Chromosomal Aberrations - 2 | Agriculture Optional Notes for UPSC PDF Download

Causes Chromosomal Aberrations

• Chromosomal aberrations primarily arise due to errors in the process of cell division. In humans, cell division occurs through two mechanisms: mitosis and, exclusively in sex chromosomes, meiosis. Mitosis involves the replication of chromosomes, resulting in daughter cells that maintain an identical number of chromosomes as the parent cell. For instance, a cell with 46 chromosomes will produce two daughter cells, each also containing 46 chromosomes.
• On the other hand, meiosis comprises two rounds of division that allow genetic material to recombine, yielding four sex cells, each with half the number of chromosomes. In this case, a cell with 46 chromosomes undergoing meiosis will generate four distinct daughter cells, each possessing 23 chromosomes.
• Aneuploidy can occur when errors take place during meiosis, with a common error called nondisjunction, where chromosomes fail to separate correctly. This can lead to sex cells having an extra chromosome or lacking one. If a fertilization event involves a sex cell affected by nondisjunction, the resulting offspring may inherit either one more or one less chromosome, potentially leading to a chromosomal disorder.
• Less frequently, structural chromosomal aberrations can also result in aneuploidy. These structural aberrations involve the loss, inversion, duplication, or fusion of part or all of a chromosome. When this occurs following meiosis in a sex chromosome, two copies or none of a chromosome may be present, potentially passing on to offspring and resulting in monosomy or trisomy.
• In some cases, chromosomal disorders are attributed to mosaicism, wherein an individual possesses two or more different cell lines. Mosaicism can arise after nondisjunction in a mitotic cell division during early embryonic development. This leads to one cell line with a chromosomal aberration while other cell lines remain unchanged.

Chromosomal Aberrations Diagnosed

• Chromosomal aberrations in offspring are frequently detectable through prenatal screenings conducted before birth. A late first-trimester or early second-trimester ultrasound can reveal an increased nuchal translucency, which measures the fluid-filled space at the back of the fetal neck. This may potentially indicate the presence of trisomy 13, 18, or 21. Another test during the late first trimester assesses the levels of pregnancy-associated plasma protein A (PAPP-A) and human chorionic gonadotropin (hCG) in the maternal blood. Deviations from the expected ranges of these markers may suggest a chromosomal disorder.
• In the second trimester, a maternal blood sample is taken to perform a "quad screen," measuring levels of inhibin A, alpha-fetoprotein (AFP), hCG, and unconjugated estriol (uE3). Different combinations of high or low results can provide insights into various types of chromosomal disorders. Beyond these traditional maternal blood screenings, similar markers can be directly tested from samples obtained through procedures like amniocentesis (sampling amniotic fluid surrounding the fetus) or chorionic villus sampling (sampling placental tissue).
• For postnatal diagnosis, a healthcare provider may consider the signs and symptoms displayed by the infant and may conduct diagnostic tests using blood samples. These tests include karyotyping, which visually displays all the chromosomes to detect significant structural abnormalities or numerical variations. Additionally, fluorescence in situ hybridization (FISH) utilizes fluorescent probes to visualize genetic material, allowing the identification of both structural and numerical abnormalities.

Reduce the Risk of Chromosomal Aberrations

• Parents can mitigate the risk of chromosomal aberrations in their children by addressing their own nutritional requirements, minimizing exposure to harmful substances, and seeking medical guidance before attempting to conceive. 
• In general, strategies for reducing the risk involve adopting a healthy diet, refraining from smoking or alcohol consumption, and taking prenatal supplements in the period leading up to pregnancy. It's important to note that the likelihood of chromosomal aberrations is higher in pregnancies where the expectant parent is aged 35 or older.
• In cases where a family has a history of chromosomal disorders, a healthcare provider might recommend genetic counseling to explore various options, including assisted reproductive techniques.

Most Important Facts to know about Chromosomal Aberrations

• Chromosomal aberrations refer to alterations in the structure or number of chromosomes. A majority of these aberrations fall under the category of aneuploidies, which involve variations in the number of chromosomes, deviating from the typical paired structure. Trisomy is a form of aneuploidy characterized by the presence of an additional chromosome, resulting in a total of three, while monosomy signifies the absence of one chromosome, leaving just one. Structural chromosomal aberrations entail modifications to specific sections of a chromosome, including deletions, insertions, inversions, and translocations.
• The most prevalent chromosomal disorder is trisomy 21, responsible for Down syndrome. Other chromosomal disorders encompass Edwards syndrome, Patau syndrome, Klinefelter syndrome, and Turner syndrome. Typically, chromosomal aberrations arise due to irregularities in the division of sex cells inherited from a parent. These aberrations can be detected before or after birth through methods like ultrasound imaging and blood tests. To mitigate the risk of chromosomal aberrations during pregnancy, parents can take measures such as consulting a healthcare provider before conception, maintaining a nutritious diet, and refraining from smoking and alcohol consumption.