Genetic Disorder | Zoology Optional Notes for UPSC PDF Download

 Page 1


Genetic 
Disorder
 
  
 
8.1 Chromosomal a bnormalities and 
s yndromes In certain situations e.g., due to environmental radiation, 
food intake or internal genetic conditions, chromosomes 
may suffer damage or may change in numbers. The 
change in structure is called structural chromosomal 
abnormality (or aberration) and the change in number is 
called numerical chromosomal abnormalities. When one 
chromosome of the pair is absent, the condition is called 
monosomy (2n-1) for that chromosome e.g., monosomy of 
chromosome 1. When a chromosome is present in three 
copies, this condition is called trisomy (2n+1) e.g., trisomy 
of chromosome X. It is important to notice that both 
monosomy and trisomy come under the broad category of 
aneuploidy. However, when the entire set of chromosome 
is multiplied (e.g., 69: 23 ´ 3, 92: 23 ´ 4), the condition 
is called polyploidy. The artificial breeding of plants has 
resulted in several polyploid varieties that we commonly 
use in our food. For example, bread wheat has six sets 
of chromosomes (hexaploid), cabbages or mustards are 
Chapter 8.indd   217 11/14/2019   10:14:02 AM
Page 2


Genetic 
Disorder
 
  
 
8.1 Chromosomal a bnormalities and 
s yndromes In certain situations e.g., due to environmental radiation, 
food intake or internal genetic conditions, chromosomes 
may suffer damage or may change in numbers. The 
change in structure is called structural chromosomal 
abnormality (or aberration) and the change in number is 
called numerical chromosomal abnormalities. When one 
chromosome of the pair is absent, the condition is called 
monosomy (2n-1) for that chromosome e.g., monosomy of 
chromosome 1. When a chromosome is present in three 
copies, this condition is called trisomy (2n+1) e.g., trisomy 
of chromosome X. It is important to notice that both 
monosomy and trisomy come under the broad category of 
aneuploidy. However, when the entire set of chromosome 
is multiplied (e.g., 69: 23 ´ 3, 92: 23 ´ 4), the condition 
is called polyploidy. The artificial breeding of plants has 
resulted in several polyploid varieties that we commonly 
use in our food. For example, bread wheat has six sets 
of chromosomes (hexaploid), cabbages or mustards are 
Chapter 8.indd   217 11/14/2019   10:14:02 AM
218
tetraploids. Likewise, banana and apple are triploid (3 sets 
of chromosomes), strawberry and sugar cane are octoploid 
(8 sets of chromosomes). Both structural or numerical 
changes can result in significant changes in phenotypic 
condition in the form of diseases or syndromes.  
8.1.1 Structural chromosomal abnormalities 
Structural chromosomal abnormalities may be of following 
types:
1. Deletion— In deletion, a segment of a chromosome 
breaks away leading to shortening of the chromosome 
(Fig. 8.1a). For example, retinoblastoma is caused due 
to deletion of a portion of chromosome 13. Sometimes 
when two ends of a chromosome are deleted, they can 
reattach to form a ring chromosome.  
2. Duplication— Duplication refers to when a segment 
of the chromosome gets repeated resulting in a longer 
chromosome (Fig. 8.1b). This can lead to conditions 
e.g., Charcot-Marie-Tooth disease caused due to 
duplication of genes on chromosome 17.
3. Inversion— In inversion, a segment of the chromosome 
breaks away, completely reverses itself and reattaches 
with the chromosome. Here the overall length of the 
chromosome remains same but the orientation of genes 
is reversed by 180 degrees (Fig. 8.1c). For example, 
RCAD syndrome caused by inversion of a segment of 
chromosome 17.
4. Translocation—In translocation, a segment of a 
chromosome breaks away and reattaches itself with 
another chromosome. If there is a mutual exchange 
of segments between two chromosomes, it is called 
Fig. 8.1: (a) Deletion (b) Duplication (c) Inversion and (d) Translocation
A
C
E
F
B
A
C
B
E
F
D D
D
C
E
F
C
A
E
F
D D
A
B
A
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D
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E
D
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(a) (b) (c) (d)
Chapter 8.indd   218 11/14/2019   10:14:02 AM
Page 3


Genetic 
Disorder
 
  
 
8.1 Chromosomal a bnormalities and 
s yndromes In certain situations e.g., due to environmental radiation, 
food intake or internal genetic conditions, chromosomes 
may suffer damage or may change in numbers. The 
change in structure is called structural chromosomal 
abnormality (or aberration) and the change in number is 
called numerical chromosomal abnormalities. When one 
chromosome of the pair is absent, the condition is called 
monosomy (2n-1) for that chromosome e.g., monosomy of 
chromosome 1. When a chromosome is present in three 
copies, this condition is called trisomy (2n+1) e.g., trisomy 
of chromosome X. It is important to notice that both 
monosomy and trisomy come under the broad category of 
aneuploidy. However, when the entire set of chromosome 
is multiplied (e.g., 69: 23 ´ 3, 92: 23 ´ 4), the condition 
is called polyploidy. The artificial breeding of plants has 
resulted in several polyploid varieties that we commonly 
use in our food. For example, bread wheat has six sets 
of chromosomes (hexaploid), cabbages or mustards are 
Chapter 8.indd   217 11/14/2019   10:14:02 AM
218
tetraploids. Likewise, banana and apple are triploid (3 sets 
of chromosomes), strawberry and sugar cane are octoploid 
(8 sets of chromosomes). Both structural or numerical 
changes can result in significant changes in phenotypic 
condition in the form of diseases or syndromes.  
8.1.1 Structural chromosomal abnormalities 
Structural chromosomal abnormalities may be of following 
types:
1. Deletion— In deletion, a segment of a chromosome 
breaks away leading to shortening of the chromosome 
(Fig. 8.1a). For example, retinoblastoma is caused due 
to deletion of a portion of chromosome 13. Sometimes 
when two ends of a chromosome are deleted, they can 
reattach to form a ring chromosome.  
2. Duplication— Duplication refers to when a segment 
of the chromosome gets repeated resulting in a longer 
chromosome (Fig. 8.1b). This can lead to conditions 
e.g., Charcot-Marie-Tooth disease caused due to 
duplication of genes on chromosome 17.
3. Inversion— In inversion, a segment of the chromosome 
breaks away, completely reverses itself and reattaches 
with the chromosome. Here the overall length of the 
chromosome remains same but the orientation of genes 
is reversed by 180 degrees (Fig. 8.1c). For example, 
RCAD syndrome caused by inversion of a segment of 
chromosome 17.
4. Translocation—In translocation, a segment of a 
chromosome breaks away and reattaches itself with 
another chromosome. If there is a mutual exchange 
of segments between two chromosomes, it is called 
Fig. 8.1: (a) Deletion (b) Duplication (c) Inversion and (d) Translocation
A
C
E
F
B
A
C
B
E
F
D D
D
C
E
F
C
A
E
F
D D
A
B
A
C
E
F
B
D
A
F
E
D
C
B
A
C
E
F
B
D
L
M
N
O
P
Q
A
C
E
F
P
Q
B
D
L
M
N
O
(a) (b) (c) (d)
Chapter 8.indd   218 11/14/2019   10:14:02 AM
reciprocal translocation. Example: Burkitt’s lymphoma, 
where exchange of materials happens between 
chromosomes 8 and 14. If a segment of a chromosome 
breaks away and attaches with another chromosome, 
without mutual exchange, it is called Robertsonian 
translocation. This may result in decrease of chromosome 
number of the cell (Fig. 8.1d). 
8.1.2 Numerical chromosomal abnormalities
Some commonly observed syndromes/diseases due to 
numerical chromosomal abberations are described in 
following section. The term syndrome is generally referred 
to a group of symptoms which consistently occur together, 
or a condition characterised by a set of associated 
symptoms. A disease refers to abnormal physiological 
response to internal or external factors e.g., fever caused 
due to microbes. 
1. Down's Syndrome
Incidence: Occurs in approx.1 per 800 live births.
Chromosomal basis: Down syndrome is a genetic condition 
that arises due to presence of an extra chromosome 21. 
Here, chromosome 21 is repeated thrice (trisomy 21), 
instead of showing up twice in a normal individual. The 
karyotype of Down syndrome is represented as 47, XX, 
+21 (females) and 47, XY, +21 (males) (Fig. 8.2a).
The trisomic condition is usually caused by an error in the 
process of cell division called non disjunction, i.e., inability 
of chromosomes to separate at the time of cell division.
Fig. 8.2: Karyogram of (a) an individual affected with down syndrome (b) an individual affected 
with Klinefelter's
(a) (b)
Chapter 8.indd   219 11/14/2019   10:14:03 AM
Page 4


Genetic 
Disorder
 
  
 
8.1 Chromosomal a bnormalities and 
s yndromes In certain situations e.g., due to environmental radiation, 
food intake or internal genetic conditions, chromosomes 
may suffer damage or may change in numbers. The 
change in structure is called structural chromosomal 
abnormality (or aberration) and the change in number is 
called numerical chromosomal abnormalities. When one 
chromosome of the pair is absent, the condition is called 
monosomy (2n-1) for that chromosome e.g., monosomy of 
chromosome 1. When a chromosome is present in three 
copies, this condition is called trisomy (2n+1) e.g., trisomy 
of chromosome X. It is important to notice that both 
monosomy and trisomy come under the broad category of 
aneuploidy. However, when the entire set of chromosome 
is multiplied (e.g., 69: 23 ´ 3, 92: 23 ´ 4), the condition 
is called polyploidy. The artificial breeding of plants has 
resulted in several polyploid varieties that we commonly 
use in our food. For example, bread wheat has six sets 
of chromosomes (hexaploid), cabbages or mustards are 
Chapter 8.indd   217 11/14/2019   10:14:02 AM
218
tetraploids. Likewise, banana and apple are triploid (3 sets 
of chromosomes), strawberry and sugar cane are octoploid 
(8 sets of chromosomes). Both structural or numerical 
changes can result in significant changes in phenotypic 
condition in the form of diseases or syndromes.  
8.1.1 Structural chromosomal abnormalities 
Structural chromosomal abnormalities may be of following 
types:
1. Deletion— In deletion, a segment of a chromosome 
breaks away leading to shortening of the chromosome 
(Fig. 8.1a). For example, retinoblastoma is caused due 
to deletion of a portion of chromosome 13. Sometimes 
when two ends of a chromosome are deleted, they can 
reattach to form a ring chromosome.  
2. Duplication— Duplication refers to when a segment 
of the chromosome gets repeated resulting in a longer 
chromosome (Fig. 8.1b). This can lead to conditions 
e.g., Charcot-Marie-Tooth disease caused due to 
duplication of genes on chromosome 17.
3. Inversion— In inversion, a segment of the chromosome 
breaks away, completely reverses itself and reattaches 
with the chromosome. Here the overall length of the 
chromosome remains same but the orientation of genes 
is reversed by 180 degrees (Fig. 8.1c). For example, 
RCAD syndrome caused by inversion of a segment of 
chromosome 17.
4. Translocation—In translocation, a segment of a 
chromosome breaks away and reattaches itself with 
another chromosome. If there is a mutual exchange 
of segments between two chromosomes, it is called 
Fig. 8.1: (a) Deletion (b) Duplication (c) Inversion and (d) Translocation
A
C
E
F
B
A
C
B
E
F
D D
D
C
E
F
C
A
E
F
D D
A
B
A
C
E
F
B
D
A
F
E
D
C
B
A
C
E
F
B
D
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N
O
P
Q
A
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E
F
P
Q
B
D
L
M
N
O
(a) (b) (c) (d)
Chapter 8.indd   218 11/14/2019   10:14:02 AM
reciprocal translocation. Example: Burkitt’s lymphoma, 
where exchange of materials happens between 
chromosomes 8 and 14. If a segment of a chromosome 
breaks away and attaches with another chromosome, 
without mutual exchange, it is called Robertsonian 
translocation. This may result in decrease of chromosome 
number of the cell (Fig. 8.1d). 
8.1.2 Numerical chromosomal abnormalities
Some commonly observed syndromes/diseases due to 
numerical chromosomal abberations are described in 
following section. The term syndrome is generally referred 
to a group of symptoms which consistently occur together, 
or a condition characterised by a set of associated 
symptoms. A disease refers to abnormal physiological 
response to internal or external factors e.g., fever caused 
due to microbes. 
1. Down's Syndrome
Incidence: Occurs in approx.1 per 800 live births.
Chromosomal basis: Down syndrome is a genetic condition 
that arises due to presence of an extra chromosome 21. 
Here, chromosome 21 is repeated thrice (trisomy 21), 
instead of showing up twice in a normal individual. The 
karyotype of Down syndrome is represented as 47, XX, 
+21 (females) and 47, XY, +21 (males) (Fig. 8.2a).
The trisomic condition is usually caused by an error in the 
process of cell division called non disjunction, i.e., inability 
of chromosomes to separate at the time of cell division.
Fig. 8.2: Karyogram of (a) an individual affected with down syndrome (b) an individual affected 
with Klinefelter's
(a) (b)
Chapter 8.indd   219 11/14/2019   10:14:03 AM
The possibility of having a Down's syndrome baby in 
the family increases with the maternal age. It has been 
reported that more than 85% Down syndrome babies 
are born in mothers over 35 years of age, at the time of 
pregnancy. 
Clinical symptoms: Some of the distinguishing features of 
Down's syndrome are: flat face, slanting eye, small mouth, 
protruding tongue, flattened nose, short neck, short arms 
and legs, single deep crease across the palm, low IQ, 
stunted growth, muscular hypotonia, under developed 
gonads. Down's syndrome babies also show breathing, 
heart or hearing problems. 
Diagnosis and Treatment: Down 
syndromes are usually diagnosed 
by an extra chromosome 21 in the 
karyotype. There is no single standard 
treatment protocol for Down syndrome. 
Treatments are tailored on specific 
set of conditions presented by these 
individuals. At early age, children with 
Down's syndrome can benefit from 
speech therapy, physiotherapy and 
taking nutritional supplements. 
In early 1900s, on an average, 
Down's syndromes used to live until 
age 9. Now with the advances in 
diagnostic and treatment technologies, 
the age expectancy has increased up to 
60 and even longer.
2. Klinefelter's syndrome 
Incidence: Occurs in approximately 1 
out of 1000 new born males.
Chromosomal basis: Genotype: 
47, XXY. Affects males. The extra 
chromosome is not transmitted 
genetically (i.e., a Klinefelter newborn 
cannot have a Klinefelter father) but 
arises from inability of X chromosome 
to detach itself from the pair during 
meiosis (at the time of gamete 
formation). Fertilisation of an XX ova 
with a Y sperm produces an XXY 
zygote. 
Fig. 8.3: Individual with 
Klinefelter's Syndrome 
Chapter 8.indd   220 11/14/2019   10:14:03 AM
Page 5


Genetic 
Disorder
 
  
 
8.1 Chromosomal a bnormalities and 
s yndromes In certain situations e.g., due to environmental radiation, 
food intake or internal genetic conditions, chromosomes 
may suffer damage or may change in numbers. The 
change in structure is called structural chromosomal 
abnormality (or aberration) and the change in number is 
called numerical chromosomal abnormalities. When one 
chromosome of the pair is absent, the condition is called 
monosomy (2n-1) for that chromosome e.g., monosomy of 
chromosome 1. When a chromosome is present in three 
copies, this condition is called trisomy (2n+1) e.g., trisomy 
of chromosome X. It is important to notice that both 
monosomy and trisomy come under the broad category of 
aneuploidy. However, when the entire set of chromosome 
is multiplied (e.g., 69: 23 ´ 3, 92: 23 ´ 4), the condition 
is called polyploidy. The artificial breeding of plants has 
resulted in several polyploid varieties that we commonly 
use in our food. For example, bread wheat has six sets 
of chromosomes (hexaploid), cabbages or mustards are 
Chapter 8.indd   217 11/14/2019   10:14:02 AM
218
tetraploids. Likewise, banana and apple are triploid (3 sets 
of chromosomes), strawberry and sugar cane are octoploid 
(8 sets of chromosomes). Both structural or numerical 
changes can result in significant changes in phenotypic 
condition in the form of diseases or syndromes.  
8.1.1 Structural chromosomal abnormalities 
Structural chromosomal abnormalities may be of following 
types:
1. Deletion— In deletion, a segment of a chromosome 
breaks away leading to shortening of the chromosome 
(Fig. 8.1a). For example, retinoblastoma is caused due 
to deletion of a portion of chromosome 13. Sometimes 
when two ends of a chromosome are deleted, they can 
reattach to form a ring chromosome.  
2. Duplication— Duplication refers to when a segment 
of the chromosome gets repeated resulting in a longer 
chromosome (Fig. 8.1b). This can lead to conditions 
e.g., Charcot-Marie-Tooth disease caused due to 
duplication of genes on chromosome 17.
3. Inversion— In inversion, a segment of the chromosome 
breaks away, completely reverses itself and reattaches 
with the chromosome. Here the overall length of the 
chromosome remains same but the orientation of genes 
is reversed by 180 degrees (Fig. 8.1c). For example, 
RCAD syndrome caused by inversion of a segment of 
chromosome 17.
4. Translocation—In translocation, a segment of a 
chromosome breaks away and reattaches itself with 
another chromosome. If there is a mutual exchange 
of segments between two chromosomes, it is called 
Fig. 8.1: (a) Deletion (b) Duplication (c) Inversion and (d) Translocation
A
C
E
F
B
A
C
B
E
F
D D
D
C
E
F
C
A
E
F
D D
A
B
A
C
E
F
B
D
A
F
E
D
C
B
A
C
E
F
B
D
L
M
N
O
P
Q
A
C
E
F
P
Q
B
D
L
M
N
O
(a) (b) (c) (d)
Chapter 8.indd   218 11/14/2019   10:14:02 AM
reciprocal translocation. Example: Burkitt’s lymphoma, 
where exchange of materials happens between 
chromosomes 8 and 14. If a segment of a chromosome 
breaks away and attaches with another chromosome, 
without mutual exchange, it is called Robertsonian 
translocation. This may result in decrease of chromosome 
number of the cell (Fig. 8.1d). 
8.1.2 Numerical chromosomal abnormalities
Some commonly observed syndromes/diseases due to 
numerical chromosomal abberations are described in 
following section. The term syndrome is generally referred 
to a group of symptoms which consistently occur together, 
or a condition characterised by a set of associated 
symptoms. A disease refers to abnormal physiological 
response to internal or external factors e.g., fever caused 
due to microbes. 
1. Down's Syndrome
Incidence: Occurs in approx.1 per 800 live births.
Chromosomal basis: Down syndrome is a genetic condition 
that arises due to presence of an extra chromosome 21. 
Here, chromosome 21 is repeated thrice (trisomy 21), 
instead of showing up twice in a normal individual. The 
karyotype of Down syndrome is represented as 47, XX, 
+21 (females) and 47, XY, +21 (males) (Fig. 8.2a).
The trisomic condition is usually caused by an error in the 
process of cell division called non disjunction, i.e., inability 
of chromosomes to separate at the time of cell division.
Fig. 8.2: Karyogram of (a) an individual affected with down syndrome (b) an individual affected 
with Klinefelter's
(a) (b)
Chapter 8.indd   219 11/14/2019   10:14:03 AM
The possibility of having a Down's syndrome baby in 
the family increases with the maternal age. It has been 
reported that more than 85% Down syndrome babies 
are born in mothers over 35 years of age, at the time of 
pregnancy. 
Clinical symptoms: Some of the distinguishing features of 
Down's syndrome are: flat face, slanting eye, small mouth, 
protruding tongue, flattened nose, short neck, short arms 
and legs, single deep crease across the palm, low IQ, 
stunted growth, muscular hypotonia, under developed 
gonads. Down's syndrome babies also show breathing, 
heart or hearing problems. 
Diagnosis and Treatment: Down 
syndromes are usually diagnosed 
by an extra chromosome 21 in the 
karyotype. There is no single standard 
treatment protocol for Down syndrome. 
Treatments are tailored on specific 
set of conditions presented by these 
individuals. At early age, children with 
Down's syndrome can benefit from 
speech therapy, physiotherapy and 
taking nutritional supplements. 
In early 1900s, on an average, 
Down's syndromes used to live until 
age 9. Now with the advances in 
diagnostic and treatment technologies, 
the age expectancy has increased up to 
60 and even longer.
2. Klinefelter's syndrome 
Incidence: Occurs in approximately 1 
out of 1000 new born males.
Chromosomal basis: Genotype: 
47, XXY. Affects males. The extra 
chromosome is not transmitted 
genetically (i.e., a Klinefelter newborn 
cannot have a Klinefelter father) but 
arises from inability of X chromosome 
to detach itself from the pair during 
meiosis (at the time of gamete 
formation). Fertilisation of an XX ova 
with a Y sperm produces an XXY 
zygote. 
Fig. 8.3: Individual with 
Klinefelter's Syndrome 
Chapter 8.indd   220 11/14/2019   10:14:03 AM
Clinical Symptoms: Klinefelter's syndrome children are 
unusually tall for their age, have reduced facial and body 
hair, smaller testes, enlarged breasts and coarse voice 
(Fig. 8.2b and 8.3). 
Diagnosis and Treatment: One of the most frequent 
methods to diagnose Klinefelter syndrome is through Barr 
body test of buccal smear. Normally no Barr body appears 
in the male buccal smear. However, in Klinefelter one 
Barr body shows up, indicating the presence of an extra X 
chromosome. 
At the time of birth, babies with Klinefelter differ a little 
with other normal babies. However, as the age increase 
the differences  become noticeable, especially at the time 
of puberty. 
People with Klinefelter's syndromes are often treated 
with testosterone to look masculine. They also need to be 
psychologically counselled to control depression leading to 
aggression.
3. Turner's Syndrome
Incidence: Occurs in 1 in 2,500 newborn girls, frequently 
observed in miscarriages and still births. 
Chromosomal basis: Affects females, arises due to the 
missing X chromosome in affected females. This is called 
monosomy X and the karyotype is represented as: 45, X. 
A cell division error during meiosis of an ovum results in 
an ovum with no X chromosome and other 
with two X chromosomes. The ovum with no 
X chromosome fuses with sperm with one 
X chromosome to generate 45, X condition. 
Mothers with Turner syndrome cannot 
pass the condition to their daughters i.e., 
this condition is not inherited. 
Clinical symptoms: Turner syndromes 
are diagnosed by following features — short 
stature, webbed neck (i.e., the neck skin is 
unusually loose and can be pulled several 
centimetres of the neck), small breasts, low 
set ears (i.e., ears are placed below the normal position), 
swollen hands and feet. Furthermore, ovaries are under 
developed and menstrual periods are usually absent  
(Fig. 8.4). 
Fig. 8.4: Turner's Syndrome
Chapter 8.indd   221 11/14/2019   10:14:03 AM