Chromosomal Abnormalities: Causes, Symptoms & Treatment

Chromosomal abnormalities can occur as a mistake when the egg or the sperm is formed or during the early stages of fetal development of the fetus. The age of the mother, along with environmental factors, can also play a role in the occurrence of genetic disorders. Prenatal screening and other testing can also be done to examine the foetal chromosomes and be able to detect some abnormalities.

This Story also Contains

1. What are Chromosomal Abnormalities?
2. Structure and Function of Chromosomes
3. Types of Chromosomes
4. Types of Chromosomal Abnormalities
5. Causes of Chromosomal Abnormalities
6. Recommended Video on Chromosomal Abnormalities
7. MCQs on Chromosomal Abnormalities

The most common type of chromosomal abnormality is known as aneuploidy, which is an abnormal chromosome number due to an extra or missing chromosome. Some common examples of aneuploidy include trisomy (three copies of a chromosome), monosomy(single copy of a chromosome). Down syndrome is one of the well-known examples of chromosomal aneuploidy. Other than trisomy, other chromosomal aneuploidies seen are trisomy 18; trisomy 13, 45, X (Turner syndrome); 47, XXY (Klinefelter syndrome); and 47, XYY. Chromosomal Abnormalities is a topic of the chapter Principles of Inheritance and Variation in Biology.

What are Chromosomal Abnormalities?

The chromosomal abnormalities are those ones which there is a change in the normal number or pattern of the chromosome structure. The genetic disorder varies in its cause and development. It might have been developed due to an error in the cell division, which significantly changed the structure of chromosomes. Those abnormalities can make a big difference in the life and other attributes of a person: growth, development, and overall health.

Most of the chromosomal abnormalities are associated with health problems of physical malformations, intellectual disabilities, and susceptibility to some diseases. A common chromosomal abnormality is Down syndrome, which results from the presence of an extra copy of chromosome 21 and leads to developmental delays. Knowing these abnormalities is very important so that early diagnosis is made, and management is done appropriately to improve the quality of life of these individuals.

Structure and Function of Chromosomes

Chromosomes are DNA and histone entities that are found in the long threads residing in the cell nucleus of a plant. These threads are associated with heredity, and it is important to ensure that genetically inherited material is transferred from one generation to the next without deterioration. They are important for the division, growth, and well-being of a cell and carry all the information needed for the construction and maintenance of an organism.

Chromosomes are, by and large, pretty complex structures, as they consist of a couple of important parts, listed below:

• Chromatid: A chromosome consists of two identical sister chromatids that stick to each other at the region known as the centromere.

• Centromere: This is the mid-point of the chromosome where the two sister chromatids are attached. It plays a very important role in cell division, as the centromere is responsible for the proper division of chromosomes into daughter cells.

• Telomere: The End of the chromosome that acts as a cap to the chromosome. Telomeres give protection to the ends of chromosomes against degradation and against sticking to each other.

Types of Chromosomes

Based on functions and genetic contents, chromosomes are divided into two types, they are:

• Autosomes: These are the chromosomes that are not directly involved in determining the sex of an individual. In a human being, there are 22 pairs of autosomes.

• Sex Chromosomes: They are known to determine the sex of an individual. In humans, the two chromosomes are expressed as XX for females and XY for males.

Types of Chromosomal Abnormalities

Chromosomal abnormalities are the abnormalities in the chromosomes of the organism, both autosomes and allosomes. The chromosomal abnormalities are categorised as:

Numerical Abnormalities

It is an abnormality caused due to a change in the number of chromosomes. The two most important numerical abnormalities are:

• Aneuploidy: It is a condition where the number of chromosomes in a cell is not a multiple of the haploid set.

• Trisomy: An extra chromosome is present. E.g. Down's Syndrome (Trisomy 21).

• Monosomy: A chromosome is missing. E.g. Turner's Syndrome (Monosomy X).

Structural Abnormalities

Structural abnormalities are caused by an actual change in a chromosome's structure. These types fall into the categories of the following:

• Deletions: A piece of the chromosome is deleted. E.g. Cri du Chat Syndrome.

• Duplications: A piece of the chromosome is duplicated, hence there is an excess of genetic material.

• Inversions: a segment of a chromosome attaches to the same chromosome where it originated, but in the reverse order.

• Translocations: a segment is taken out from one chromosome and inserted into a different chromosome. Example: Chronic Myelogenous Leukaemia.

Causes of Chromosomal Abnormalities

Dysregulation in cell division or environmental factors can cause chromosomal abnormalities. The same is described below:

Errors in Cell Division

• Nondisjunction: Chromosomes do not perfectly separate from each other during cell division, creating an irregular number of chromosomes.

• Errors in Meiosis and Mitosis: During the stages of meiosis, errors can be of consequence in gametes and cause a chromosomal abnormality; in the case of mitosis, it is with somatic cells.

Environmental Factors

• Radiation: High doses of radiation can damage the chromosomes and cause an abnormality.

• Chemicals: Some have also been believed to damage the human chromosome; these include chemicals found in tobacco smoke and other pollutants.

• Viruses: Infections from some viruses can interfere with chromosome structure and function.

Recommended Video on Chromosomal Abnormalities

MCQs on Chromosomal Abnormalities

Q1. A human female with Turner's syndrome:

Option 1: Has 45 chromosomes with XO.

Option 2: Has one additional X chromosome.

Option 3: Exhibits male characters.

Option 4: Can produce children with a normal husband.

Correct answer: 1)Has 45 chromosomes with XO.

Explanation:

One of the two X chromosomes is absent or partial in Turner's syndrome, making the total number of chromosomes 45 rather than the normal 46. With "X" standing for the single X chromosome present and the second sex chromosome lacking, the situation is represented by the notation 45, X. Females with this syndrome may experience small stature, infertility, and possible heart and kidney issues, among other physical and developmental abnormalities. Turner's syndrome sufferers, however, frequently possess normal intelligence.

Hence, the correct answer is option 1)Has 45 chromosomes with XO.

Q2. Pick out the correct statements:

(a) Haemophilia is a sex-linked recessive disease.

(b) Down's syndrome is due to aneuploidy.

(c) Sickle cell anaemia is an X-linked recessive gene disorder.

(d) Phenylketonuria is an autosomal recessive gene disorder.

Option 1: (a) and (d) are correct.

Option 2: (b) and (d) are correct.

Option 3: (a), (c) and (d) are correct.

Option 4: (a), (b) and (d) are correct.

Correct answer: 4) (a), (b) and (d) are correct.

Explanation:

(a) Haemophilia is a sex-linked recessive disease. The abnormal gene responsible for haemophilia is carried on the X chromosome.

(b) Down's syndrome is due to aneuploidy. Down Syndrome is a disorder that results from an extra copy of 1 chromosome.

(c) Phenylketonuria is an autosomal recessive gene disorder. It is caused by mutations in both alleles of the gene for phenylalanine hydroxylase (PAH), found on chromosome 12.

(d) Sickle cell anaemia is an autosomal recessive disorder. Two copies of the gene must be mutated for a person to be affected by an autosomal recessive disorder.

The statements a, b and c are correct.

Hence, the correct answer is option 4) (a), (b) and (d) are correct.

Q3. X chromosome is one of the ________.

Option 1: Allosome

Option 2: Autosome

Option 3: Both (1) and (2)

Option 4: None of the above

Correct answer: 1)Allosome.

Explanation:

The X chromosome is one of the sex chromosomes or allosome (the other being the Y chromosome), X-linked inheritance is determined by the sex of the parent carrying a specific gene and can often seem complex. This is due to the fact that, typically, females have two copies of the X chromosome, while males have only one copy.

Hence, the correct answer is option 1)Allosome.

Also Read:

• MCQ Practice on Chromosomal Abnormalities

• Evolution

• Molecular Basis of Inheritance