Chromosomal disorders occur when there are changes in the number or structure of chromosomes. These changes can result in problems with growth, development and body functioning. There are two main types of chromosomal disorders: numerical aberrations which are changes in chromosome number, and structural aberrations which are changes in chromosome structure such as deletions, duplications, translocations, inversions, and ring chromosomes. Common chromosomal disorders include Down syndrome, Klinefelter syndrome, and Turner syndrome.

1. CHROMOSOMAL DISORDERS
By –Lovnish Thakur
ASU2014010100099
Integrated Biotech- 3rd sem
Subject- Genetics

2. WHAT ARE CHROMOSOMAL
DISORDERS
• A chromosomal disorder occurs when there is
a change in the number or structure of the
chromosomes.
• This change in the amount, or arrangement of,
the genetic information in the cells may result
in problems in growth, development and/or
functioning of the body systems.

3. Studying Human Chromosomes
• Clinical Cytogenetics: study of
chromosomes, their structure and inheritance
• Karyotype: chromosome constitution (46,XX
or 46,XY)
• Chromosomes are identified by their size,
position of the centromere and their banding
pattern.

5. Chromosome nomenclature
short arms are labeled “p” (petit)
long arms are labeled “q” (queue)
• Each chromosome is divided into regions, labeled
p1,p2,p3…. and q1,q2,q3…. counting outwards from
the centromere. Regions are divided into bands and
sub-bands labeled p11.1, 012.3, p13.5 ….) (read as
one-one.one, not eleven.one)
• Centromere is designated ‘cen’ and telomere ‘ter’

6. http://www.google.co.in/imgres?imgurl=http://web2.mendelu.cz/af_291_projekty2/vseo/files/26/661.png&imgrefurl=http://web2.mendelu.cz/af_291_projekty2/vseo/print.php?page%3D250%26typ%3Dhtml&h=547&w=557&tbnid=Ki2Fp08u_YkcLM:&docid=KoJ
Xl1hjasFOtM&ei=cJ8tVvu6BsGo0AT8kZ3wAQ&tbm=isch&ved=0CE0QMygVMBVqFQoTCPuy0bCa38gCFUEUlAod_EgHHg

7. Types of Chromosomal Abnormalities
There are two main types of chromosomal
disorders:
• Numerical Aberrations-: changes in
chromosome number
• Structural Aberrations-:changes in
chromosome structure

8. Structural
Aberrations
• Structural chromosome rearrangements are changes in the physical
structure of chromosomes that may result in-:
• birth defects
• mental retardation
• increased risk for infertility
• pregnancy loss.
Due to-:
chromosomal breakage or unequal crossing over which result in
• Deletions
• ring chromosomes
• Duplications
• Translocations
• insertions
• inversions.

9. Deletions
• Abnormalities in which a portion of chromatin
from a single chromosome is lost are called
deletions.

10. Duplications
• Duplications are unbalanced rearrangements
that result in partial trisomy.
• Duplications are believed to
result primarily from
unequal crossing over
especially in regions of
the genome where repeat
sequences are found.

11. Translocations
• Translocations involve breaks in two different
chromosomes with an exchange of segments.
In humans, there are two major types of
translocation:
• Reciprocal translocations-: in which there is no
visual loss of chromatin
• Robertsonian translocations-: in which the long
arms of two acrocentric chromosomes are joined
with loss of the two short arms.

13. Inversions
• Inversions are formed by two breaks in the
same chromosome with exchange of the two
ends.
Types-:
• Pericentric Inversions
• Paracentric Inversions

14. Pericentric -
chromosome in
which one break
occurs in each arm
of the chromosome
and, thus, the
centromere is
included in the
inverted segment
Paracentric-
chromosome is
formed when both
breaks occur in the
same arm and,
therefore, the
centromere is not
included in the
inverted
segment

15. Numerical Aberrations
• Numerical aberrations represent a significant
proportion of chromosomal changes found in
humans.
Constitutive (present in all cells) due to meiosis
Mosaic (present in some cells) due to mitosis
They represent a significant cause of pregnancy loss
as well as abnormalities found in live births.

16. Meiotic Non-disjunction
• Chromosome non-disjunction is defined as the
unbalanced segregation of chromosomes
leading to aneuploidy
• may occur at-
• meiosis I(when homologous chromosomes
segregate to opposite poles)
• meiosis II (when the two sister chromatids
separate at the centromere and segregate to
opposite poles)

17. http://www.biology.iupui.edu/biocourses/N10
0/2k2humancsomaldisorders.html

18. Sex Chromosome Aneuploidies
45,X (Turner syndrome)
Features of Turner syndrome present in the
 newborn include redundant neck skin and
peripheral lymphoedema
 Congenital heart disease
 There is an increased risk of
 adrenal anomalies, hypertension, autoimmune,
thyroid disease, and obesity.

19. Klinefelter syndrome
• 47,XXY
• is the most common cause of hypogonadism
and male infertility, accounting for about 10% of
infertile men.
http://www.klinefelters.org/links.htm

20. Autosomal Aneuploidies
Down syndrome (trisomy 21)
 Dysmorphic facial appearance,
 with a flattened facial profile
 flattened nasal bridge
 protruding tongue.

21. Polyploidy
Occurs either as the result of a post fertilization
cleavage error
• Triploidy
• Tetraploidy

22. Uniparental Disomy
• Cells with UPD may have a numerically
balanced
• chromosome complement but are unbalanced
in terms of parental contribution.
• For e.g. -:an egg with an extra chromosome is
fertilized by a sperm which was missing that
same chromosome

23. Reference
• http://web.udl.es/usuaris/e4650869/docencia/segoncicle/g
enclin98/recursos_classe_(pdf)/revisionsPDF/estruchromo.
pdf
• http://www2.biology.uoc.gr/courses/BIO303_genetiki_anth
ropou/PDF/Chromosomal%20aberrations.pdf
• https://www.med.unc.edu/neurology/files/documents/chil
d-teaching-pdf/CHROMOSOMES%20AND%20DISEASE.pdf
• http://web.udl.es/usuaris/e4650869/docencia/GenClin/con
tent/recursos_classe_(pdf)/revisionsPDF/numcromo.pdf
• http://www.els.net/WileyCDA/ElsArticle/refId-
a0001451.html