Heredity - Heredity and Evolution - Class 10 Science Video Lecture

now to understand heredity and selection
and variation and evolution you first
need to understand at the root level the
difference between phenotype and
genotype
so what's phenotype what are the
characteristics that you can call
phenotype basically all that you can see
is the phenotype in humans the eye color
and hair color are examples of the
phenotype so the phenotype of an
organism is an organisms actually
observed properties like appearance
development or behavior and genotype is
an organisms full charity three
information now the part of the genotype
that is expressed is called the
phenotype expressed big word what does
it mean nothing much
when the information that is there in
your genes gets manifested or shown it's
called gene expression now the phenotype
that you see is because of the
combination of the genotype that you
have and the environmental factors that
affect it we are going to see how
exactly in a short while now a genotype
is nothing but the genetic information
that you carry now each trait in you
which you can see has two parts or two
alleles one is from mom and dad now
let's go back to a little bit of basics
about the nucleus the hotspot of the
cells you can file all this blue printed
information that apparently decides what
you're going to look like a lot more
features that make you so special so if
I take a typical cell over here the
nucleus is right here in the middle
let's zoom in and see what's happening
this darker part over here this is all
the nucleolus the nucleus takes
decisions and makes important laws for
the cell based on the information it
gets from the chromosomes inside the
nucleolus which you can identify as a
darker portion of the nucleus now all
the information or the important
information is blue printed in the
chromosomes which are like specials Qian
like structures made up of protein and
DNA PN II was that deoxyribonucleic acid
now the DNA has important
edited the information by hereditary
information I again mean the information
that you have inherited from your
parents that besides all your features
your genotype the protein called
histones keep the DNA neatly wrapped up
and coiled because if it was not for the
proteins the DNA would not fit you know
in this way in fact if you take out the
DNA all the DNA from one body of one
person's body it would stretch to the
Sun not once not twice but an astounding
600 times ok so it's that long and it's
bound so tightly in your nucleus and
since life starts as a single cell it
keeps on dividing to produce new cells
which come together to give you tissues
and organs and organ systems and then
you so all the information that is
required form all these tissues organs
and organ systems is found in D E n a
now the cells in your body keep on
dividing so that is important sending
information is passed on and the portion
of the DNA segment that contains all
this information is called
a gene ge and E okay we look a bit like
our parents too because of our DNA and
same is the story with you a puppy would
carry its parents DNA to and would hence
look like them it's the same story for
every single creature that reproduces
sexually what determines each one of us
at the gene level is our DNA our
genotype I hope it's really really clear
now coming to chromosomes all eukaryotes
have them and they have them in
different numbers we humans have 23
pairs of chromosomes hair because there
are two of each type so in total we have
22 to 46 chromosomes because they come
in pairs they are called diploid okay
now since these pairs of chromosomes are
so similar
we call them homologous chromosome pairs
homologous is a simple word once you
break it up for more similar logos
relation so
same relation or similar relation there
are only one pair of cells that are
haploid they have exactly half that
information and they are found in yes
the sex cells that is the sperm and the
egg so that when they come together the
resulting cell for the zygote gets one
full set of 46 chromosomes and then this
keeps on and on in fighting now not long
after homologous chromosomes were
discovered someone noticed one exception
human males have one pair that is not
homologous the larger of these
chromosomes was called X and the smaller
was called Y so we boil down to one big
basic genetic difference between human
males and females females have two X
chromosomes males have One X and one Y
the other 22 pairs of chromosomes are
the same not to confuse you anything but
with birds is exactly the opposite xx is
male and XY is female some organisms
have more than two pairs of each
chromosomes and these are called
polyploid because Polly's many some
plants are polyploid like potatoes there
polyploid don't worry just this fact
does not make potatoes better than you
so when there is a single set of
unpaired chromosomes
it's called haploid and bees can be said
to be very very very be easy okay males
develop from unfertilized eggs they're
all haploid well as all sea veins are
diploid what is the significance of
diploid remember I told you that you
inherit traits from mom and dad so it so
happens that you get one half of your
DNA from mom and one half from dad now
all your body cells will have the same
mix of DNA so the 46 chromosomes grouped
into 23 pairs are pairs with good reason
one in each pair from mom and one from
dad those pairs of chromosomes are
pretty similar but they're not identical
they contain versions the same gene
called alleles and these alleles have
found same spots on the chromosomes so
it
won't talk about any kind features like
having dimples or the update control
your tongue or the fact that you your
love the free you're attached all of
these have different alleles this
essentially means that if you look into
chromosome you will find different parts
of the DNA of the chromosome responsible
for different traits but the beauty is
you get one copy from mom and one copy
from dad and if mom's earlobes are free
and dads are attached how does your body
decide which one you have all of this is
related to something called dominance
and recessive Ness which just simply
means that in nature there are some
traits that are more favorable and thus
when they interact with the recessive
form of the trade the dominant one is
expressed and as I told you before
expressed is not a big word expressed
just means visible turns out that having
dimples is dominant over not having them
rolling a tongue is dominant over not
rolling and one more very interesting
one having wet earwax turns out to be
dominant over having dry earwax cellular
DNA is the information source for making
proteins in the cell a section of the
DNA that provides information for one
protein is called the gene of that
protein brings me down to my next big
important question why exactly are
proteins this important to answer that
let me take an example of tallness
tallness is a characteristic now we know
that plants have hormones that can
trigger growth plant height can depend
on the amount of a particular plant
hormone the amount of the plant hormone
made again will depend on the efficiency
of the process for making it obviously
now consider an enzyme that is important
for this process
if this enzyme works well a lot of
hormone will be made and the plant will
be tall if the gene for that enzyme has
an alteration that makes the enzyme less
efficient the amount of hormone will be
lesser and the plant will be short and
that's exactly how we can confidently
say that genes control characteristics
or traits so as you must guess there has
to be the smart
and scientist or scientist who first
realized and discovered that these
things exist how these things work and
surprise surprise
this smart plant scientist was a
scientist as when as a monk he was an
Augustinian monk from Austria who in his
spare time for your breath dead he
plants who he must have loved so dearly
and was totally obsessed with a
little-known fact is that Mendel began
his studies on heredity using mice he
was at sin Samus is a bee but his bishop
did not like one of his friar studying
animal sex so Mendel you know sadly
switched to plants now you must be
assuming that he would be quite a
gardener but no he was not just any
amateur gardener but the scientist who
studied his pea plant so carefully so
carefully he called them his children oh
my god what kind of dad does experiments
on his kids and why exactly choose bees
it was pure luck actually but they
turned out to be perfect models for
genetic research with so many stable
varieties there was a tall variety and a
short one
one type made smooth round Peas while
others were lumpy and wrinkled there
were green peas yellow peas gray seed
coats and white seed coats white flowers
and purple flowers and so on since every
pea flower has both male and female
parts by default they would fertilize
themselves so men will practice some
family planning on them and set about
his tasks on creating hybrid varieties
first he snipped off the anthers that's
the male part of the flower that
produces pollen while still immature -
he dusted the stigma that is the female
part with pollen are taken from the
appropriate or desired for the plant and
finally he tied bags over the flowers to
keep out any stray pollen this way the
monk was playing God and controlling the
parentage of each generation his first
big discovery was dominance when he
crossed at all
P blonde with a short one one would
expect medium-sized plants but no they
were all tall so he concluded that
tallness is superior or dominant over
shortness so the trait of tallness is
dominant and the trait of shortness is
recessive in every case in each and
every case one trait was found to be
dominant brown seeds are dominant over
wrinkled ones plump over pinched gray
seed coats over white seed coats so it
did not matter which parent contributed
the pollen and which parent contributed
the eggs at all short hybrid was all
based on and then came the complication
what if you cross two hybrids
when hybrids were self fertilized about
1/4 of their offspring were short the
recessive trait appeared the other 3/4
were tall when he further continued the
self-fertilization that is the short
with the short and the tall with the
tall's he found that about one tall out
of three produced only tall's while the
others yielded both stalls and shorts in
a ratio of 3 to 1 and the shorts they
pred only shorts what did he interpret
there is something in the pollen and the
egg that determines the height of the
pea plants this something is called G
each pollen grain and egg has one high G
so the plant formed by their union has
two so the gene is of two distinct types
or alleles one allele a for tallness and
one a for shortness so depending on what
they inherited a plant can have same or
different alleles the allele a is
dominant over a so a plant with a
combination of a a would still be tall
the alleles do not blend now let's look
at a case ok when a a breeds with a a
fall in an egg get one copy of each
because sex sells off the number of
chromosomes right taking you back to
what I taught you a few minutes back
a haploid and all other cells will be
deployed now in the case of a a and E a
the sex cells respectively get a and a
as I mentioned so the offspring will be
a a or tall now in the case of a a or a
a that is small is the sex cells again
will get a and a and the varieties
yielded will be stable and short in
Mendel's first hybrid experiment the
cross was between a a and a a so the
pollen and the egg carried a and E so
the cross was a a which was tall you
remember that I told you everything
revolves around phenotype and genotype
so in this case the tallness or
shortness is the phenotype and the
combination of alleles at the gene level
a a a or a a is the genotype so even
though the first hybrid experiment gave
us a tall plant at the gene level it was
a a and this is called heterozygous
because head throw is different and
zygous for zygote and then homozygous is
same there's nothing to memorize here
they are all just logical names derived
from various languages just give you
that extra twist okay let's continue
with our story when the hybrid cell
fertilizes its alleles a and a are
sorted out randomly among the pollen
grain and eggs both a and a appear and
roughly in equal proportion so what are
the four possibilities short fallen
short egg tall fallen short egg short
pollen tall egg and tall pollen da one
egg these are the only four
possibilities that can happen now this
is too much to write let's just put it
in a square let's just simplify it okay
and it's got a fancy name it's called a
Punnett square okay eun and ET now let's
write down the four alleles and then
it's just simple combination
you have the egg here you have the
pollen here you have capital a via small
a here here and a small here bring them
together and what do you get a a e a
a and a this is every type of offspring
possible and so when the hybrids were
crossed
he got one fourth through breeding
tall's half-turns which may breach arts
and one fourth true reading shots and
Mendel did not stop here being the
inquisitive scientist that he was he
wondered what would happen if I play God
between plants differing in two
characteristics say a tall plant with
smooth seeds and a short plant its
wrinkled seeds are height and smoothness
correlated somehow to depend on each
other or they act independently when
their plant reproduces what exactly
happens that's what you wanted to see
let's call the alleles for smooth seeds
as s and that for wrinkles it says s
because smooth is dominant over wrinkled
now let's cross them and see what
happens you have a a SS and a a SS the
pollen and egg will have a s and a s
first generation will be a a s s and yes
the offspring will be tall and but
smooth pea pods as this is what is
dominant and when this hybrid was self
pollinated let's just put it down in a
Punnett square and it would look like
this now as you can see a constant ratio
of nine is to 3 is to 3 is to 1 in each
case this experiment proved an important
thing that the chains for height sort
out independently of each other
they do not blend or mix just to brief
you again an organism is homozygous with
respect to a given gene if it's 2
alleles are the same and heterozygous if
they are different enough of pea plants
let's talk about you and me clasp your
hands together without thinking about it
most people place their left thumb on
top of their right as this happens to be
the dominant phenotype now for fun
try clasping your hand so that the
opposite thumb is on top
please strange and unnatural does it
so dominant is left thumb on top and
recessive is right thumb on top looks
like we are recessive out here same is
the case with infants the dominant trait
is having a dimple so if your mom and a
dad both have a dimple
you will definitely sport one once
you're checked out your smile in the
mirror you also know if you were paying
attention what our high five modern
geneticist means by phenotypically
smooth and Gino typically heterozygous
the first few steps towards the
wonderful world of genetics and
biotechnology and let's answer that big
question we asked you right at the
beginning of the chapter about identical
twins are they free not typically the
same and what about Gino typically when
it comes to identical twins since
phenotype also depends on the
environment if one twin being the
aggressive teenager that she is decides
to color her hair hang out on the
beaches and sports some major suntan and
the other one decides to just eat eat
and eat they wouldn't be phenotypically
the same but Gino typically they aren't
the same as they have the same DNA note
that I am talking about identical twins
or monozygotic twins and what is
fascinating is that even though they are
G not typically the same because of the
movements in the fetus during pregnancy
they would have developed this thing
grooves on their fingers and because of
this they have distinct fingerprints so
this says one very important thing about
phenotype it is the interaction of the
genotype with the environment so
genotype plus environment will give you
the phenotype and this is very important
to understand because something called
variation in phenotype or phenotypic
variation is the reason this world is so
colorful between populations among
populations the differences you can see
big differences small differences all of
this is nothing but variation