Chromosomal Disorders: Ploidy, Chromosomal Aberrations, Common Chromosomal Disorders, Practice Problems and FAQs

Did you know, about 1 in 150 babies born around the globe has a chromosomal disorder? You must have seen individuals with Down’s syndrome around you or on television. Did you know that Down’s syndrome is caused due to a chromosomal disorder?
You must be wondering what does chromosomal disorder mean? Are all genetic disorders caused due to chromosomal disorders? Well, chromosomal disorders are those that are caused due to the addition, deletion or structural alteration of one or more chromosomes. 
But genetic disorders such as thalassemia, colour blindness, haemophilia, etc are not chromosomal disorders. Why not?? This is because these diseases are rather caused due to mutations in individual genes whereas in chromosomal disorders the number or the entire structure of chromosomes of an individual is affected.
Can you name some other chromosomal disorders? Did you know that Klinefelter’s syndrome, Turner’s syndrome, etc are also examples of chromosomal disorders? But what actually causes these disorders? Let us help you understand as you keep reading.

Table of contents

• What is Ploidy?
• Chromosomal Aberrations
• Common Chromosomal Disorders 
• Practice Problems of Chromosomal Disorders
• FAQs of Chromosomal Disorders

What is Ploidy?

Before we get into the details of chromosomal disorders, let us understand the concept of ploidy. Ploidy is the number or sets of chromosomes present in a cell. Ploid is the term given to the cell and the condition is called ploidy. The ploidy present in human somatic body cells is diploidy whereas that in human gametes is haploidy. 
Human karyotype is the representation of the chromosomes present in each human cell. The given figure shows the human karyotype which tells us that each and every cell in the human body has 46 chromosomes which occur in 23 pairs of homologous chromosomes. Out of these 23 pairs, one set is received from the father and the other from the mother. As we have two sets of chromosomes, human cells are said to be diploid and their ploidy is said to be diploidy.

There are two types of ploidy – euploidy and aneuploidy.

Euploidy

A single set of chromosomes, present in a cell is known as the basic set. Euploidy is a condition in which multiples of the basic set are present. There are different types of euploidy, based on how many sets of chromosomes are present and it is denoted as n - monoploidy, 2n - Diploidy, 3n and more - Polyploidy.

Types of Euploidy

1. Monoploidy or haploidy - It is the condition in which the cell has a single set of chromosomes. The cell or organism is called monoploid  or haploid. The ploidy is denoted as : n 

2. Diploidy - It is the condition in which the cell has two sets of chromosomes. The cell or organism is called diploid. It is denoted as : 2n 

3. Polyploidy - It is the condition in which the cell has multiple sets of chromosomes. The cell or organism is called polyploid. It is denoted  as : 3n (triploidy), 4n (tetraploidy), 5n (pentaploidy), etc.

Aneuploidy

Aneuploidy is the condition in which there is addition or removal of one or more chromosomes and not a whole set of chromosomes. 

Types of Aneuploidy 

1. Hypoploidy - [removal of chromosome(s)]
    a. Monosomy - It is the condition in which one chromosome is removed. It is denoted by (2n-1) in diploid organisms.
    b. Nullisomy - It is the condition in which two chromosomes are removed. In diploid organisms it is denoted by (2n-2).

2. Hyperploidy - [addition of chromosome(s)]
    a. Trisomy - It is the condition in which one chromosome is added. It is denoted by (2n+1) in diploid organisms.
    b. Tetrasomy - It is the condition in which two chromosomes are added to a diploid set of chromosomes. It is denoted by (2n+2) in diploid  organisms.

Chromosomal Aberrations

Chromosomal aberrations refer to the changes in the number or structure of chromosomes in an individual that can lead to chromosomal disorders.

The changes in structure can be due to duplication, deletion, inversion or translocation of segments of a chromosome.

The alterations in the number of chromosomes can be due to improper segregation of chromosomes during anaphase of cell division, known as nondisjunction of chromosomes, or due to failure of cytokinesis post telophase of cell division. These malfunctions during gamete formation results in increase or decrease in chromosome number of the gametes. Fusion of such abnormal gametes leads to changes in the ploidy of the cell and causes chromosomal aberrations.

Types of Chromosomal Disorders

Polyploidy

This is caused due to the addition of one or more sets of chromosomes to the diploid chromosome number of an organism. This is more commonly seen in plants.

Aneuploidy

This is caused due to nondisjunction of chromosomes during anaphase of cell division while gamete formation. This results in increase or decrease in one or more chromosomes. Aneuploidy can be of the following types in human human beings:

1. Nullisomy - It is the condition in which there is removal of two chromosomes (2n-2). Total number of chromosomes is 44 (46-2). Human beings with nullisomy conditions do not survive.

2. Monosomy - It is the condition in which there is removal of one chromosome (2n-1). Total number of chromosomes is 45. The disorders associated with monosomy are Turner’s syndrome and Cri-du-chat syndrome.

3. Polysomy - It is caused due to addition of one or more chromosomes to a haploid set of chromosomes in a diploid cell. Polysomics can be trisomics (2n+1), tetrasomics (2n+2), etc. Examples of such disorders include Down’s syndrome, Klinefelter’s syndrome, etc.

Common Chromosomal Disorders

Turner’s Syndrome

It is the condition in which there is removal of one X chromosome from the sex chromosomes. It occurs only in females. The chromosome composition associated with turner’s syndrome is 44A+XO .

The characteristic feature includes short stature, characteristic facial features, low hairline, fold of skin, constriction of aorta, poor breast development, widely spaced nipples, elbow deformity, rudimentary ovaries, underdeveloped gonads, small fingernails, brown spots (nevi), and no menstruation.

Cri-du-chat Syndrome

It is caused due to the deletion of a part of the 5th chromosome. Since only a part of the chromosome is deleted and not the full chromosome, this type of aneuploidy is called partial monosomy. Infant's cry is very high pitched which sounds like a cat’s cry.

Down’s Syndrome

It is the trisomy of the 21st chromosome, i.e, an extra copy of the 21st chromosome results in Down's syndrome. The total number of chromosomes in this case is 47 (2n+1). This syndrome occurs in both females and males. 

The characteristic features of Down’s syndrome are growth failure, broad flat face, slanting eyes, short nose, flat back of head, abnormal ears, many "loops" on fingertips, short and broad hands, small and arched palate, big wrinkled tongue, dental anomalies, palm crease, special skin ridge patterns, congenital heart disease, intestinal blockage, enlarged colon, abnormal pelvis, big toes widely spaced.

Edward’s Syndrome

It is the trisomy of the 18th chromosome. This condition is seen in males and females.

The characteristic features seen in this syndrome are that the back part of the skull is prominent, mouth is small with small jaw, short neck, dysplastic or malformed ears, shield chest, or short and prominent sternum, wide-set nipples, clenched hands with overlapping fingers, flexed big toe, prominent heels.

Triple X Syndrome/Super Female

It is the condition in which, trisomy in the sex chromosome of females, i.e, an extra X chromosome, is present. It is present only in females and is also known as superfemale syndrome due to the presence of extra X chromosomes. The chromosomal composition of super females is 44A+XXX.

The characteristic features of this syndrome are delayed motor development, delayed speech, low IQ, wide set eyes, etc.

Klinefelter’s Syndrome 

It is the condition in males in which there is presence of extra X chromosome in sex chromosome. The chromosomal composition: 44A+XXY.

The characteristic features of Klinefelter’s syndrome are absence of frontal baldness, tall stature, poor beard growth, slightly feminised physique, breast development (in 30% of cases), tendency to lose chest hair, small testes, female type pubic hair pattern.

Practice Problems of Chromosomal Disorders

Question 1. Polyploidy is distinguished by

a. presence of two sets of chromosomes
b. removal of a copy of chromosome
c. presence of more than two sets of chromosomes
d. presence of a single set of chromosomes

Solution: Ploidy is the number of sets of chromosomes in a cell. Presence of more than two sets of chromosomes in a cell is called Polyploidy. Triploidy (3n) and tetraploidy (4n) are types of polyploidy.
Hence the correct option is c.

Question 2. Down’s syndrome is caused due to

a. trisomy of X chromosome
b. monosomy of X chromosome
c. trisomy of 18th chromosome
d. trisomy of 21st chromosome

Solution: Down’s syndrome is a chromosomal abnormality in humans in which the affected individuals (can be males or females) have three copies of the 21st chromosome instead of two copies in a diploid cell. This condition is called trisomy (2n+1) of the 21st chromosome.
Hence the correct option is d.

Question 3. Monosomic females who lack secondary sexual characters and possess rudimentary ovaries suffer from which chromosomal disorder ?

a. Down’s syndrome
b. Edwards syndrome
c. Turner’s syndrome
d. Klinefelter’s syndrome

Solution: Turner’s syndrome is a chromosomal disorder in human females which is characterised by the absence of one of the X chromosomes. The females with Turner’s syndrome are sterile with rudimentary ovaries and do not exhibit secondary sexual characters. Such a condition in which one chromosome is absent is called monosomy (2n-1) and affected females are called monosomic females. Females with turner’s syndrome have 44+XO chromosomes and the number of chromosomes is 45.

Hence the correct option is c.

Question 4. If a married couple has a baby with Down’s syndrome, what will be the possible reason for the condition?

a. Young age of the mother
b. Old age of the mother
c. Age does not play any role
d. Father’s age is the key factor

Solution: A woman's chance of delivering a baby with Down’s syndrome, increases with the increasing age. The risk of improper chromosome disjunction during gamete formation increases after 35 years of age. Thus, the chances of producing gametes with an extra copy of the 21st chromosome will increase in women of age more than 35. 
Hence the correct option is b.

FAQs of Chromosomal Disorders

Question 1. What is Down’s syndrome? Give its symptoms and cause.

Solution: Down syndrome is a genetic disorder that occurs due to an error in cell division that generates an extra copy of chromosome 21 in an individual. 

Symptoms are round face, broad forehead, short statured with small round head, furrowed tongue and partially opened mouth, broad palm with a characteristic palm crease, retareded physical, psychomotor, and mental development, low intelligence. 

It is caused due to trisomy of the 21st chromosome. 

Question 2. Define euploidy.

Solution: A single set of chromosomes, present in a cell is known as the basic set. Euploidy is a condition in which multiples of the basic set are present. There are different types of euploidy, based on how many sets of chromosomes are present and it is denoted as n - monoploidy, 2n - Diploidy, 3n and more - Polyploidy.

Question 3. Define aneuploidy. How does it differ from polyploidy?

Solution: Aneuploidy is the chromosomal variation that occurs due to a gain or a loss of chromosome resulting in the change from the normal chromosome count in a particular species. For example, (2n+1) condition in Down’s syndrome. 

It differs from polyploidy as in polyploidy there is an increase in the whole set of chromosomes.

Question 4. Describe the individuals having XXY chromosomal abnormalities.

Solution: XXY - Klinefelter’s syndrome is caused by XXY genotype. The individual has 47 (2n+1) chromosomes(44+XXY). It is due to trisomy of the X chromosome in males. Symptoms of Klinefelter’s syndrome include the following: Sterile males having small testes. The individuals are with some feminine characteristics such as sparse body hair, high pitched voice and enlarged breasts . They also show mental retardation.