Loading

Alison's New App is now available on iOS and Android! Download Now

Study Reminders
Support
Text Version

Set your study reminders

We will email you at these times to remind you to study.
  • Monday

    -

    7am

    +

    Tuesday

    -

    7am

    +

    Wednesday

    -

    7am

    +

    Thursday

    -

    7am

    +

    Friday

    -

    7am

    +

    Saturday

    -

    7am

    +

    Sunday

    -

    7am

    +

Today, we are going to look at how localization also plays a role; where an mRNA is present
and when it is present also matters. So, here is one good example of Drosophila oocytes during
its development. So even before fertilization, there is an asymmetric localization of molecules; a
good example is the Nanos mRNA. So, it is deposited in the oocytes by the nurse cells. As the
oocyte grows, the Nanos mRNA molecules move towards the posterior primarily by diffusion,
but then at the posterior, they are bound by a protein complex primarily by another RNA binding
protein called Oskar. This protects the Nanos RNA from degradation.

Oskar also activates Nanos translation, so RNA is being localized within the cell at the posterior
region, and it gets translated there. So, the unlocalized Nanos RNA is not translationally active.
(Refer Slide Time: 02:57)

And another example we are going to see is again from Drosophila itself. The hsp83 mRNA is
protected from degradation only when it is in the posterior. In other places, the Deadenylase
complex is going to bind and degrade the RNA. So, therefore, localized protection.
(Refer Slide Time: 03:17)

The third aspect of this is the transport of mRNA, so in Drosophila oocytes, Oskar mRNA is
transported on these microtubules by the motor proteins to the posterior side. At the same time,
on microtubules, motor proteins transfer Bicoid mRNA to the anterior. So, this is how
asymmetric localization happens in the very early embryo itself; in this case, it is oocyte; it is not
even fertilized. So, therefore, when you keep following, like yesterday’s question, the
transcription cascade, like this one is specific to this tissue, then what made it specific to that,
like if you keep going back, you will all the way go back to a one-cell embryo, the zygote.
Therefore, the differential gene expression starts in the one-cell embryo by asymmetric
localization of molecules in a single cell.

So symmetric cytoplasm is converted into an asymmetric cytoplasm meaning different parts of
the cytoplasm have different components. So, people think this is the starting point, and they are
excited about embryonic polarity, and they focus on that. So, a lot of labs focus on how polarity
is established in the early embryo in different model systems.

So, this is, for now, our brief overview of how differential gene expression is brought about.
First, we learned that it is primarily the nuclear material that seems to be controlling
development. The early genetic evidence told the same and then other experiments like, for
example, somatic cloning experiments told there is a genomic equivalence. This convinced us
that development must be brought about by differential regulation of gene expression.

So, now we saw how that might be accomplished. So various aspects starting from chromatin
structure to mRNA asymmetric localization. So now we are going to take a huge detour we are
moving from developmental biology to genetics because the whole of development has been
learned only through the methods of genetics.
(Refer Slide Time: 06:40)

So, Gregor Mendel was an Austrian monk. He seriously tried to become a teacher and could not
pass the exams and could never become a teacher. He failed multiple attempts and could not
succeed; then, he realized the monastery is probably the place to spend the rest of the time. So
before moving on to his experiments, I would like to tell the historical context in which his
experiments were done. So, at that time, people knew genetic information is transmitted from
one generation to another generation. Only a Homo sapiens can make another Homo sapiens
similarly only a blackbuck makes a blackbuck so that already tells you that the blackbuck has the
information to make another blackbuck, only humans have the information to make another
human being. So obviously there is information, and that is being transmitted from generation to
generation.

So the branch of science that tries to focus on how this information is transmitted? And what is
this information? Are there general rules that govern this transmission process? That is genetics.

So people knew for transmission to happen information must exist, but they did not know how it
happens. There are multiple theories, including the preformation theory, that is familiar to you.

But the predominant prevailing theory at that time was called a blending theory, meaning the
information coming from the maternal and paternal source are blended to generate the offspring.
But then there was a contradiction to this in everyday observation. For example, let us say your
father and mother have straight hair, but your paternal grandmother had curly hair, and you have
curly hair, so how blending will explain that? So, the curly hairiness has never been blended with
straight hair and became intermediately curly in your parents, they both have straight hair. So,
blending theory could not support this. In the F2 generation, that is in your generation; you get
your grandmother’s curliness in your hair. So certain features which resemble one of the
grandparents are not there in your parents. So many features like that exist in nature.

For example, plant breeders quickly realized hybrid vigor happens in the hybrid, and then in F2
again, you get the grandparental qualities. So, like this, multiple pieces of the evidence
contradicted blending theory. So, people were interested in finding out how exactly genetic
information gets transmitted. So that is the context in which we need to look at Mendel’s
experiments.
(Refer Slide Time: 11:45)

So first, let us look at his experimental strategy. So, garden pea plants were readily cultivatable
earlier, and locally among the farmers. He could find seeds for garden pea having different
phenotypes, for example, flower color variation, plant height, the position of the flowers, etc.
Many variations were readily available, and therefore, he took them and cultivated and tried to
do genetic crosses and observe what happens.
(Refer Slide Time: 12:57)

So, this is the experimental strategy, unlike us, humans, many flowering plants have both
gametes in the same flowers, like pollen grains and ovum are present in the stigma and carpel.
Since these flowers are bisexual, both the gametes come from the same genome.

So, if you take a garden pea variety, that is giving purple flower for several generations, then its
genome contains the information to make only purple color. So, you cross a white one and purple
one and follow the flower color to discover what rules govern genetic transmission.

Before that, we have to make sure gametes from the same flower are not fusing. So, for that, the
flower is opened up even before it is ready for pollination, and the stamen is removed; this
prevents the ovules in that flower from being fertilized by the pollen of the same flower. Also, it
is covered with a paper bag that prevents fertilization by random pollen grain in the environment.
Now you take a flower of a plant that you like, the white color one in this experiment, then with
a paintbrush, you dab a little bit of the pollen grains from that, then open up the cover and coat it

on that carpel. So, this helps in controlling the pollen that will fertilize the oocytes of the plant
that you have chosen as the female. So, this is how the genetic crosses are performed. So now the
fertilized one is going to grow, so the ovary is going to form this pod; it makes more oocytes
there, and therefore it is going to make several seeds. So, each seed is an embryo, it had one
pollen grain’s genome and one egg’s genome fusing to create a diploid.
So, this is the experimental setup. When it grows now, if you are focusing on flower color, then
you have to wait till the plant grows up to the flowering stage. And the flower color here is the
phenotype, and the genetic information that is contributing to the color is called the genotype.

During performing these experiments, Mendel ensured that the seeds are pure breeding variety,
which means for several generations in a row, that phenotype only was displayed by that
particular set of seeds. Let us call it a strain, so that produce only that one variety, it was not
giving rise to other phenotypes. So that is called pure breeding. So, as you see in the cartoon, all
the flowers are like one of the two parents; in this case, purple color and he called this as the
filial generation one, and therefore, this is F1, where this is the first generation.
(Refer Slide Time: 18:49)

So, now let us look at what happens when you go further. So, this is the parental generation or P
generation than filial generation one where all plants produced purple flowers. So, now what he
does is; he does not open up and cover it and selectively brings stamen from somewhere; instead,
he allows them to self-pollinate among themselves, and then collected the seeds and sowed it.

And when they grew, and new flowers came the F2 generation comes there, he ended up seeing
roughly 3⁄4 of the plants produced flowers that were purple color; the same color was there in the
F1 generation, not the color that disappeared in F1. The color that was or the phenotype that was
absent in F1 appeared in F2 in 1⁄4 of the plants.

So, this is all the experiment and the result. So, the summary for one cross is there in this slide.
So the parents are pure-breeding parents, one is always purple for generations, and the other was
always white for generations. Then in F1, only purple is present, and white is absent, and in F2,
when selfing takes place within its stamen and eggs, results in 3⁄4 of the purple color and 1⁄4 of the
white color.
(Refer Slide Time: 21:08)

So the experiment was not done just with the flower color, it was done with various aspects of
the pea plant as follows: axial bud versus terminal bud, yellow seed coat color versus green
color, seed shape round or wrinkled, pod shape inflated or constricted, green or yellow pod, tall
plant or short plant; like that he took multiple phenotypes. One phenotype at a time and did
mating with the opposite or contrasting phenotype on the same thing like color or height or shape
where you had a distinguishable another feature like purple color and white color.

So, he did multiple experiments, and in all of them, he got the same kind of numbers. He got
705:224, and therefore it is 3.15:1. So we never saw experimental data like this, so here if you
see the numbers, these are the actual numbers that were counted. The color that was not there in
F1 reappears in the F2 killed the blending theory.

If blending theory is wrong, then we have to come with another theory. You have to interpret the
observed results right, so why in F1 one did not show up? Why 3:4 ratio, all the time, it is not
random. So, the first thing is you need to assume that I am probably getting information from
both parents. Parent 1 gave the information to make a purple color; parent 2 gave the information
to make the white color. So, then you need to invoke this dominance and recessive to explain
why one of them is absent in F1. So, to get to a 3:1 ratio, two sets of information are crucial. And
only then this can be worked out.
(Refer Slide Time: 23:30)

So, this is what Mendel worked out. He assumed that a sexually reproducing organism has two
parents, and it probably has two copies of the biological information or genetic information. He
assumed because this was before meiosis, cytogenetics is still not there, and microscopes are still
not being used extensively. So, he believed that during gametogenesis, only one copy of the
information goes to a gamete and not the other one, and therefore, two letters are used to denote
a genotype. So, the reason we use two letters is based on the assumption that there were two

parents for that individual; therefore, two copies of the biological information and in
gametogenesis each gamete gets only one copy. If you assume that I use P to denote a purple
color and the gamete contributed by this, therefore, bring one copy of that. Now the other one
similarly brings one copy of the white color, which is denoted by p. Now when these two fuses
and make a diploid cell that is going to be Pp. Now, let us say the purple is dominant like purple
is going to show up if it is present even in one copy, then this Pp will be purple.

Now let us look at gametogenesis in the F1 generation; half of the gametes will have P and half
will have p that both eggs and sperm, now if there is a random fusion of P and p, then you will
have this Punnett square representation. 50% will be P, and 50% will be p. Now you have a
random fusion that I mean is during gametogenesis, there is no bias to have P or p, so therefore
50% of the gametes are P, 50% of the gametes are p. And similarly, when the two gametes fuse
again, there is no bias, P is not looking for sperm with P. It fuses with either one of the two.
Similarly, sperm does not have a preference for either one of the two types of oocytes. So, when
you have total randomness, then you will end up getting this combination giving you 3:1, so this
is what Mendel deduced from this experimental observation. And by considering that the parents
chose had two parents, each he proposed the law of segregation. Meaning particular genetic
information let us now use for convenience the word gene, probably exists in multiple variants;
for example, one variant makes this purple color phenotype another variant makes this white
color phenotype. And these two variants, how many ever variants exist they do not influence
each other when present together as you see in the F1 generation, the white did not make purple
to become something else or purple did not influence the other. They stayed together during
which one does not alter the other one, and they independently segregate when gametes are
made. One does not affect the other one, and then there is a random fusion of the gametes. So,
that is what is the law of segregation.
(Refer Slide Time: 28:35)

So now I should introduce these words, but you already know, you know what a genotype is?
And what is phenotype? The genetic information or the informational constitution is what you
call the genotype and phenotype is what is the visible biological outcome, biological function, in
this case, the flower color. So, when you have the same type, you know both the variants are
identical, so variants are alleles. When both the alleles are identical, it is homozygous, and when
they are not, it is heterozygous. The genotypic ratio can be different from the phenotypic ratio;
here, the genotypic ratio is 1:2, which means 1 is homozygous for the P, 2 is heterozygous, and 1
is homozygous for p, so 1:2:1. But the phenotypic ratio is 3:1, and this difference is explained by
one of them being dominant over the other.
(Refer Slide Time: 29:55)

So, to find whether a given genotype is PP or Pp, a test cross is done. A test cross is when you
have an organism displaying the dominant phenotype and if you want to find out whether its
genotype is homozygous or heterozygous for the dominant allele. To address that, you do a
genetic cross with a parent that is homozygous for the recessive phenotype. It is also called a
back cross; because crossing the selected one with the parent with a recessive phenotype. Since
you are crossing back with the parent many times, it is called a back cross, but a good way of
saying it would be a test cross because it is a cross to test. So, this cross will result as follows: PP
will make only one type of gamete, whereas Pp will make two types of gametes. If the unknown
genotype is homozygous for P, it will produce all purple flowers, and if it is heterozygous, it will
produce 1⁄4 homozygous for the white flowers, which is the recessive phenotype.

So, if it is heterozygous, you will get a 1:1 ratio; if it is homozygous, you will only see the
dominant phenotype. So, this is how you will determine the genotype whether it is homozygous
or heterozygous for the dominant allele.

So, this is a back cross or test cross, so you remember crossing with the homozygous recessive
parental phenotype.
(Refer Slide Time: 32:34)

So, the next experiment is relatively easy to follow, so in the previous one, it is only looking at
one particular phenotype, flower color alone, not two phenotypes at the same time. Now if we
consider two phenotypes like color and height of the plant, will one influence the other?

Let us take a tall plant with a purple flower and a short plant with a white flower. Now, will the
inheritance of white demand going to the short plant, or it can be on a tall plant too and vice
versa. So, does the genetic information for one phenotype influence the transmission pattern of
the genetic information for another phenotype? So that is what is this second experiment, the
dihybrid cross.

Let us assume it will influence, then the tall and purple will always go together, then it will boil
down to 3:1 ratio, it will be like a monohybrid cross, and that is what is illustrated in this cartoon.
Here we are taking round and a yellow seed; the seed shape is one phenotype, and seed color is
another phenotype. So, you assume there are two copies for each of those two phenotypes. It is
also called a trait. Now the yellow and round seed plant is crossed with the green and wrinkled
seed plant. During gametogenesis, you will find one copy for each, and when they fuse, you will
get the combination in the slide. F1 is heterozygous for both; it is round, and yellow, meaning
yellow is dominant over green round shape is dominant over the wrinkled shape and when you
go to F2, if the round and yellow always were going together, then you will only get this 3:1

ratio of round yellow to wrinkled green. But when Mendel experimented, he found that they do
not influence each other. So, he found 9:3:3:1 ratio, 9, 3, 3, 1 ratio of all possible four
combinations. So yellow and round did happen like one of the two parents, and you got green and wrinkled like the other parent, but you got the mix like green and round, yellow and wrinkled. This Punnett square explains how that might happen.

To explain this, one should assume that there is no bias in which allele goes into the gamete with
respect to both of the phenotypes. So, a gamete that inherits the Y does not worry whether it is
going to have R or r for the other information. So during fertilization, gamete fusion happens
randomly, only then you will get 9:3:3:1 ratio, so when the inheritance pattern of alleles of one
gene does not influence alleles of another gene, that is called independent assortment, and when
alleles of a given gene do not influence each other, and they go separately; we call that as
segregation, that is the law of segregation.

So, the independent assortment is the second law of Mendel. So, these are the basic principles
like rules of addition and subtraction in math. The entire set of genetics is derived from these two
laws. If you know this then you know genetics, now everything else is derivative of this. So that
is why it is worth taking a lot of time to comprehend this completely.
(Refer Slide Time: 38:00)

So whatever I already highlighted is shown here, so it is random; there are no biases.
(Refer Slide Time: 38:09)

So now, we are going to consider the situation that appears as if they do not follow Mendel’s
laws, and we will try to understand how Mendel’s rules are followed there. One case is what we
call as incomplete dominant. The F1 generation phenotype does not resemble the phenotypes of
either one of the two parents; instead, it is like an intermediate phenotype. When you go to F2,
the phenotype accurately reflects the genotype 1:2:1.

To explain this, we need to get into biochemistry a little bit; then, it is easy to understand. So let
us assume a particular enzyme that was catalyzing the reaction to make the pigment to make the
red color. When you had double the dose of it, remember we have already learned dosage
compensation, and when you have double the amount, then it makes enough pigment to make the
flower red. But if you have only one copy that means half the quantity of the enzyme, now the
pigment produced is let us say only half of what would be made normally, and if that is visible,
the diluted state is readily recognizable then that is called Incomplete dominance. But Mendel’s
laws are strictly followed with respect to the genotype here, only in the phenotype, you see a
variation, and this is called incomplete.
(Refer Slide Time: 40:01)

And so this is carnation, so that is Snapdragon you know Darwin’s favourite plant, then this is
carnation having a very similar thing.
(Refer Slide Time: 40:15)

And then there is an opposite situation where both the alleles show their presence. For example,
cell surface markers you can have two markers at the same time. Let us say there are 100
receptors on the surface of a cell, now 50 receptors could come from one allele, and 50 receptors
could come from the other allele. Both are there on the same cell, and if one receptor bind one
particular variant of a ligand of another one bind another variant of the ligand now, this cell will

bind both of them. So, in such a situation, both can show their presence, and we call that as co-
dominance. So, blood group antigen reaction is one such thing, so that is shown in the slide. I am not going to go into detail because you will readily understand this.

So for example, if you have blood group A, the RBC surface will have particular glycosylation,
so you call that A, and that could be homozygous or heterozygous, similarly, blood group B has
the B variant, a different sugar attached and that could be homozygous or heterozygous. As I just
explained when you have multiple receptors, some could come from one allele; some could
come from the other, and both may not be present, you probably lack both the modifications. So
now, the way it reacts with the corresponding antibodies is where you find that it is co-dominant.

So when it is AB, both the modifications are present, so they have antigens for both, and lack
antiserum for both. Therefore, you do not get any reaction. So that is co-dominance.

(Refer Slide Time: 42:42)

You understood how co-dominance is possible, so in both cases, we are getting to biochemistry
to explain this, so this is the actual blood test I will ignore that.