1. Chromosome Structure and Nomenclature

2. Cytogenetic: The study of chromosomes
and their abnormalities and cell division.
Chromosomes: is threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.

3. Picture 1.
Chromosomes as they appear under a Microscope.

4. Picture 2. The chromosomes arranged according to size;
chromosome 1 is the largest.
The last two chromosomes are the sex chromosomes .

5. Structure: chromosomes are not uniform in width through
out their length ,with each Chromosomes having
a construction, centromere it is responsible for
the movement of the chromosome at the
cell division.
The centromere divide the chromosome in to
short and long arms, and by international
Convention, the short arm is termed the “p arm”
while the long arm of the chromosome is termed
the “q arm”.

6. The tip of each end is called telomere.

7. Chromosomes are classified according
to the position of centromere:
Metacentric: centromere in the middle
of chromosome.
Submetacentric: centromere divides
the chromosome into 1\3 and 2\3.
Acrocentric: centromere near the end

9. Chromosomal Abnormalities:
Chromosome abnormalities are responsible of
genetic diseases, occurring in approximately
1 of every 150 live births. They are the leading
known cause of both mental retardation and
pregnancy loss. Chromosome abnormalities
are seen in 50% of first-trimester and
20% of second-trimester spontaneous abortions.

10. There are 2 types of chromosomal abnormalities
Numerical
Aneuploidy (Trisomy& Monosomy).
Polyploidy (Mosaic aneuploidy).
2. Structural
Deletion.
Duplication.
Translocation .
Inversion.

11. Numerical Anomalies:”chr abnormality”
Aneuploidy: involve the gain or loss
of one or more chromosome.
Trisomy: presence (i.e. gain ) of an extra
chromosome. Ex. Trisomy 21, this trisomy
produces Down syndrome that its caused
by the presence of an extra copy of
chromosome 21. we write “ 47,+ 21” .

13. Monosomy: absence (i.e. loss )
of a single chromosome. Ex. sterile female
with Turner syndrome. We write “ 45 , x”
Note:
Chromosome number in
Normal male is “ 46 , XY “
Normal female is “ 46 , XX”

14. Polyploidy: Chromosome abnormality in
which the number of chromosomes in a cell
is a multiple of 23 but is greater than the
diploid number.
Examples
triploidy [69 chromosomes in the human]
caused by fertilization of an egg by
two sperm. 1n + 2n=3n
tetraploidy[92 chromosomes in the human]
caused by failure maturation of division
in ovum\sperm. 2n+2n=4n

15. Picture show Triploidy

17. Structural Abnormalities:
structural abnormalities result from chromosomal
breakage with subsequent reunion in a different
configuration, and they may be balanced or
un balanced.
In balanced rearrangement the chromosome
complement is complete which no gain or loss
of genetic material, i.e. they are harmless.
However, the carriers of balanced rearrangements
are often at risk of producing children with
unbalanced chromosomal complement. .

18. In unbalanced rearrangements the
chromosomal complement contains
an incorrect amount of chromosomal material.
Translocation:
involves two non homologous chromosomes
(e.g., chromosome 2 and chromosome 6).
Following a break in each of the chromosomes,
and subsequent reunion, a segment of
chromosome 2 becomes attached to
chromosome 6 and vice versa.

20. Reciprocal translocation:
A break occurs in each of
two chromosomes with segment being
exchanged. Ex. balanced translocation
involve the long arms of chromosome
11 and 22. The resulting chromosomes
are called derivative chromosomes.
The carrier of a reciprocal translocation
is usually unaffected.

22. Robertsonian translocation:
The short arms of two non homologous
chromosomes are lost and the long arms
fuse at the centromere to form a single
chromosome.

23. Deletion:
Caused by a chromosome break and
subsequent loss of genetic material.
A single break leading to a loss that includes
the chromosome's tip is called a terminal
deletion. When two breaks occur and
the material between the breaks is lost,
an interstitial deletion.

25. Inversion:
involve only one chromosome in which two breaks occur and in the process of repair, the intervening segment is rejoined in an inverted or opposite manner.
An inversion is paracentric if the inverted segment is on the long arm or the short arm and does not include the centromere.

26. The inversion is pericentric if breaks occur
on both the short arm and the long arm
and the inverted segment contains
the centromere.

27. Duplications:
refers to an extra chromosomal
segment within the same homologous
chromosome or an extra chromosomal
segment on another non-homologous
chromosome.