Dihybrid Cross : Sweet Pea Video Lecture - NEET

hello friends this video on principles
of inheritance part 29 is brought to you
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no more fear from exam now does that
mean that there would be no reform the
names formed when think things are
involved so in so that means if you talk
like that so wherever you have linked
genes they will only have the same
traits being carried on them to the next
generation in the same way so there is
no question of having recombinants but
still when we perform with an experiment
on a dihybrid cross by Morgan we saw
that there was some one person for
recombinants so from where does that
recombinant come into picture yes even
though the genes are linked when the
genes are linked the more linked to the
genes are the less possibility of having
recombinant so the more linked the genes
are the more possibility of having the
same parental traits over the generation
but now that depends on how much link
are the genes whether the links and
whether the genes are very much linked
they are like very close to each other
and completely linked or they are quite
far and then they are not they are
linked but they are not completely
linked so it depends so that is why we
will look at another experiment which
was perform on sweet pea and the results
from this experiment will actually tell
you the the scenario where you get
recombinant such a sort that if you want
agree for penance as the genes unlinked
so here you will see that the genes are
linked but still you tend to get really
four minutes so once we see that then we
will try to analyze from how does that
recombinant form so now when this
experiment or this dihybrid cross was
done in sweet pea again it is a guy
hybrid cross so two traits needed to be
considered so what were the two traits
which were considered flower color and
pollen shape so flower color in an two
possibilities the flower cousin would be
you or the flower color could be red so
blue being the dominant one is denoted
by capital D and delay is denoted by a
small B similarly for the pollen shape
the cordon shape could be round or the
pollen shape could be long
so here long is the dominant one so
denoted by capital L and roundels the
recessive and each so it is denoted by a
small n so these are the two traits
which were considered so what was done
was heterozygous dominant was crossed
with homozygous recessive so basically
it was a test cross so whenever we
perform a cross between a heterozygous
and the homozygous recessive that type
of process one test cross why is it one
test for us because it is generally used
to determine the genotype of an unknown
organism for example if I suppose in
this case we know it is heterozygous but
how do we not know this if we cross it
with homozygous recessive we get to know
if this is heterozygous dominant or it
is homozygous dominant okay so that is
above test cross so now let us see what
happened here
so now as per the previous concept on
linked genes and unlinked genes what we
see we saw that one linked genes the
knife hybrid ratio should be one is to
one that is 50 percent each so let us
see what happens in case of in piece of
this if now there can be two
possibilities we don't know whether the
genes are linked or they are unlinked
so let us first explore possibility one
then we assume that the genes of the
clips are unlived
so both the genes are not at all linked
to each other so they will show the
principle of independent assortment so
they will assort independently so what
does it what happens in this so in this
case the parental generation what does
the parental generation here so as I
said a heterozygous dominant so
heterozygous dominant will be something
like this this will be crossed with a
zyphus recessive so homozygous recessive
will be this now since here the genes
are not at all linked so this capital B
can combine with capital n the capital B
can also combine with swollen and the
same is true with small V so this alone
can produce four types of gametes and
what are they
capital v capital L capital v is fall
and small V capital L and small V small
n so in the central use for damage
whereas this can produce just one gamete
that is B n right so now these are the
Gammons which have been produced now
when you have these gametes now you can
easily get the f1 generation so what
would be the f1 generation so each of
these can combine with this one right so
what would be the F my generation one
option is BL BL the next option is B and
BL third option is BL BL and the fourth
option is BL BL so these are the four
types of offsprings that will be
produced in this case right okay so what
about the phenotypes of these offspring
so if you talked about this one this is
going to be red and round what about
this one
this is again going to be red but it is
going to be long what about this one
this is going to be blue but it is going
to be round and finally this one this is
going to be blue and law so in this case
if you see four different types of
phenotypes are being produced and these
are their genotypes so this is the
result of the scenario when we assume
that the genes are unlinked now let us
explore the second possibility when we
see that the genes of the two capes are
completely lived so the genes are
completely linked that means they will
always remain together so when I say
completely linked that means if this is
one chromosome this is B
this is it so they are located on the
same chromosome and they are completely
linked to each other so be and then it
will always be together
so whether it be and then we'll always
be together so in this case what will
happen again the same cross so we ll
crossed with the homozygous recessive so
in this case the gametes which will get
produced out of this there will be only
two graphics that is PL and B in because
the other combinations are not possible
as they are linked genes and this is
this just one gamete so this is what
what the various possibilities for
offsprings just two possibility be and
BL or BL BL
so this what is the phenotype for this
this is going to be blue long and this
is going to be red and round so now if
you compare possibility 1 and
possibility to what do you see in
possibility 1 what was what was the
parent the parent was blue long and red
round so we had the parental traits in
half of the cases that is in 50% of the
cases we had the parental traits and in
the remaining 50% cases we had new
combinations or recombinations
impossibility to when the genes have
completely lame we saw that in 100% of
the offsprings we have the parental
traits so this is like parent this is
also like parent so you do not have any
recombinations so in this case there is
no recombinations in this case there are
50% recombination so here
nobody combinations but please remember
that both of these scenarios were just
our assumptions now we had to do a
dihybrid cross we just assumed that what
will happen if the two genes are
unlinked what will happen if the genes
for the two traits are completely thing
but what actually happens in reality is
this cross was actually performed what
was the design so let us look at the
real result so the designs which were
all came in reality was that
recombinants and clear so recombinant
ceci
so in our previous a where did we see
the recombinants impossibility one where
the genes were unlinked so does that
mean that the genes are unthinking this
is let us see it was also observed that
the parent and the recombinant types of
ten where in the ratio seven is to 1 is
to 1 is to 7 now this is a very
different ratio because it shows that
these two figures denote the parental
types and this denotes the recombinant
so this shows that there are very few
recombinants which are being formed and
most of the outputs on most of the
offsprings of the parent ok so that
means the real results which were
obtained that is somewhere in between
the possibility 1 and possibility - so
as far as possibility one was unlinked
genes in unlinked genes we get
recombinants but 50% of the whole motion
even if not completely 50% but a good
amount of percentage and like
recombinants so if they are completely
not linked in that is you actually end
up having a good amount of recombinants
but in this case the recombinants and
very less so in one way it is not linked
but in some way is if it doesn't fall
under the category of unlinked genes if
you look at possibility - when we say
that they are linked genes in that case
the parental types were whole in the hot
schools but there was no recombinants at
all but here we have recombinant but
very little recombinant so that means
this is somewhere midway in between the
unlinked genes and completely linked
genes so from this and first it was
concluded that basically there are three
options possible one is I'm thinking
where the genes are not at all linked to
each other so in that case they can
independently assort themselves so that
is one option the second option is
completely linking so the genes will be
completely linked with each other in
that case there will be no D
combinations and the third option is in
completely linked that means they are
linked but not completely so they
landing but still there will be some
recombinations for now how much
recombinations will be formed I mean how
many how many reformulations will be
there that might depend on the amount of
linkage so more the linkages less
recombinants will be so we've got to see
that these dreams when neither unlinked
nor they were completely linked and
there came the concept of incomplete
linkage so this is also possible that
genes are linked but they are not
completely there so there is something
in between unlinked and completely
linked and in case of incomplete linkage
you tend to see recombinations like if
they are completely lling you don't see
much reformulations but the recut the
linkage is the more the recombinations
you tend to see so now the question
would be how are these recombinations
form from there exactly the
recombinations are coming up and I am
sure you would like to know more about
incomplete linkage as well thank you
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