Genetics is the study of heredity and variation in living organisms. Some key events in genetics include Mendel discovering the principles of heredity in 1865 and Watson and Crick determining the structure of DNA in 1953. In humans, each cell normally contains 23 pairs of chromosomes, including 22 pairs of autosomes and one pair of sex chromosomes. Important parts of chromosomes include the centromere and telomeres. Karyotyping involves analyzing an individual's chromosomes to detect abnormalities. Lyon's hypothesis explains X chromosome inactivation in females.

2. Genetics:-
Defn :-
“Genetics is the branch of bioscience which deals with
the study of the underlying principles of heredity.”
Importance-
• Parent to offspring
• Inherited diseases / medical genetics
• Fertility genetics---- programmed baby
• First test tube baby---- Durga

3. Historical events:
 Mendel (1865) : discovered principles of heredity.
 Karl Landsteiner (1930): blood groups & immunology
 William Bateson (1903):coined the term Genetics
 Herman Joseph Muller (1946): induction of mutation
by X- rays
 Arthur korenberg, Severo Ochoa(1959): study on
chemistry of DNA & RNA

4.  James watson, Francis Crick: Maurice wilkins(1961):
structure of DNA
 Marshell Nirenberg, Har Gobind Khorana, Robert
Holley (1968): “cracking the genetic code” & revelling
the means by which a gene determines the
sequence of amino acid in a protein.
And many more researcher who added the
drops of their knowledge into ocean of genetics.

5. Gregor Mendel(1822-1884)

6. Mendel & Mendelism
 1853 conducted
experiments on garden
peas (pisum sativum)
 7 pairs of contrasting
characters
 Crossed these varieties
considering one pair of
contrasting character
 F1 generation: allowed to
self pollinate
 F2 generation : allowed to
self pollinate.

7. On analysis of progeny of generations in garden
pea, Mendel proposed his concepts.
Mendel’s Laws:
1. Law of unit inheritance:-
it states that the characters do not blend ; if they do
not expressed in the first generation they can
reappear without change in the subsequent
generations.

8. 2. Law of Segregation:
It states that the members of gene pair segregate
and pass to different gametes.
3. Law of independent assortment:
It states that the members of different gene pairs
assort independently of one another during
gametogenesis.

9. Terms in Genetics
 Allele: One of alternative forms of a gene at a particular
locus.
 Aneuploid: A set of chromosomes which does not
contain an exact multiple of haploid sets of
chromosomes.
 Centromere: Constricted region where sister chromatids
are attached in mitotic chromosomes. The centromere is
generally flanked by repetitive DNA sequences and it is
late to replicate. The centromere is an A-T region of
about 130 bp. It binds several proteins with high affinity to
form the kinetochore which is the anchor for the mitotic
spindle.

10.  Codon: A group of three consecutive nucleotides in
mRNA which specifies an amino acid to be
incorporated in the polypeptide product of the gene.
 Deletion: A mutation resulting in the loss of normal
DNA sequence. A deletion may be of any size from 1
nucleotide pair to the loss of most of a chromosome.
 Dominant: An allele is dominant if its effect can be
observed in the phenotype of a heterozygote.
 Genome: The complete DNA sequence of an
organism (though it may sometimes be used in the
sense of mitochondrial genome or nuclear genome).

11.  Haploid: In the human case, having exactly one
copy of each of the autosomes and one sex
chromosome.
 Chimaera (chimera): An animal (or plant) made up
from a mixture of cells from more than one species.
Also (in human genetics), an individual made up
from cells derived from two or more zygotes.
 Mosaic: An individual composed of more than one
genetically distinguishable cell population derived
from a single zygote.

12.  Plasmid: A short circular DNA sequence which is
replicated within a host bacterium (or yeast) and
which usually confers a selective advantage (such
as antibiotic resistance) to the host. Plasmids have
been engineered to serve as vectors for the
propagation of DNA which is foreign to the host
species.
 Polymerase chain reaction (PCR): A technique by
which a relatively small piece of DNA of known
sequence can be amplified (often from a complex
mixture) by successive cycles of strand separation
followed by DNA synthesis.

13. Human chromosome
It is a combination of two words, i.e., “Chroma”-
means „colour‟ and “Somes”-means „body‟.
So the coloured thread like bodies present in the
nucleoplasm of the living cells, which helps in the
inheritance (transmission) of characters in form of Genes
from generation to generation are known as
CHROMOSOMES.
 1956- Tjio & Levan : demonstrated 46 chromosomes in
human
 22 pairs – autosomes
 1 pair - sex chromosome (XX / XY)

14. Morphology
 Uncoiled in interphase
 chromatin granules : some remain coiled
 Euchromatin / heterochromatin
 Rod like – end of interphase
 Centromere
 p- arm , q- arm
 Two sister chromatid-
in late prophase
 Telomers
 Satellite bodies

15. Classification of Chromosomes
According to position of centromere

16. According to “Denever system” of classification
Group Chromosomes Description
A 1–3 Largest; 1 and 3 are metacentric but 2 is submetacentric
B 4,5 Large; submetacentric with two arms very different in
size
C 6–12,X Medium size; submetacentric
D 13–15 Medium size; acrocentric with satellites
E 16–18 Small; 16 is metacentric but 17 and 18 are
submetacentric
F 19,20 Small; metacentric
G 21,22,Y Small; acrocentric, with satellites on 21 and 22 but not
on the Y
Autosomes are numbered from largest to smallest, except that chromosome 21 is
smaller than chromosome 22.

18.  Paris nomencleature (1971,75)
 More accurate way of identification of chromosome
based on various banding pattern.

19. Karyotyping / Karyotype
Definition:-
Photomicrograph of an individual‟s
chromosomes arranged in a standard manner.
Indications:
1) Turner Syndrome (45,X and variants)
2) Klinefelter Syndrome (47,XXY and variants)
3) Down Syndrome (Trisomy 21, translocation and
mosaicism)
4) Couples with infertility of unknown cause

20. 5) Couples with recurrent spontaneous abortions
6) Children with ambiguous genitalia
7) Female children with inguinal hernia
8) Children with mental subnormality and dysmorphic
features
9) Suspected cases of Fragile X syndrome, Fanconi
anaemia, Ataxia, Telangiectasia, Bloom syndrome
etc.
10) Bone marrow analysis in leukemias (blood cancer).
11) Prenatal Diagnosis of fetal chromosome disorders in
high-risk pregnancies.

21. Karyotyping Procedure
Blood sample
5ml of venous blood
(centrifuge)
Nutrient medium
(phytohaemagglutinin)
Stimulates T- lymphocyte to divide

22. Culture at 370C for 3 days
Colchicine added to arrest cell cycle in metaphase
Add hypotonic saline- causes swelling of cells
Cell lyses
Mounted on slide & stained
(cells are dropped on a cold, clean slide, to cause
rupture and stained with giemsa)

23. Photomicrograph
Arrange chromosome in
decreasing order of their length

24. Methods of study of chromosomes
A) Chromosome Banding
• Q banding
• G banding
• R banding
B) Flow cytometry
C) Molecular cytogenetics: FISH

25. Barr body / Sex Chromatin
Introduction
1949- Barr & Bertram.
Found chromatin mass in female cat neurons.
They found it only in females sex.
Best site to examine - Buccal mucosa.

26. Procedure of examining Barr body
Scrapping from inner side of cheek.
Smeared evenly on slide
Fixed in alcohol
Stained with thionin
Observed under microscope.

27. No. of Barr bodies = no. of X chromosomes – 1.
eg. Individual with 47,XXX complement
No. of barr bodies are 3-1= 2.
In Turners syndrome (45,XO) it is 0.
 Barr body represents one of two X chromosomes of
female cell.
 It remains condensed & inactive in interphase.

28. Lyon’s hypothesis
It states that:
1) In female somatic cells, only one X chromosome is
active. The second is inactive, condensed and
appears in the form of sex chromatin in interphase.
2) Inactivation occurs early in embryonic life.

29. 3) Inactivation is random but fixed.
inactive X can be maternal or paternal in
different cells of same individual.
but once decision is made then it is followed by
it‟s descendent cells.
• It is detected in blastocyst at 9-12 days.
• First in syncytotrophoblast, then chorionic villi
then in yolk sac
• In embryo proper- after 18 days.

30. Mechanism of inactivation of X chromosome
Methylation of DNA. Mainly cytosine.
5- methyl cytosine at certain sites in DNA.
Inactivation centre:-
Long arm of X chromosome.
In some translocation involving X chromosome &
autosomes. The intact X chromosome forms Barr body.

31. Genetic significance of X inactivation
1) Dosage compensation
2) Variability of expression
3) Mosaicism