Basic medical genetics for undergraduate MBBS students. I have tried to make it easier for the undergrad students. Hope this helps. Happy learning.

2. Basic Human Genetics
Maruf Ibne Monowar
MBBS, MS
Assistant Professor
Department of Anatomy
Enam Medical College & Hospital

3. Terminologies

4. Genetics
It is a branch of biology concerned with
the study of genes, genetic variations,
and heredity in organisms.

8. What is the difference between
a DNA & a chromosome?

9. Chromosome=DNA

10. Chromosome=DNA
Chromosomes are deeply stained thread-like
structures within the nucleus of each cell.
Individual chromosomes are visualized under
microscope ONLY during cell division
And in the rest of the times, they are seen as
chromatin threads

11. Genes are parts of specific DNA molecules
The position of a gene in the chromosome or
DNA is its locus (pl. loci)

12. Homologous chromosomes are
chromosome pairs inherited
from each parent. They are
similar in gene position.

13. The genes occupying the identical
loci in a pair of homologous
chromosomes are known as alleles
Different forms of gene are known
as alleles
Paired parental genes are known as
alleles

14. When the two parental genes
(alleles) are identical (same) for a
particular character,
they are called homozygous
When the two parental genes
(alleles) are of contrasting features
(different) for a particular character,
they are called heterozygous

15. A genome is an organism's complete set of DNA,
including all of its genes.
The genome contains all of the information
needed to build and maintain an organism.
The entire human genome contain more than 3
billion DNA base pairs & is contained within a
nucleated cell.

16. Genotype is the genetic code/constitution of an
individual organism.
Phenotype is the set of observable characteristics of an
individual resulting from the interaction of its genotype
with the environment.
The phenotype of an organism is its physical
(observable) characteristics or traits.
i.e. Phenotype is the expression of the genotype

17. A trait is a specific characteristic of an organism.
Traits can be determined by the interactions
between the genes & the environment.
e.g. hair colour, eye colour, height etc.
The genetic contribution to a trait is called the
genotype.

18. T
t
T t

20. Karyotyping
It is a process of arranging each pair of
homologous chromosomes in a sequence,
the longest chromosome being placed at the
beginning and the shortest at the end.
It is usually done manually from the
photograph of the metaphase spread

21. Chromosome
Chromosomes are deeply stained thread like structures
within the nucleus, which acts as the carriers of units of
inheritance.

22. Types of Chromosome
According to the position of the centromere,

23. According to function, chromosomes can be divided into 2 types:
Autosomes (body chromosomes) and
Allosome (Sex chromosome).
Certain genetic traits are linked to a person's sex and are passed
on through the sex chromosomes. The autosomes contain the
rest of the genetic hereditary information.
All act in the same way during cell division. Human cells have 23
pairs of chromosomes (22 pairs of autosomes and one pair of sex
chromosomes), giving a total of 46 per cell.
Types of Chromosome (contd.)

24. Structure of Chromosomes
Each chromosome is composed of a single
continuous molecule of double helix of a DNA along
with proteins (histone and non-histone) (Primary
coiling)
The DNA-protein complex exists as a highly coiled
(folded) structure

25. 8 histone proteins combine to
form a spherical octomer known
as nucleosome core.
Each nucleosome is wrapped
nearly twice by the helical DNA.
This forms the appearance of
‘beads-on-a-string’
Further aggregation of 6
nucleosomes per turn forms
solenoids and increases the
thickness of the chromatin fiber.
This condensed chromatin is
further folded several times into
giant supercoiled loops, which
finally form a chromosome
Nucleosome
beads-on-a-string/
Secondary coiling
Solenoids/Tertiary
coiling
supercoiled
loops/Quaternary
coiling
DNA/ Primary
coiling

26. Orders of DNA coiling
Primary coiling:
of double helix of DNA
Secondary coiling:
around the nucleosomes
Tertiary coiling:
in the form of solenoids
Quaternary supercoiling:
in the form of chromosomes
Nucleosome
beads-on-a-string/
Secondary coiling
Solenoids/Tertiary
coiling
supercoiled
loops/Quaternary
coiling
DNA/ Primary
coiling

27. Bio-chemical Structure of DNA
Deoxyribonucleic acid (DNA) is a molecule
composed of two polynucleotide chains that
coil around each other to form a double
helix. The two DNA strands are known
as polynucleotides as they are composed of
monomeric units called nucleotides. Each
nucleotide is composed of a nucleoside
(nitrogenous base, and deoxyribose sugar)
And a phosphate group.
N.B.:
Nucleotides=Base+sugar+Phosphate
Nucleoside= Base+sugar
Strand 1 Strand 2
Nucleotide
Nucleoside

28. Each strand consists of a back bone
of alternate deoxy-ribose sugar and
phosphate molecules, and the 2
strands are held together by
hydrogen bonds between the
nitrogenous bases attached to the
sugars as side groups and point
towards the centre of the helix.
Bases are of 2 types, Purine (A & G)
& pyrimidine (T & C)
Strand 1 Strand 2
Nucleotide
Covalent
bond

29. Functions of DNA molecule
DNA molecule possess the following potentialities:
DNA replication
Biosynthesis of RNA & proteins
Recombination
Mutation

30. Chromosomal Abnormalities
Structural Numerical Chimaeras
Deletion Aneuploidy: Dispermic
Insertion Monosomy Blood
Inversion Trisomy
Isochromosome Tetrasomy
Ring chromosome Polyploidy:
Translocation Mono/Tri/Tetraploidy
Duplication *Di-ploidy is normal

32. For
better
understanding

33. Numerical Abnormalities
Aneuploidy is the presence of an abnormal number of chromosomes in a
cell, for example a cell having 45 or 47 chromosomes instead of the usual
46. An extra or missing chromosome is a common feature of aneuploidy,
i.e.: 46 ± 1
e.g.: Down’s syndrome (trisomy 21, there is an extra chromosome 21)
Polyploidy is the state of a cell or organism having more than two paired
complete sets of chromosomes. For example, when a human cell has an
extra set of 23 chromosomes. i.e.: 46+23, so, the chromosome number
may be 3n, 4n, 5n instead of 2n

34. Chimaeras
An unusual condition, when an
individual presents two or more
genetically distinct cell lines derived
from more than one zygote.
Dispermic chimaera: results from
double fertilization, where, two
different X-bearing and Y-bearing
sperms fertilise two ova, and the
resulting two zygotes fuse to form
a single embryo.

35. Blood chimaera: it happens when two non-
identical twins in utero exchange their cells
through placental barrier.
Chimaeras (contd.)

36. RNA

37. Biosynthesis of RNA
Only one of the 2 strands of DNA acts as a template for RNA
The synthesized RNA conveys the genetic message and interprets
into the synthesis of protein by linear linkage of amino acids.
Central dogma of molecular genetics:
DNA RNA Protein
(Transcription) (Translation)

38. DNA vs. RNA
Traits DNA RNA
Number of
strand
Double Single
Pentose sugar Deoxy-ribose D-ribose
Nitrogenous
Bases
Adenine, Guanine,
Cytosine, Thymine
Adenine, Guanine,
Cytosine, Uracyl

39. Protein Synthesis
It is a “better to know” topic.
Please have a look at:
1. “Transcription and Translation Overview” by Armando Hasudungan
https://www.youtube.com/watch?v=6YqPLgNjR4Q
2. “Transcription and Translation: From DNA to Protein” by Professor Dave
Explains
https://www.youtube.com/watch?v=bKIpDtJdK8Q