This document discusses chromosomal aberrations, which are changes to the structure or number of chromosomes. It begins by introducing chromosomes and chromosomal aberrations. It then describes the main types of chromosomal aberrations including numerical (changes in number) and structural aberrations. Common chromosomal disorders like Down syndrome are also discussed. The causes of chromosomal aberrations during cell division are explained. Methods for diagnosing chromosomal aberrations both prenatally and after birth through tests like karyotyping and FISH are summarized.
2. TABLE OF CONTENT
Introduction
Types of chromosomal aberrations
Common chromosomal disorders
Causes and diagnosis of chromosomal
aberrations
Conclusion
References
3. Introduction
Chromosomal aberrations, or abnormalities, are changes
to the structure or number of chromosomes, which are
strands of condensed genetic material. Humans typically
have 23 pairs of chromosomes, of which 22 pairs are
autosomal, numbered 1 through 22.
The last pair of chromosomes are sex chromosomes,
which determine an individual’s sex assignment. At birth,
most people with XY sex chromosomes are assigned
male, and most individuals with XX are assigned female.
In general, each parent contributes one set of
chromosomes to their offspring, which collectively make
up the 23 pairs of chromosomes. A change to any of the
chromosomes, in number or structure, creates
a chromosomal aberration and may cause medical
disorders.
5. Common chromosomal disorders
Down syndrome
Edwards syndrome
Patau syndrome
Klinefelter syndrome
Turner syndrome
6. Causes of chromosomal aberrations
Chromosomal aberrations are most often caused by errors
during cell division. Cell division in humans occurs
via mitosis or, only in sex chromosomes, meiosis.
In mitosis, cells duplicate their chromosomes and produce
daughter cells with an identical number of chromosomes as the
original cell. In other words, a cell with 46 chromosomes will
produce two cells, each with 46 identical chromosomes.
Meanwhile, cell division by meiosis involves two rounds of cell
division that allow for the recombination of genetic material,
resulting in four sex cells with only half of the number of
chromosomes.
Causes of chromosomal aberrations
7. Diagnosis of chromosomal aberrations
Chromosomal aberrations in offspring can often be diagnosed
during prenatal screenings prior to birth. An ultrasound near the
end of the first trimester or early in the second trimester can
show increased nuchal translucency, which is a measure of the
fluid-filled space in the back of the neck, potentially
indicating trisomy 13, 18, or 21.
A blood screening may also be conducted in the late first
trimester to test for pregnancy-associated plasma protein A
(PAPP-A) and human chorionic gonadotropin (hCG), which may
indicate a chromosomal disorder if the levels are out of their
expected ranges.
During the second trimester, a blood sample may be taken from
a pregnant woman to conduct a “quad screen,” measuring levels
of inhibin A, alpha-fetoprotein (AFP), hCG, and unconjugated
estriol (uE3). A combination of high or low results can indicate
different types of chromosomal disorders.
8. Diagnosis of chromosomal aberrations
For diagnosis after birth, a clinician may consider the infant’s
signs and symptoms and may use blood samples to run
diagnostic tests, such as karyotyping and fluorescence in situ
hybridization (FISH).
Through karyotyping, all of the chromosomes are displayed
visually in order to reveal any large structural abnormalities or
numerical differences. Similarly, FISH uses fluorescent
probes to visualize the genetic material, which can show both
structural and numerical abnormalities.
