a tendency to aggregate and degenerate. They, therefore,
clog blood capillaries. Blood circulation and oxygen supply
to the affected tissues is disturbed. Spleen and brain can
get damaged. The patient feels acute physical weakness.
The homozygotes having only haemoglobin-S usually die
before reaching maturity because erythrocyte distortion and
degeneration occur even under normal oxygen tension.(b) A sickle-cell anaemic child can be born only to a carrier
couple. Thus, Aabha cannot solely be blamed for the birth
of a sickle-cell anaemic child and Ravi is equally responsible
for this disorder. The counselling can encourage the family to
understand that both mother and father are responsible for
any autosomal disorders. A child inherits its characters from
both of the parents and not just from mother only. It will help
Aabha to upgrade her condition in such an orthodox family.
- In humans, ABO system of blood group is a case of
multiple allelism, codominance and dominance. In the
ABO system, there are four blood groups A, B, AB and
O. ABO blood groups are controlled by gene I. The
gene I has three alleles IA, IB and i, this phenomenon
is known as multiple allelism. IA and IB are completely
dominant over i, this is known as dominance. When IA^
and IB^ are present together they both express themselves
and produce blood group AB, this phenomenon is known
as codominance. The blood groups and their possible
genotypes are given below in the table :
Blood group Genotypes
(possible)
A
B
AB
OIAIA or IAi
IBIB or IBi
IAIB
ii
ORAlfred Hershey and Martha Chase worked with viruses
that infect bacteria and are called bacteriophages. The
bacteriophage attaches to the bacteria and its genetic
material then enters the bacterial cell. The bacterial
cell treats the viral genetic material as its own and
manufactures more virus particle. They grew some viruses
on a medium that contained radioactive phosphorus^32 P
and some on medium containing radioactive sulphur^35 S.
Viruses grown in the presence of radioactive phosphorus
contained radioactive DNA but not radioactive protein
because DNA contains phosphorus but protein does not.
Viruses grown on radioactive sulphur contained radioactive
protein but not radioactive DNA because DNA does not
contain sulphur. Radioactive phages were allowed to attach
to E.coli bacteria. The viral coats were removed from the
bacteria by agitating them in a blender. The virus particles
were separated from the bacteria by spinning them in a
centrifuge. Bacteria which was infected with viruses that
had radioactive DNA were radioactive, indicating that DNA
was the genetic material that passed from the virus to the
bacteria. Bacteria that were infected with viruses that had
radioactive proteins, were not radioactive. This experiment
proved that DNA is the genetic material that is passed from
virus to bacteria.- (a) Oparin discovered coacervates. The biological properties
of coacervates are as follows : (i) Coacervates were non-living
structures that led to the information of the first living cells
from which the more complex cells have today evolved. (ii) It
consists of a collection of organic macromolecules surround
by a film of water molecules. (iii) Coacervates have been
synthesised in the laboratory. They can selectively absorb
chemicals from the surrounding water and incorporate
them into their structure. (iv) Coacervates lack a definite
membrane, no one claims coacervates are alive, but they do
exhibit some lifelike characters can remain in solution for
extended periods and have the ability to increase in size.
(b) Vestigial organs are the organs which are present in
reduced form and do not perform any function in the body
but correspond to the fully developed functional organs of
related animals. They are believed to be remnants of organs
which were complete and functional in their ancestors.
Examples : vermiform appendix of human nictitating
membrane, nipples in males, wings of flightless birds etc.
OR
DNA fingerprinting is a technique used for determining
nucleotide sequences of certain areas of DNA which are
unique to each individual. It identifies a person on the basis
of his/her DNA specificity. This technique was invented by
Alec Jeffreys.
The major steps in DNA fingerprinting are :
(i) Isolation of DNA from the cells if DNA is in small
amounts, then it can be amplified by polymerase chain
reaction.
(ii) Digestion of DNA by restriction endonucleases.
(iii) Separation of DNA fragments by electrophoresis.
(iv) Transferring of separated DNA fragments to synthetic
membranes, such as nitrocellulose or nylon.
(v) Hybridisation using VNTR probe.
(vi) Detection of hybridised DNA fragments by auto-
radiography. - (a) The salient features of genetic code which can be
inferred are as follows:
(i) The codon is triplet.
(ii) There are no punctuation marks (i.e., commaless) so
genetic code is read continuously.