The document provides information about chromosomal aberrations including definitions of key terms like chromosomes, chromatids, centromere, karyotype, autosomes, and sex chromosomes. It then describes different types of chromosomal abnormalities including structural abnormalities like deletions, duplications, inversions, translocations, and numerical abnormalities involving aneuploidy and polyploidy. Specific chromosomal disorders are summarized, such as Down syndrome, Klinefelter syndrome, Turner syndrome, and others resulting from aneuploidy. The roles of non-disjunction and translocations in conditions like Down syndrome are explained. Overall features and implications of various chromosomal aberrations are concisely outlined.
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1. S.Y.B.Sc Semester III
Botany Paper II
Unit II :Cytogenetics
Chromosomal aberrations
By Mrs. Mandakini R Ingle
Department of Botany
Satish Pradhan Dnyanasadhana College, Thane (w)
2. Basic definitions
• Chromosomes, DNA and genes
• Chromatides, and centromere
• Arms of a chromosome (p and q)
• Karyotype
• Autosomes and sex chromosomes
• Genotype and phenotype
3. Human Chromosomes
• The chromosome carries the genetic
information.
• composed of deoxyribonucieic acid
(DNA) on framework of protein .
• Segments of DNA molecules
comprise the genes; the units of
heredity.
4. • During cell division,
the chromosome can
be seen to consist of
2 parallel strands; the
chromatids, held
together at one point,
the centromere.
Chromosomes
6. Karyotype
• It is the set of chromosomes of an individual.
• It is the systematized arrangement of the
chromosomes of a single cell.
• In the human cell, there are 46 chromosomes or 23
pairs (diploid number); of these 23 pairs, 22 are
similar in both sexes and are called the autosomes.
The remaining pair is called sex chromosomes : XX in
the female cells and XY in the male cells .
• Chromosomes are arranged in groups A to G
according to their shape & size.
10. Chromosomal Abnormalities
• Chromosomal abnormalities are either numerical or
structural.
• They are a very common cause of early spontaneous
miscarriage.
• Usually, but not always, cause multiple congenital
anomalies and learning difficulties.
11. Chromosomal Aberrations (abnormalities)
• Structural Aberrations
o Deletion
o Duplication
o Inversion
o Translocation
• Numerical Aberrations (abnormalities)
o Polyploidy: Multiple of the haploid (> Diploid)
o Aneuploidy: Abnormal number
12. Structural abnormalities
1) Deletion : loss of a portion of a chromosome
2) Duplication : extra piece of a chromosome. .
3) Inversion : fragmentation of a chromosome
followed by reconstitution with a section inverted.
4) Translocation :
the transfer of a chromosome or a segment of it to a
non-homologous chromosome
13. Chromosomal Deletions
• a deletion results in a lost portion of a
chromosome,
• Deletion Causative Agents: Heat, radiation,
viruses, chemicals, errors in recombination. result
in partial monosomy,
• the organism is monosomic for the portion of the
chromosome that is deleted,
• as in monosomy, most segmental deletions are
deleterious.
19. Duplication
Causes:
– duplications often result from unequal crossing
over,
– can occur via errors in replication during S-Phase.
Effects:
– results in gene redundancy,
– produces phenotypic variation,
– may provide an important source for genetic
variability during evolution.
23. Duplication in Evolution
essential genes do not tolerate mutation,
duplications of essential genes, then subsequent
mutations, confers adaptive potential to the organism,
new gene family members are ‘recruited’ to perform new
functions.
25. Paracentric Inversion
...an inversion in which the centomere is
not included,
A B C
...a paracentric inversion does not
change arm length ratio.
B
A C
B A
26.
27. Pericentric Inversion
...an inversion in which the centromere is
included,
A B C
A B
C
...a pericentric inversion results in a
change in chromosome arm length.
33. Paracentric Outcomes
1 Normal Gamete, 1 Inversion Gamete, No Crossover Classes
Recombination is not inhibited, but recombinant gametes are selected against.
37. Acentric
…a chromosome having no centromeres,
…segregates to daughter cells randomly,
or is lost during cell division,
…deletions impart partial monosomy.
39. Recombination and Inversions
• Paracentric and Pericentric;
– 1 Normal Gamete,
– 1 Inverted Gamete,
– No Crossover Classes = No Recombination,
Inversions select against recombinant
gametes, thus preserves co-segregation of
specific alleles.
40. Inversions and Evolution
• Inversions ‘lock’ specific alleles
together,
– all offspring get the alleles from either a
wild-type, or inverted chromosome,
• If the ‘set of alleles’ is advantageous,
the set can be maintained in the
population.
44. Down Syndrome
• 95% of Down Syndrome individuals are a result
of Trisomy 21,
– the probability of having a second Down Syndrome
child is usually similar to the population at large,
• However, there is second cause of Down
Syndrome caused by a Robertsonian
translocations that is heritable.
46. Reciprocal translocations
• An exchange of material between two different
chromosomes is called a reciprocal translocation.
When this exchange involves no loss or gain of
chromosomal material, the translocation is 'balanced'
and has no phenotypic effect.
• Balanced reciprocal translocations are relatively
common, occurring in 1 in 500 of the general
population.
47. Reciprocal translocations
•Finding a balanced translocation in one parent
indicates a recurrence risk for future pregnancies and
antenatal diagnosis by chorionic villus sampling or
amniocentesis should be offered as well as testing of
relatives.
51. Figure 8.4 Translocation Down's syndrome. There is a translocation between
chromosomes 21 and 14 inherited from a parent
52. Numerical Aberration
• Autosomal
- Trisomies: 1 ch extra (e.g. trisomy 21-13-18)
- Monosomies: 1 ch is missing
• Sex chromosome
- Klinefelter syndrome (47, XXY male)
- Turner syndrome (45, XO female)
53. Overview
I. Chromosomal Variations
A. Polyploidy
B. Aneuploidy
II. Syndromes resulting from Aneuploidy
A. Trisomy 13
B. Trisomy 18
C. Down Syndrome
D. Turner Syndrome
E. Klinefelter Syndrome
F. XYY Syndrome
54. Numerical abnormalities
Trisomy i.e. 47 chromosomes
- Trisomy 21 (the extrachromosome is No 21)
- Klinefelter syndrome ( 47, XXY male)
Monosomy i.e. 45 chromosomes
- Monosomy 21
- Turner syndrome (the missing chromosome is
X in female : 45, X or 45 XO )
55. Variations in Chromosomal
Number
• Euploidy – the usual number and sets of
chromosomes
• Polyploidy – the presence of three or more
complete sets of chromosomes
• Aneuploidy – the presence of additional
chromosomes or missing individual
chromosomes
56. Types of Polyploidy
• Triploidy – three sets of chromosomes
23 x 3 = 69
• Tetraploidy – four sets of chromosomes
23 x 4 = 92
57. Types of Aneuploidy
• Monosomy – one less chromosome
(23 x 2) – 1 = 45
• Trisomy – one additional chromosome
(23 x 2) + 1 = 47
58. Aneuploidy
• When aneuploidy occurs in humans,
syndromes can result. Examples include
the following:
1. Trisomy 13
2. Trisomy 18
3. Down Syndrome
3. Turner Syndrome
4. Klinefelter Syndrome
5. XYY Syndrome
59. Trisomy of Autosomes
• Trisomy 13 or D-trisomy (Patau syndrome)
• Trisomy 18 or E-trisomy (Edward syndrome)
• Trisomy 21 or G-trisomy (Down syndrome)
60. Trisomy 13 (Patau Syndrome)
• 1st described by Bartholin (1657) & redefined by
Patau (1960).
• Chromosomal complement: 47,XX,+13 (female) or
47,XY,+13 (male)
• Phenotype: Male or female
• Incidence: 1:12,000 (increases with the age of
mother)
64. Trisomy 18 (Edward Syndrome)
• Chromosomal complement: 47,XX,+18
(female) or
47,XY,+18
(male)
• Phenotype: Male or female
• Incidence: 1:8000
65.
66. Features of Edward Syndrome
• Mental deficiency
• Growth retardation
• Prominent occiput with
elongated head
• Webbing of the neck
• Short sternum
• Micrognathia
• Low-set malformed ears
• Ventricular septal
defects
• Renal anomalies
• Clenched fists with
overlapping of fingers
• Hypoplastic nails
67. Trisomy 18, Edward Syndrome
Overlapping of the fingers
inEdwards' syndrome Short broad hand
69. Pathogenic mechanisms
Disruption
Involves destruction of a fetal part which
initially formed normally; e.g. amniotic
membrane rupture may lead to amniotic bands
which may cause limb reduction defects.
Dysplasia
Refers to abnormal cellular organization or
function of specific tissue types, e.g. skeletal
dysplasias and dysplastic kidney disease.
71. Down Syndrome (Mongolism) Trisomy 21
Incidence :
• The most common chromosomal aberration
• Incidence 1/700 live birth & 10 % of M.R.
Definition : It is trisomy 21 i.e. the cell contain an extra
chromosome, number 21 i.e. the cell contains three
21 chromosomes instead of two .
72.
73. Genetic types (Cytogenetics)
(1) Non-disjunction : “ 95 % of cases”
• It is due to failure of disjunction of the 2
chromosomes of the pair No 21 during division, the
extra 21 chromosome is separate and so total no. in
cell is 47.
• Incidence is higher with increasing maternal age & so
it is age-dependent
76. Genetic types (Cytogenetics)
Translocation : “ 4 % of cases”
• The extra 21 chromosome is translocated (attached)
to another chromosome e.g. (15/21) so total no. of
chromosomes is 46 but the genetic material is that of
47 chromosomes .
• Incidence is usually in young mothers & risk of
recurrence is high & mother is called translocation
carrier
77. Down syndrome
Translocation carrier
• One parent contains a 14/21 translocation
and has only 45 chromosomes, and is a
phenotypically normal carrier.
• 1/4 of the individual's gametes will have
almost 2 copies of chromosome 21.
• The resulting zygote has 46 chromosomes, but
almost 3 copies of chromosome 21, and
exhibits Down syndrome.
79. Genetic types (Cytogenetics)
Mosaicism : “ 1 % of cases”
• Some cells are normal (46 chromosomes) & others
are trisomic (47 chromosomes)
Clinical feature are less evident & M.R. is mild .
Characteristic physical features
(in the absence of mental retardation, they should
NEVER be considered diagnostic)
80. Hands
• Simian crease (transverse palmar crease)
• Clinodactly (incurved little finger)
• Short broad hand
Feet
• Big space between the first and second toes
Clinical Features
83. Features of Down Syndrome
• Short height
• Severe mental deficiency
with decline in the IQ with
age
• Brachycephaly with flat face
and occiput
• Flat and low nasal bridge
• Upward slant to palpebral
fissures
• Malformed large ears
• Epicanthal folds of the eyes
• Brushfield spots in iris
• Renal anomalies
• Prominent and protruding
tongue (scrotal tongue)
• Simian crease
• Clinodactyly of 5th digit
85. Klinefelter Syndrome
• Trisomy of sex
chromosome - XXY
(An additional X
chromosome in males)
• Occurrence – 1 in 500-
1000 males
86. Features of Klinefelter Syndrome
• Tall , Comparatively low weight relative to stature
• Infertility , Sexually underdeveloped
• Sparse facial and body hair
• Developmental delays
• Increased risk of autoimmune disorders, breast cancer,
osteoporosis, leg ulcers, depression, and dental
problems
• Larger craniofacial dimensions
• Severe acne in adolescence
• Behavior problems Learning disabilities
• Slightly lower IQ than normal
91. Turner syndrome
(Gonadal Dysgenesis)
At Birth .. Edema of dorsum of hand & feet
.. Webbing of neck.
Childhood period
- Short stature
- Head .. Low posterior hairline
- Neck .. Webbing of the neck
- Chest .. Broad chest & wide spaced nipples
- Limbs .. Cubitus valgus
92. Turner syndrome
Adolescence
- Failure of development of 2ry sex characters
- 1ry amenorrhea (streaked ovary)
- Normal mentality, Some learning disability
Associated anomalies & complications:
1- Cardiac anomalies: Bicuspid aortic valve –
AS - coarctation of aorta
2- Renal anomalies: Horseshoe kidney
3- Ear: Recurrent OM – SNH loss
