Download presentation on factors which influence genes, prevention and

PRESENTATION ON FACTORS WHICH
INFLUENCE GENES, PREVENTION AND
SOCIAL MEASURES
INTRODUCTION
Human genetics had its foundation laid down around 300
B.C when Aristotle observed that characters like pecularities of
hair, nails or even gait pass from parents to the offspring’s. A
scientific approach towards geneties came in the latter half of 19th
century when Greyor Mendel discovered principles of heredity.
DEFINITIONS
GENETICS: Genetics is the science that deals with transmission
of characters from parents to offspring.
GENE: A gene is the structural unit which is responsible for
transmission of a particular character from parents to offspring.
Genes are the units of heredity. They contain hereditary
information encoded in their chemical structure for transmission
from generation to generation. They affect development and
function, both normal and abnormal. Though genes are not seen
with a microscope much is known about them by indirect method.
it is said that we inherit about 50,000 genes from the father and
50,000 genes from the father and 50,000 from the mother. The
genes occupy a specific position or locus in the chromosomes.
Since genes are contained in the chromosomy genes are occur in
pairs. If the genes comprising a pair are alike (AA) the individual is
described as homozygous for that gene and if it different (Aa). The
individual is described as heteroszygous.
A gene is said to be dominant when itmanifests its effects both in
the heterozyyour and homozygous state. A gene is said to be
recessive when it manifests its effect only in the homozygous
state.
Genes are usually stable, but sometimes normal genes
may be converted into abnormal ones. This change is called
mutation. Mutation is a regular phenomenon in nature. The natural
mutation rate is increased b y exposure to mutagens such as
ultraviolet rays radiation or chemical carcinogens.
POPULATION GENETICS
Population has been defined as the study of the precise
genetic composition of population and various factors determining
the incidence of inherited traits in them.
Population
is
founded
on
a
principle
enunciated
independently by hardy in England and Weinberg in Germony in
1908. When human population consisting of tall (TT), intermediate
(Tt) and short (tt) individuals were allowed to mate at random,
even after several generations of interbreeding, it will be found
that there will be some individuals who are tall (TT), some
intermediate (Tt) an some short (tt). We cannot produce a race
which is pure or uniform in height.
The Hardy – Weinberg law state that “The relative frequencies of
each gene allele tend to remain constant from “generation to
generation” in absence of forces that change gene frequencies.
Thus the study of gene frequencies and the influences which
operate to alter the gene pool and their long term consequences
are central theme in population genetics.
FACTORS WHICH INFLUENCE GENES
The Hardly –Weinberg law assumes that human population
is static. But is reality, human population and consequently human
gene pool is never static. There are several factors which
influence the human gene pool.
1.MUTATION
Mutation implies a change in the genetic material of an
organism which results in a new inherited variation.
It is a regular phenomenon in nature. It is now believed that
mutant genes are so widespread in their occurrence that everyone
of us night be harbouring a few or many of them. According to
modern geneticists. The entire body structure of man and every
other animal and plant alls have been built through hundreds of
millions of year by means of a long succession of mutation. The
cause of spontaneous mutation is not yet known.
CAUSES
Ionizing radiation
Chemical agents
Most mutant genes are harmful. But there are instances
where mutant genes could be beneficial, eg. Sickle cell anaemia.
The heterozygotes of sickle cell trait were found to be resistant to
falciparum malaria some mutant genes remain neutral. They do
not harm or impair the survival ability of the carriers. Each gene
has its own characteristics mutation rate which is estimated
anywhere from 104 to 106 per generation.
2. NATURAL SELECTION
Natural selection is the process whereby harmful genes are
eliminated from the gene pool and genes favourable to an
individual tend to be preserved and passed on to the offspring.
Darvin proposed the theory of natural select ion or survival of the
fittest to explain evolution. When DDT was first used it was lethal
to houseflies, today not many houseflies are killed by DDT. This is
an example of natural selection in response of DDT. It doesnot
apply in humanpopulation because man, by his superior
intelligence
has
interfered
with
natural
selection
in
conceivable way by changing the environmental conditions.
very
3. POPULATION MOVEMENTS
Because of industrialization, increased facilities for earning,
ways of living and education people are moving – sometimes on a
large scale – from rural to urban areas. There is also a migration
of people between countries. Such population movements will
lead to changes in the distribution of genes, affecting both the
areas of immigration and emigration. The inter-mixing of people
makes new genetic combination possible.
4. BREEDING STRUCTURE
If all marriages were to occur in a random fashion. The
effect would
be the attainment of a genetic equilibrium. In
practice, however mating tends to occur selectively within various
subgroups based on religion, economic and educational status
and family relationship, for instance, doctors tend to marry doctors
or nurse, musicians tend to marry musicians. This type of mating
is called assortative mating. The genetic consequences of
assortative mating have not been adequately studied.
5. PUBLIC HEALTH MEASURES
Advances in public health and medical care services do
affect the genetic endowment of people as a whole. Ramstedt’s
operation which was introduced in 1912 has saved many children
suffering from congenital pyloric stenosis. Individuals with
genetically conditioned retinoblastoma may e saved by timely
surgery. The carries of hereditary diseases, malformation and
constitutional weaknesses are able to survive and pass their
genes to their progeny.
PREVENTIVE AND SOCIAL MEASURES
1. HEALTH PROMOTIONAL MEASURES
a. Eugenics: Galton proposed the term eugenics for the science
which aims to improve the genetic endowment of human
population. Eugenics has both negative and positive aspects.
b. Negative eugenics : Hitler sought to improve the German race
by killing the weak and defective. This was negative eugenics. It is
not an approved measure to improve human race. The aim of
negative eugenics is to reduce.
The frequency of hereditary disease and disability in the community
to as low as possible. Inspite of sterilization of people who are
suffering serious hereditary diseases or debarred from producing
children, new cases of hereditary diseases will continue to arise in
the population partly because of fresh mutations and partly because
of marital alliances between hidden carriers (heterozygotes) o
recessive defects.
2. Positive Eugenics : It seeks to improve the genetic
composition of the population by encouraging carriers to
desirable genotypes to assume the burden of parenthood. At
present positive eugenics have very little application.
Its realization is difficult for two reasons.
Majority of socially valuable traits, let us say intelligence and
positive
character
features,
though
partially
determined
biologically are not inherited in such a simple way as blood
group.
We cannot determine which gene we transmit to our children.
B. EUTHENICS : Mere improvement of the genotype is of no use
unless the improved genotype is given access to a suitable
environment, an environment which will enable the genes to
express themselves readily studies with mentally retarded (mild)
children indicated that exposure to environmental stimulation
improved their IQ. Thus the solution of improving the human race
does not lie in contrasting heredity and environment but rather in
the mutual interaction of heredity and environmental factors. This
environmental
manipulation
is
called
considerable prospectus for success.
euthenics
and
has
C. GENETIC COUNSELLING : The most immediate and practical
service that genetics can render in medicine and surgery is
genetic counseling. Genetic counseling may be prospective or
retrospective.
1. PROSPECTIVE GENETIC COUNSELLING: This allows for the
true prevention of disease. This requires identifying. The
heterozygous individuals for any particular defect by screening
procedures and explaining to them the risk of their having affected
children if they marry another heterozygote for the same gene.
For eg. Sickle cell anaemia and thalassemia. This kind of
prevention may find wider application to cover a number of other
recessive defects.
11. RETROSPECTIVE GENETIC COUNSELLING : It refers to the
counseling given after the occurrence of a hereditary disorder in
the family. A survey carried out by the WHO showed that
genetic counseling was chiefly sought in connection with
congenital abnormalities, mental retardation, psychiatric illness
and inborn errors of metabolism. Only very few sought premarital advice.
Methods suggested under retrospective genetic counseling are:
a. Conctraception
b. Pregnancy termination
c. Sterilization
depending
upon
the
environment of the couples involved.
attitudes
and
cultural
D. OTHER GENETIC PREVENTIVE MEASURES
1. Consanguineous marriage : When blood relatives marry each
other. There is an increased risk in the offspring of traits controlled
by recessive genes. Therefore a lowering of consanguineous
marriage would be advantages to the health of the community.
Late marriage : Mongolism is more common in children born of
elderly mothers. Hence early marriage of females in better than
late marriage from the point of view of preventing mongolism. Its
incidence in a mother at age 20 is 1:3000 by the age 40 it is 1:40.
2. Specific protection
Protection of individuals and whole communities against
mutagens such as :
X-ray examination of the pregnant uterus is strongly depicted.
Parents undergoing x-ray examination should be protected
against unnecessary exposure of the gonads to radiation.
Haemolytic disease of the newborn is to be prevented by
immunization by Anti-D globulin.
3. Early diagnosis and treatment
a. Detection of genetic carriers : It is now possible to detect the
healthy carriers of a number of genetic disorders especially the
inborn errors of metabolism. The female carriers of Dunchenne
type of muscular dystrophy, an X-linked disorder, can now be
detected by elevated levels of serum creation kinase in 80% of
carriers.
b. Prenatal diagnosis : Ammiocentesis in early pregnancy (1416wks) has now made it possible for prenatal diagnosis of
conditions associated with chromosomal anomalies. E.g Down’s
syndrome. The diagnosis of chromosomal anomalies is made by
culture and karyotyping of foetal cells from the amniotic fluid, and
of metabolic defects by biochemical analysis of the fluid.
Ammocentesis : is called for in the following circumstances if the
parents are prepared to consider abortion.
1. A mother aged ≥35 (because of high risk of Down’s syndrome
with advanced maternal age).
2. Parents who have had a child with down’s syndrome or other
chromosomal anomalies.
3. Parents who are known to have chromosomal trans-location
1. Parents who have had a child with a metabolic defect
detectable by amniocentesis. The most common one defect of
the neural tube, on encephaly and spinal bifida which can be
detected by an elevation of alpha – fetoprotein in the amniolic
fluid.
2. When determination of the sex is warranted, given a family
history of a sex linked genetic disease eg. Certain muscular
dystrophies.
For neural tube defect the maternal blood can be tested for the
level for alpha –faeto protein. If the test is positive, it can be
confirmed by amniocentesis.
C. Screening of newborn infants : There are many screening
tests available for the early diagnosis of genetic abnormalities, sex
chromosome abnormality, congenital dislocation of hip. PKU
congenital hypothyroidism cystic fibrosis.
d. Recognizing pre clinical cases : There are quite a number of
screening tests for the early diagnosis of hereditary diseases.
Examples
-Heterozygotes for phenylketonurier
phenylalamine tolerance test.
can
be
detected
by
-A simple urine examination of glucose for diabetes
-A raised serum uric acid in case of govt.
-Sickle cell anaemirtriat can be uncovered by subjecting the red
cells to reduced O2 tension.
Thalassemia minor can be detected by studying the blood
picture.
-Genetic counseling an have the greatest impact when individuals
or couples at genetic risk are identified prospectively.
-Once diagnosed, some of the genetic conditions can be treated
with complete or partial success by medical and surgical
measures, for example, diets low in phenylalanine are now
prescribed as treatment for PKU children.
-Persons suffering from haemophilia can be greatly helped by
administering antihaemolytic globulin which promotes clotting of
blood.
-Modern surgical techniques have brought great improvement in
dealing with cases of spinapifida.
REHABILITATION
With many genetic or partially genetic conditions causing
physical or mental disability, much can be done for the patients
and for his family in helping him to lead a better and more useful
life.
GENETIC SERVICE IN THE THIRD WORLD
Medical genetic in the world began to develop in 1970’s
primarily in Latin America and India. It grew in big cities and
centred around medical colleges.
The third world children born with defects ergenetic disease
are likely to be causalities of infectious diseases and malnutrition
CONCLUSION
Hereditary influences on health and disease are assuming
increasing importance to persons in the health profession. Onethird of all pediatric hospital admission and substantial number of
deaths in paediatric ICU are due in part to genetic cause, although
the underlying genetic disorder may not be recognized. Alterations
of a whole past of a chromosome or even a single gene can
manifest as a genetic disorder.
BIBLIOGRAPHY
1. Trula mayers gornic, Emily Slone Mckinney, Sharon Smith
Murray, “Hereditary and environmental influences on child
bearing. Page 83-87.
2. Lowdermilk and perry Bobak, “Maternity Women’s health care”,
Mosby St. Louis London 7th edition 325-331.
Journal
Clinical obstetrics and gynaecology, December 7, 1996 Vol 39,
No :4 “Screening for genetic disorders.