An Introduction to Chromosomal Disorders
Chromosomes are thread-like structures present within the nucleus that carry hereditary information within the sort of genes which are passed from parents to offspring. Every species features a characteristic structure and number of chromosomes present. Due to certain irregularities at the time of cell division, alteration in the structure or number of chromosomes may happen. Even the slightest alteration can lead to various abnormalities. Changes in one chromosome parts, whole chromosome or chromosomal sets are known as “chromosomal aberrations”.
Chromosomal Disorders in Humans
There are 46 chromosomes in each human cell present as 23 pairs (n pairs), out of which 22 are autosomes and 1 pair of sex chromosomes.
Chromosomal disorders are caused because of the structural changes or numerical changes in chromosomes.
Chromosomal Disorders Due to Numerical Abnormalities
Chromosomal disorders are caused due to the change in the number of chromosomes present. This can be categorised into various types:-
Aneuploidy: loss or gain of a chromosome. This happens due to non-disjunction of chromatids when chromatids fail to separate during cell division. This results in one gamete having two copies of one chromosome and the other having no chromosome.
Trisomy: The cell has one extra chromosome (2n+1)
Monosomy: The cell has one chromosome less (2n-1)
Aneuploidy can be due to nondisjunction of autosomes i.e. chromosomes 1-22 or sex chromosomes.
Chromosomal Disorders Due to Aneuploidy: This is the cause of most of the genetically inherited disorders and abortion during pregnancy
Euploidy: Loss or gain of the entire set of chromosomes. Mostly occurs in plants.
Haploid: Loss of one set of the chromosomes, i.e. ‘n’ number of chromosomes
Polyploid: Addition of one or more sets of chromosomes, e.g. ‘3n (triploid)’, ‘6n (hexaploid)’ etc.
Chromosomal Disorders Due to Structural Abnormalities
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This happens when an outsized set of genes are deleted, duplicated or rearranged causing structural changes within the chromosome. Structural abnormalities can be due to:
Deletion:
A portion of the chromosome is lost during cell division. A portion of the chromosome without the centromere lags during anaphase movement and are lost from reorganising nuclei or digested by nucleases. The resulting chromosome lacks certain genes that get inherited by offspring. This condition is typically lethal thanks to missing genes.
Deletions are often terminal, where a terminal portion of a chromosome breaks leading to one break
Intercalary deletion, where an intermediate portion is lost resulting from two breaks, which results in 3 pieces. The middle piece is lost and the other two parts rejoin
Example of Disorder Due to Deletion:
Cri du Chat (Cry of the Cat):
There occurs deletion of a small portion of the 5th chromosome. Children with this disease have a small head with unusual facial features, severe mental retardation and make a sound like a cat while crying.
Duplication:
The presence of part of a chromosome in excess is known as duplication. If the duplication is present only in one of the homologous pairs of a chromosome, the duplicated part makes a loop to maximise the juxtaposition of homologous regions during pairing. The extra segment is often arranged in many ways:
Tandem duplication, where the duplicated region is present side by side (ABCDEF→ABCDEF)
The reverse tandem, here duplicated region is simply reverse of the traditional sequence (ABCDEF→ABCDEF)
Displaced duplication, here duplicated region isn't situated adjacent to the traditional sequence.
Transposed duplication is a way in which the duplicated part becomes attached to a non-homologous chromosome.
Extra-chromosomal duplication, here duplicated part acts as an independent chromosome within the presence of centromere
Example of Disorder Due to Duplication:
Fragile X: Affects 1:1500 males and 1:2500 females. This is the most common form of mental retardation. Many people have around 29 repeats at the tip of the X chromosome.
Inversion:
inversion results from breakage and reunion of a part of the chromosome rotating by 180° on its own axis. So there occurs a rearrangement of genes. Its effects are not as severe as in other structural defects.
Translocation:
The shifting or transfer of a set of genes or part of a chromosome to a non-homologous one is known as translocation. There is no addition or loss of genes, only the rearrangement occurs. This rearrangement may lead to phenotype changes pertaining to the new environment. It can cause difficulties in the development of eggs, sperm or zygote. These often result in miscarriages and children born with disabilities.
Reciprocal translocation, in this segment of two chromosome, gets interchanged
Robertsonian translocation, here a whole chromosome attaches to a different chromosome.
Example of Disorder Due to Translocation:
Acute Myelogenous Leukemia: during this sort of cancer, bone marrow and cells derived from it show the presence of a brief chromosome named as “Philadelphia (Ph1) chromosome”. The 22nd chromosome loses a neighbourhood of its arm which gets translocated to the distal end of the 9th chromosome. It is not transmitted to the offspring.
The article discusses all the important points related to chromic disorders in humans. After going through the article, students will get good information that would be helpful for them for their exams as well as for general awareness.
FAQs on Chromosomal Disorders in Humans
1. What are the causes of chromosomal abnormalities?
The specific cause is unknown, but we do know that when a cell divides in two, chromosome abnormalities are common (a normal process that a cell goes through). Chromosome abnormalities can occur during the development of an egg or sperm cell (known as germline), or they can occur after conception (called somatic). The right amount of chromosomes is expected to end up in the resultant cells during cell division. Every cell in the body has chromosome abnormalities that start in the gametes.
2. What are chromosomes?
Inside your body's cells, chromosomes are ordered bundles of DNA. Genes in your DNA govern how your body develops and functions. Humans have 23 chromosomal pairs (46 in total). Your mother gives you one of each chromosomal pair, and your father gives you the other. The size of chromosomes varies. Each chromosome has a centromere that separates it into two unequal portions. The p arm is the shorter section, while the q arm is the longer section.
3. What are the indications and symptoms of uncommon chromosomal disorders?
Rare chromosomal abnormalities have a wide range of effects. Symptoms of chromosomal material loss or gain can include a mix of physical issues, health issues, learning challenges, and problematic behaviour. The symptoms vary depending on which chromosomes are affected. Losing a chromosomal section is usually more dangerous than having an extra copy of the same segment. This is because when you lose a chromosomal segment, you may also lose one copy of a critical gene that your body needs to function.
4. Name a few disorders caused by changes in chromosome number?
Disorders caused by changes in chromosome number are:
Down's syndrome is caused by chromosome number 21 trisomy. It is characterized by mental impairment and intellectual incapacity. They have a lot of heart problems, as well as learning and growth impairments.
Turner's syndrome, often known as XO syndrome, is caused by a missing second sex chromosome. Individuals born with this disease suffer mental problems and sexual development delays, as well as both male and female genitalia.
5. Humans have how many chromosomes?
In a human cell, the average number of chromosomes is 46:23 pairs, with an estimated total of 20,000 to 25,000 genes. The biological mother (from the egg) inherits one set of 23 chromosomes, whereas the biological father inherits the other (from the sperm).
The first 22 pairs of chromosomes are referred to as "autosomes." The "sex chromosomes" are the final pair. Males have an X and a Y chromosome, while females have two X chromosomes.
