The study of genetics is incomplete without
understanding the Laws put forth by Mendel.
After his experiments of Monohybrid and Dihybrid
crosses, Mendel concluded that there are certain
“factors” which are passed from one generation
to the other.
But he explained this better with the help
of his first law in genetics!
The first law in Genetics is called the “Law
of segregation”.
To understand this law, we first need to understand
the concept of Alleles!
We know that genes, in simple terms, are the
units of heredity that carry genetic information.
Alleles are nothing but the DIFFERENT FORMS
of genes.
Let’s take a simple example to understand this!
Assume we have these two chromosomes.
Here, we have genes for the character HEIGHT.
Can you tell me which genes could possibly
be present here?
We can have either both capital “TT” or
both lower case “tt”… and we can even
have one capital and one lower case “Tt”
as the gene set.
Now here, the individual “T”, that is
the single gene is nothing but the allele.
So the alternative form, that is either dominant
or recessive, is called allele.
In simple words, one single “T” or “t”
is an allele.
Now that we know what alleles are, let us
try to understand the law!
There are different definitions you will find
for this law, but let’s pick the simplest
one!
It says that, during gamete formation, the
alleles for each gene segregate from each
other such that, each gamete formed carries
only one allele for each gene.
It may still sound a bit confusing.
To understand this, let us take a simple example.
Let’s assume we have this single cell with
only two chromosomes in it.
The chromosomes have a set of genes “Cc”
for the skin colour in an individual.
Now to understand the law, let us break it into parts.
The first part says “During gamete formation”.
This is the process of meiosis.
It is the type of cell division which gives
us four cells with half the number of chromosomes.
The second part says, “The alleles of each gene”.
So here, “C” and “c” is what we are referring to.
That means one allele from the gene set.
Now what is the next part?
The next part of the statement is “Segregate
from each other”.
This means the alleles will separate.
They will not remain together in the process.
And now comes the last part!
It says that, “one gamete carries only one
allele for each gene”.
Now this part means that at the end of the
meiosis process, we will get four cells, such
that each cell will have only one allele in it.
So here, two cells will have “capital C”
and the other two cells will have the “lower
case c” allele.
Thus each gamete gets only one allele.
To summarize, we can say that the alleles
for a gene segregate during gamete formation!
To be honest, this law sounds very obvious
to us now!
It’s like having a big book with many chapters
in it, and then we make smaller books for
each chapter.
It is obvious that the smaller books will
have the content resembling the content of
the big book!
This law sounds obvious also because we know
the process of Meiosis.
But nobody knew this back in the earlier times.
Hence it was hypothesised by Mendel and then
it was accepted as a law later.
Which is the next law?
It is the law of independent assortment.
Does this mean that the genes assort independently?
Or is there anything else that the law specifies?
Let us find that out in the next part!
There is one common thing among sea waves,
dust particles, and a dice rolled on floor.
Any guesses what that common thing could be?
It’s “RANDOMNESS”!
Randomness is commonly found in nature and
in several things around us.
But can you state an example of Randomness
that occurs inside us?
Though our body seems to be organised, there
are a few processes that have no fixed pattern.
One of the processes is that of the alignment
of chromosomes on the metaphase plate!
Let me elaborate!
Imagine this to be a cell that is about to
undergo division.
For now let us assume it is undergoing meiosis.
We know that during cell division, a stage
named Metaphase is reached, where the chromosomes
align themselves on the equatorial plane.
In other words, on the metaphase plate!
So can you tell me how these chromosomes will
align before they line up on the plate?
Will all the red chromosomes from each pair
line up on one side, or will they align in
an alternate manner?
Or maybe like this?
Or will the alignment be like this?
There are numerous ways in which this can happen.
And this is Random!
There is no fixed pattern or a principle of
sequence that the chromosomes follow when
they align.
Now if the chromosomes get separated randomly,
it is obvious that the genes present on them
will also get separated randomly.
So let’s say this chromosome has the genes
“capital A” and “lower case a”, this
chromosome has the gene set “capital B”
and “lower case b”, while this has “capital
C” and “lower case c” on it.
We are considering all the sets to be heterozygous
in order to understand the concept better!
So when the chromosomes get separated in the
respective cells, it is obvious that the genes
will assort independent of each other.
There’s no thumb rule that all the dominant
alleles will perpetuate in one cell and all
the recessive alleles in the other.
They can assort independent of each other!
This is exactly what Mendel had to say in
his second law of genetics!
It is named the “Law of independent assortment”.
The law says that “Genes for the different
traits assort independent of each other during
gamete formation.”
Let us understand this with the help of this cell.
The first part says, “Genes for different traits”.
This means we are addressing the genes which
represent diverse traits.
So here, we have the gene sets “Aa”, “Bb”
and “Cc” on these respective chromosomes.
These are the genes for different traits.
The second part states, “Assort independent
of each other”.
This means the genes will not have a fixed
pattern to follow when they segregate in the
cells.
So here, the alleles of the gene set “Aa”
will assort independent of the other two gene
sets.
It is not mandatory that all the dominant
alleles will get assorted in the cells together.
So the allele “ capital A” can get assorted
with any allele from the gene set “Bb”
and “Cc”.
That is the reason why we get the gametes
with different possible combinations at the
end!
To quickly review, the assortment of alleles
is purely random and does not have a predictable
and fixed pattern during the process of gamete
formation!
This is what the law says!
There can be more possibilities of obtaining
different combinations depending upon how
the chromosomes align themselves on the metaphase
plate.
Now tell me, which type of a cross will help
us understand this law better?
A monohybrid cross or a Dihybrid cross?
Think about it!
Let me help you with this!
If it’s a monohybrid cross, then there will
be no independent assortment of the different
alleles.
And why so?
That is because we are considering only one
character at a time.
So we will be dealing with a single gene set!
And thus this law is best understood when
there are more than one characters studied
at a time.
This is nothing but the DIHYBRID CROSS or
higher crosses which involve MORE characters.
Now that we are well versed with the first
two laws of genetics, let us have a look at
the third law in our upcoming part!
A scene of lush green trees covered with yellow
or brown mesh is not new to us.
We have come across it many times.
Any idea what this mesh is all about?
These are also plants: Parasitic plants to be precise.
Being parasitic, they derive all the nutrients
from the host and grow to form a huge structure.
These plants are like creepers and overpower
the huge trees as they grow!
This is one of the best examples for understanding
the fact that “Dominance” is not a matter
of size!
Rather, it is never affected by size!
It’s the nature of the dominating component
that matters!
Let’s take our genes for example!
We know how small genes and their constituent
alleles are!
Yet, there is one form which dominates the other!
Yes, we are familiar with the concept of Dominant
genes and recessive genes!
But this wasn’t known back then!
Not even to Gregor Mendel.
That is the reason why he put forth the concept
of Dominant and recessive alleles with the
help of his law.
This law is called the “Law of Dominance”.
After the Monohybrid cross, Mendel concluded
that a few genes are dominant while the others
are recessive.
Let us have a look at what the law has to say!
The third law of genetics, known as the Law
of Dominance, states that “Some alleles
are dominant while others are recessive.
An organism with at least one dominant allele
displays the effect irrespective of the presence
of the recessive one”.
The law is actually very simple!
Let us take an example and understand it.
Let’s consider a cross of a tall and a dwarf plant.
The first part of the law says that, “some
alleles are dominant while others are recessive.”
Here, can you tell me why is this plant tall?
Well that is because it has both the dominant alleles.
We represent it with the capital letters “TT”.
Similarly, this is a dwarf plant as it has
both the recessive alleles denoted by “tt”
in lower case.
Now the next part of the law states that,
“An organism with at least one dominant
allele, displays the effect irrespective of
the presence of the recessive one”.
What does this mean?
Here, on crossing the two, we get these plants
in the F1 generation.
The phenotype of all these plants is “tall”.
And what is their genotype?
As we can see, they are all heterozygous with
one dominant and one recessive allele.
Doesn’t the tall phenotype clearly indicate
that the dominant allele has masked the presence
of the recessive one?
Yes!
That’s what the second part of our law says!
The presence of a single dominant allele is
enough to express the trait phenotypically.
And what if both the genes are dominant?
It is obvious that the phenotype will be dominant.
The law states “at least one dominant allele”,
which means if one or even both the alleles
are dominant, then we get the respective dominant
phenotype.
Let us cross this F1 generation to check whether
we have understood the law correctly!
The cross of these F1 hybrids gives us these
offsprings in the F two generation.
Can you help me with the phenotype and the
genotype of these offsprings?
Here, we get 3 tall plants and one dwarf plant.
Genotypically, there is one plant that is
homozygous for the dominant trait.
There are two plants which are heterozygous
and this last one is again homozygous, but
for the recessive trait.
So can we relate these results with our law
of dominance?
There are two types of alleles, one dominant
and one recessive!
This is what the first part says.
And the second part says that “at least
one dominant allele displays the effect, irrespective
of the presence of the recessive one”.
So as we can see here that these three plants
are tall, because they contain the allele
“T” for tall trait.
This one has both the dominant alleles and
these two have one allele for the dominant
trait.
Its presence masks or supresses the effect
of the recessive allele.
This was about the third law of genetics called
the “Law of Dominance”.
