Dihybrid & Test Cross Ratio in Linked Genes Video Lecture - NEET

hello friends this video on principles
of inheritance part 28 is brought to you
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let us try to find out how the link the
genes give again hybrid ratio of 3 is to
1 so for that purpose let us consider
that we have a proposal here where we
have two genes a and B and this
represent the homologous pair of
chromosome the left counter parts of the
gene so we say that a and B are linked
and a and B are again linked so with
this idea in mind let us try to do a
dihybrid cross so for that we should
have a parental generation so let us
suppose we cross a homozygous two
homozygous parents a ABB with a a b b so
this is what we cross so what are the
gametes that can be produced from this a
B what are the gametes that can be
produced from this a B and they both
will combine to form the f1 generation
which will be a a b b and this is going
to be heterozygous so now the most
important question that what are the
gametes that can be form produced by
this so this can form only two gametes
that is capital e capital B and small is
small that's because now they are linked
so a will combine only with B again
small a will combine only with small U
so capital e cannot combine with small B
similarly small a cannot combine with
capital T so these are the only possible
gametes now when this self-pollinate or
we when we cross the f1 generations
amongst themselves so that is we cross
it with another a in V B so what happens
again this also produces the same
cabbage so now what would be the f2
generation so for f2 generation system
combined with this August and combined
with this on this tank of my withers all
this and combine with this these are the
only possibilities right so what would
be the output you can get either a B a B
or you can get a B a B or you can
maybe 80 or 80b right so now if you look
at their phenotypes now this is going to
be a V because all the dominant traits
are present so even this is going to be
thus of the same phenotype because you
have the dominant traits this is also
going to be on the same phenotypes but
this has all the recessive trait so this
is going to be different so what is
going to be the phenotypic ratio so the
phenotypic ratio in this case that is
the ratio of dominant is to recessive so
the dominant is to recessive ratio in
this case is going to be 3 is to 1
because this this and this they are all
going to be dominant because the
dominant traits the dominant allele of
both the shapes are present here and
this is going to be recessive so this is
1 so 3 is 2 one guy hybrid ratios of in
with linked genes but if I did not mean
a linked gene in that case you would
have got two more Gammons here so when
you get two more gap into what happens
you get more combination so when you get
more populations your f2 generation
output also changes now similarly let us
try to perform a test cross what is a
test cross so a test cross is nothing
but a cross between a heterozygous and
homozygous recessive so what is
heterozygous so heterozygous would be
something like what we got in the f1
generation so this is basically a
heterozygous
so we take a heterozygous and then we
cross it with a homozygous recessive so
homozygous recessive would be something
like this so what would be the possible
gametes in this case so let us try to
find out here there will be two possible
gametes and is a B and baby again you
cannot get more damaged because they are
named in this case just one gamete is
possible and that is a B so one of the
possible combinations that you can
as a result of this test cross just two
types of combination one is a B a B and
the other one is a B a B okay so now if
you look at it you will see that this is
dominant and this is recessive so there
is 50% chance that it will be like this
there's 50% chance that it will be like
this that assessing one so basically
both of them exactly resemble the parent
so if you see this exactly resemble the
parents whether you talk about the
phenotype and genotype similar in this
exactly resemble the parents so what
does the test cross ratio that you
obtain so the test cross ratio here is
one is to one and whatever you see here
here you only get parental phenotypes
and there is no recombinant so you do
not see any recombinants in this case
correct so if you talk about the first
dihybrid cross also there also if you
see the phenotypes and the in terms of
the phenotypes it is all parent on the
same parental phenotypes however the
genotype you get an additional genotype
that is av AV so this is the additional
genotype but otherwise they are all the
parental phenotypes and in this case the
phenotype as well as the genotype both
phenotype ratio genotype ratio is 1 is
to 1 and you see only the parental
combinations being formed in the
offspring and there is no recombinants
name form so whenever we talk about
linkage we see that what happens is
wherever the genes tend to get linked
the possibility of getting any
combinations reduce because when the
genes are linked they will always get
inherited together so you tend to get
the same parental combinations over the
generations so the chances of adam
recombinants becomes less thank you
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