Chromosomal Aberrations - 1 | Agriculture Optional Notes for UPSC PDF Download

Chromosomal Aberrations

• Chromosomal aberrations, also known as abnormalities, involve alterations in the structure or quantity of chromosomes, which are tightly packed genetic material strands. In the case of humans, the typical chromosome count is 23 pairs, with 22 pairs referred to as autosomal (numbered 1 through 22). The final pair consists of sex chromosomes, determining an individual's sex assignment, where most with XY sex chromosomes are assigned as males at birth, and most with XX chromosomes are assigned as females.
• In essence, each parent contributes one set of chromosomes to their offspring, forming the complete set of 23 chromosome pairs. Any change in the number or structure of these chromosomes results in a chromosomal aberration, which can lead to various medical disorders.

Different Types of Chromosomal Aberrations

• Chromosomal aberrations can be classified into two categories: numerical and structural aberrations. Numerical aberrations, which involve changes in the count of chromosomes, are known as aneuploidies. The most prevalent types of aneuploidy are monosomies, where only one of a chromosome pair is present, and trisomies, where there are three copies of a chromosome instead of the usual pair.
• Structural chromosomal aberrations come in four main forms: deletion, duplication, inversion, and translocation. Deletions involve the removal of a segment of a chromosome, potentially impairing its functionality. For instance, the deletion of part of the short arm of chromosome 5 leads to Cri-du-chat syndrome, characterized by symptoms like reduced head size and high-pitched crying in infants.
• In duplication, a portion of the chromosome is copied, resulting in extra genetic material. This is seen in conditions like Charcot-Marie-Tooth disease type I, where part of chromosome 17 is duplicated, causing muscle weakness.
• Inversion of a chromosome occurs when the genetic material is flipped in the opposite direction. Inversions are generally not strongly associated with diseases and frequently affect chromosome 2.
• Translocations happen when a segment of one chromosome breaks off and attaches to another chromosome. A common example is Robertsonian translocation, which occurs when two acrocentric chromosomes (ones with arms of unequal lengths due to an off-center centromere) lose their short arms, leading to the fusion of their long arms. Robertsonian translocations are one potential cause of trisomies.

Most Common Chromosomal Disorders

• Chromosomal disorders are frequently the result of aneuploidies, with trisomies being the most common. Notably, Down syndrome, caused by trisomy 21 or the presence of an extra chromosome 21, is a well-known example. Down syndrome can manifest with various signs and symptoms, including small white/grey spots called brushfield spots on the iris, distinct facial features such as a small head or flat face, gastrointestinal issues like vomiting, heart problems, neuromuscular challenges like decreased muscle tone, pale skin, fatigue, and breathing difficulties.
• The other two prevalent autosomal trisomies are trisomy 18, leading to Edwards syndrome, and trisomy 13, causing Patau syndrome. Edwards syndrome often involves severe intellectual disability, clenched hands, a prominent back of the head, a small mouth, low-set ears, and rocker-bottom feet characterized by prominent heels. Patau syndrome is associated with severe intellectual disability, a small head, small eyes, cleft lips or palate, polydactyly (more than five fingers on a hand), rocker-bottom feet, and malformation of the forebrain, a condition known as holoprosencephaly. Advanced maternal age is a risk factor for all three trisomy disorders and chromosomal aberrations in general.
• Aneuploidies of the sex chromosomes can also occur but typically exhibit different symptoms compared to autosomal disorders. Common signs and symptoms of sex chromosome aneuploidies include delayed puberty, absence of menstruation, ambiguous genitalia, and infertility. The most frequent sex chromosome disorder is Klinefelter syndrome, where individuals with XY chromosomes have an additional X chromosome, resulting in the XXY genotype, and are often assigned male at birth. The other two sex chromosome trisomies are XYY syndrome and XXX syndrome. Turner syndrome, the sole sex chromosome monosomy, arises when a person inherits only one X chromosome and is typically assigned female at birth.