two
red genes then you are going to have a
genotype of red red and what kind of a
phenotype are you're going to have well
using socks what we can do is if I talk
one inside the other because you really
only see that one thing you really see
only see that phenotype of the
individual so again if you have two red
genes you're going to have red hair
likewise if I have two non-red genes
what color hair are you going to have
well let me tuck this in
to make our phenotype basically you're
going to have non red hair and so that's
pretty easy to read genes you're going
to have red hair to non-red genes you're
not going to have red hair but what
happens if we were to have an individual
that has one red gene and one non red
gene well since this one is dominant to
this gene that basically means that this
other gene is going to remain hidden and
so when we look at this individual we
can't tell what that other gene is
they're going to have non red hair and
so if I were to pick up another
individual and look at them so this is
their phenotype this is physically what
they look like you don't know what this
individual is going to have for it's two
genes because that other one is hidden
and until we actually pull that gene out
and we see that oh this individual had
on non red and one red gene we really
don't know what their genotype is and so
it's very important when you're solving
problems that we go to the individuals
who have two of the recessive alleles
because if you have two of the recessive
alleles in other words you have the
recessive trait we know not only what
your phenotype is but we know what your
genotype let's look at a sample
population of ten individuals in this
case we have five individuals who have
red hair and five individuals who have
non red hair this is their phenotype now
what we learned from the Sox is that you
only know the genotype of individuals
who have both of those recessive alleles
and so now let's take a look at their
genotypes and what we'll find is that
all the individuals who had red hair are
going to have both of those red alleles
but if you have non red hair you might
have one of the alleles for red and one
of the alleles for non red or you might
have both of those for the non red
allele and so these are their genotypes
now the alleles in a population when
they're all combined makes something
called a gene pool so a gene pool is
simply a collection of all the alleles
in a population and when you're solving
these problems it's important that you
calculate the frequency of each and so
frequency is simply going to be a ratio
the number of those alleles compared to
the total number of alleles in the
population and we use letters to
represent these two frequencies P will
always represent the frequency of the
dominant allele and Q is going to
represent the frequency of the recessive
allele so it's kind of hard to count
these so let me stack them up so we can
look at them and so basically what this
means is that P is going to be the
frequency of the dominant alleles and so
what was dominant
remember the non-red genes are going to
be dominant so if I count those there
are six of those non-red alleles divided
by a total of 20 alleles and so that's
going to be 0.3 Q then is going to give
us the frequency of the recessive
alleles well there are 14 total
recessive alleles
divided by
total number of 20 and so our Q value is
going to be 0.7 and what you should
notice right away this is very important
that P plus Q will always equal 1
in other words 0.3 plus 0.7 is going to
equal 1 and in any problem that you
solve P plus Q will always equal 1 and
that allows you to solve some some tough
problems and so basically let's get to
the hardy-weinberg equation
hardy-weinberg equation is going to tell
us the frequency of different genotypes
in a population and so it's written as
this P squared plus 2pq plus q squared
equals 1 now lots of times when biology
students are trying to solve problems
related to hardy-weinberg equilibrium
they will simply try to plug in numbers
in and then solve this equation and
that's not how it works basically P
squared is going to represent the
frequency of the individuals in a
population that are homozygous dominant
now that's a new term what does it mean
they have both copies of this dominant
gene q squared is going to tell us the
individuals who are homozygous recessive
that means they have both copies of the
recessive gene and 2 PQ is simply going
to tell us the frequency of individuals
in a population that have one of each
and we call those heterozygous for this
trait and so let's get to a problem and
so that's say the problem presented is
this 2 percent of humans on the planet
have red hair what percent of humans are
heterozygous for this trait well the
first thing you need to do is figure out
what the question is giving us and so
they're saying that 2 percent of humans
have red hair and so what are they
telling us well they're telling us a
number here but when they're telling us
that individuals who have red hair are
they telling us a phenotype or are they
telling us a genotype well red hair is
going to be a phenotype and so they're
telling us that but the nice thing is
that since this is a recessive trait red
hair is recessive not only are they
telling us the phenotype that they're
telling us the genotype and so in this
case that's going to be Q squared since
red
there is recessive q squared is going to
be the individuals who have red hair and
so when you're solving hardy-weinberg
problems you want to make sure that you
always convert that to a decimal value
otherwise you're going to get results
that really won't make sense and so
we've got Q squared equals point zero
two why did I use Q squared well we know
that these are the individuals who are
homozygous recessive for that trait so
how do I solve this well basically I
could take the square root of this and I
could take the square root of that and
so Q is going to equal the square root
of 0.02 and so that's going to be
roughly 0.14 once I figured out my Q
value and I know in the back of my head
that P plus Q will always equal 1 I
automatically know what P is in this
case P is going to be 0.86
okay so now I've already determined P
and Q when you're solving hardy-weinberg
equations the first thing you want to do
is find your P and your Q value so let's
kind of talk about what this means if 2
percent of the humans on the planet have
red hair that means if we were to take
all the alleles of all the people on our
planet and put them together in this one
gene pool we'd find that point 1 4 or
14% of those genes are going to be
actual red hair now let's get to the
question that they're asking they're
saying what percent of individuals are
heterozygous for that trait well looking
back to our hardy-weinberg problem we
know that the heterozygous individuals
are always going to equal to P Q and so
that's going to be 2 times our p value
which is 0.8 6 times our Q value which
is going to be 0.14 and if I plug that
into my calculator I'm going to get a
value of around point 2 4 or since
they're asking as a percent that about
24 percent and so on the planet 24
percent of the individuals or almost 1/4
of the individuals on our planet are
going to be heterozygous for that trait
or they're going to be carriers of that
trait so now let me give you a problem
let's say that 12% of the individuals in
Ireland have red hair what percent of
this population is going to be
homozygous for non red hair and so what
you should do is figure out what the
question is asking you should figure out
what they're giving you and then you
should use the hardy-weinberg equation
to give this a try