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Biology · 2024 · 5 marks
CBSE 2024 · Region 3 · Set 1 · Q32
(a) "The influence of both the alleles in a heterozygous state is clearly expressed in codominance." Explain with the help of inheritance of ABO…
(a)
"The influence of both the alleles in a heterozygous state is clearly expressed in codominance." Explain with the help of inheritance of ABO blood group in humans.
OR
(b)
"A group of genes are regulated and expressed together as a unit in lac operon."
(i)
Explain the mechanism of switching 'on' of the structural genes of lac operon.
(ii)
"Regulation of 'lac operon' is referred to be negatively regulated." Justify giving a reason.
Marking-scheme solution
The plasma membrane of the red blood cell has sugar polymers that protrude from its
surface and the kind of sugar is controlled by the gene I present in blood.
The gene (I) has three alleles IA IB and i
The alleles IA and IB produce a slightly different form of the sugar.
IA and IB are completely dominant over i
When IA and IB are present as in AB blood group they both express
their own type of sugars and show codominance.
(i)
Switching ‘on’ of the lac operon
-The lac operon consists of one regulatory gene and three structural genes
(z, y and a)
-The i gene codes for the repressor of the lac operon
-The Z gene codes for beta – galactosidase (β – gal)
(βeta galactosidase is responsible for hydrolysis of (disaccharide) lactose into galactose and glucose)
-y gene codes for permease (which increases the permeability of the cell to
β-galactosides/lactose)
-The repressor protein binds to the operator region and prevents RNA
polymerase from transcribing the operon
-Lactose is the inducer and regulates switching on and off of the operon
-In the presence of inducer, lactose or allolactose, the repressor is
inactivated by interaction with the inducer
-This allows RNA polymerase to access the promoter and transcription
proceeds
Repressor binds to the operator to inhibit gene expression therefore it is referred to be negatively regulated.
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