Gene Mapping Video Lecture - NEET

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of inheritance part 32 is brought to you
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so now this was another important
contribution which was given by more
than so more than talked about linkage
more than talked about sex-linked
inheritance so these were some of the
very important contribution of Morgan
apart from this Morgan also talked about
gene mapping so G knapping mapping is an
extremely useful technique now once I
talk about you to be able to understand
its significance so what is the meaning
of gene mapping so it is maybe the name
as well as the name is maybe a map line
structure then we can locate genes so
yes exactly what is a map map basically
helps you to locate places right so if
you look at the map of the world you can
actually look at the different countries
where they are located what is their in
their neighborhood etc similarly what
will you do if you want to locate gene
so when will you locate the genes they
are located on chromosome so we will try
to locate gene on chromosome so if now
when we talk about linkage we talk about
whether the genes are nearer or the
genes are far apart so when can we talk
about the distance between the genes
when we unable to locate them on the
chromosome so gene model is all about
locating genes on chromosomes so the
linkage and crossing over between two
genes and inversely related that's what
I was talking about right that the more
linked the two genes are the lesser the
possibility of cross Rover and hence
lesser the possibility of recombinations
so that is linkage and crossover and
inversely related if two genes are very
close on the same chromosome then what
will happen if they are very close that
is the scenario of complete linkage so
there will be no crossing over they will
be inherited together and there will be
no recombination at all similarly if the
two genes are far apart on the same
chromosome then the possibility of
crossing over will increase right the
possibility of crossing over is
increasing that means greater the
distance between the genes
greater the frequency of crossing over
this is the simple relationship so as
the distance between the gene increases
now let us suppose if this is a
chromosome if this is gene am this is
gene beam so that is one scenario there
is not a scenario where this is gene a
and this is Jun B now here the distance
between gene a and gene B is less as
compared to the distance between a and B
here so that means the possibility of
crossing over this road in this case
because the distance between the gene is
small in this case the same friends
example so when the friends are loosely
connected they are far away from each
other there is more possibility that
they will join some other group so they
will they will recombine with some other
group recombination will happen
recombination means it is happening to
cross you over okay so this is the
simple concept based on which the entire
concept or the entire launching of gene
mapping was found so the gist of
whatever we discussed so far is that if
it is a complete linkage so when you we
say a complete linkage it is going to be
extremely close and linked may be
something like this so if this is the
scenario in that is only peripherally it
will be seen and there will be no
recombinant so that is the first
scenario the second scenario is in
complete linkage now in in complete
linkage what is going to happen in
complete language linkage you have two
genes that is suppose E and E they are
linked but not completely so that is you
have more parental traits but you have
some recombinations but less
recombinations so that is the scenario
for incomplete linkage the third option
is
unlinked genes so in that what happens
that they are they show independent
assortment and independent assortment
what happens is they are not at all
linked visible so in this case they have
50% parental and
person really continents so the now a
frequency of recombinants increase in
this case now with respect to this there
is a term quality combination frequency
and what is it it is the frequency that
determines the number of recombinants
form in the progeny during a test cross
so if this will actually keep track of
the number of recombinants now depending
on the number of recombinants you can
actually decide the distance between the
two genes on a chromosome because if the
number of recombinants are more that
means more crossing over has taken place
the more crossing over has taken place
what does that mean that means the genes
are far apart even if they are located
on the same chromosome they are far
apart whereas it the number of
recombinants unless that means crossing
over has not taken place much that means
that these things are both words
complete linkage so then genes are
closer together so the distance between
the genes will can be governed by the
combination frequency and how do we
calculate recombination frequency we
calculate in the same number of
recombinants form divided by the total
population form multiplied by 100 so
this gives you the percentage of
recombination frequency now how does
this convey this recombination
percentage helps us to locate genes on a
chromosome now more than hundred student
athletes 220 they used this concept to
create genetic Maps now how did they
create genetic Maps basically they
calculated the recombination frequency
like wherever you have to locate two
genes what they did they performed a
test cross or once the perform a test
cross they got a recombination frequency
because if you perform a test cross it
is very easy to find out the output
order on an offspring population and
then it is easy to calculate the
combination frequency
once you have that recombination
frequency that can be utilized to locate
the genes of the chromosomes how right
status what we see now so genetic map is
a technique to determine the location
and relative distance between
genes on a chromosome so what happens in
this case is the basis of genetic
mapping our genes are linearly arranged
on the chromosome now it is not that
they are arranged in some pattern they
are all located one after another so if
this is a chromosome this is how the
genes are located one after another in a
linear sequence and crossing over
between two genes is proportional to the
distance between them so the same things
again so they are the business of
genetic map so the basic concept behind
designing the genetic map was that one
percent of the combination was conceded
as one map unit so in the previous slide
we saw that how do we calculate the
recombination frequency so now let us
suppose in in any of the process the
recombination frequency comes out to be
12 percent so that would actually mean
12 map units with 12 units on a map echo
you have graph graph also you have
bosses right each box is like one unit
so similarly when you say map unit it is
trying to say that on a map of
chromosome 1 Judit so the 12 percent
will correspond to 12 units that means
the distance between those two genes
will be 12 units on a Crow's on a
genetic map clear okay so let us try to
see how exactly a genetic map is created
so genes are actually placed on a
hypothetical chromosome base from the
map units so this is how we design the
chromosome and then we see that on here
so if the map units is true that means
the distance between the two genes is to
live on the same chromosome so this is
how it looks like so this is the
hypothetical chromosome and on this you
have the various map units right so you
can actually divide it into several
units so if I say that the distance
between G a and G B is 3 units so we
consider that this is going to be 3
units that means the recombination
frequency that will come out if you
perform a dihybrid cross of these two
genes is going to be 3% now by now you
go how do we perform
cross between these two so that's what I
was talking about right and the previous
line
so basically this is how the genetic
maps are constructed so let us look at
some of the examples and suppose we say
that genes a b c and d occur on the same
chromosome okay
crossover values for the pairs of genes
are
if you cross over a and B you get 35%
chance be 5 percent C and B 40% and
Beyond with 10% and C and D is 30% so
what is the order of jeans on linkage
map so thank you please visit
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