2. Introduction
• Genetics is defined as “the branch of medical sciences that deals
with the study of transmission of characters from parents to
offspring”.
• With gradual successful tackling of infectious diseases by the
mankind, the importance of genetic diseases is becoming more
evident.
• It is now believed that a large number of human diseases have
either a predominantly or atleast a contributory genetic
component.
3. History
1859: Theory of natural selection- members of
a population who are better adapted to the
environment survive and pass on their traits.
Gregor Mendel (1822 – 1884)
Charles Darwin (1809 - 1882)
1866: Gregor Mendel published his
work “Experiments in Plant
Hybridization”, which set out the
basic theory of genetics.
4. Basis of Inheritance
• In nucleus of human cells, the chromatin is coiled in the form of
rod like structures called chromosomes
• Human body cells contain 46 chromosomes in 23 pairs – one of
each pair is inherited from each parent
• Chromosomes are made of DNA
• Each chromosome contains genes in a linear order
• Genes are codes for cells to make proteins
• Alterations in genes or chromosomes alter the protein produced
and can hence cause disease
5. Genes
• Each complete diploid set of chromosomes contains the
hereditary instructions of the cell, called “genome”
• Every gene lies at a particular position on the chromosome,
called the “locus”
• Like chromosomes, genes are also arranged in pairs
• Each and every function in a human being is specifically coded
by a particular “pair” of genes
• In every such pair of genes, the two alternative forms of genes
(one from father & one from mother) are called “alleles”
6. • An individual in whom the two alternative forms of genes in an
allele are identical to each other, is called “homozygous”
• If there is difference in the two genes of an allele, the person is
called “heterozygous”
7. • The genes are responsible for determining the various traits of an
individual, such as skin colour, height, intelligence, etc...
• Characters which are transmitted exclusively by a particular gene
are “monogenic” characters, while those transmitted by multiple
genes located at different loci are called “polygenic” characters
• The overall genetic constitution of a person is known as the
“genotype” while the outward expression of this genetic
constitution is called the “phenotype”
• Genotype is permanent for an individual & will not change during
the lifetime, except for rare instances like mutations
• Phenotype is determined by both the genotype as well as
interaction of the individual with the environment
• Ex – Diabetes mellitus
8. Classification of Genetic Disorders
Genetic
disorders
Inheritance
or Gene
disorders
Autosomalin
heritance
disorders
Sex-
linkedinherit
ance
disorders
Mutation
of genes
Chromosomal
disorders
Structurala
bnormaliti
es
Abnormality
innumberof
chromosomes
Multifactor
ialdisorder
s
9. Chromosomal disorders
Structural abnormalities of chromosomes:
• These may occur due to deletion, duplication, translocation or
inversion of a particular chromosome
• Ex- Cridu Chat Syndrome – due to deletion of a portion of the 5th
chromosome
• Mental & growth retardation, microcephaly, wide spaced eyes
and cardiac defects
Abnormalities in the number of chromosomes:
Involving autosomes
Involving sex chromosomes
10. Involving autosomes:
• Ex- Down’s syndrome – occurs since 1 chromosome is added to
the diploid number of chromosomes of 21st pair
• ( 2n + 1 ) = 47
• Also called “Trisomy”
• Advanced maternal age (>35yrs) at time of conception is a strong
risk factor
• Mental retardation, brachycephaly, mongoloid faces, high arched
palate & cardiac defects
11. Involving sex chromosomes:
• Ex- Turner’s syndrome – instead of a pair of sex chromosomes in
a female being “XX” it remains as only “X” – “XO”
• Thus only 45 chromosomes instead of 46
• Affected female - Dysgenesis of ovaries, primary amenorrhoea,
sexual infantilism,etc...
• Ex- Klinefelter’s syndrome – due to one additional sex
chromosome “X”
• Instead of normal “XY” in a male, it will be “XXY”
• Thus 47 chromosomes instead of 46
• Affected male – underdeveloped sexual characters and
gynaecomastia
