Question

How do chromosomal abnormalities due to changes in the number of chromosomes arise? Give three examples of occurrence of such abnormalities in humans.

Answer 1

Hint: A chromosomal abnormalities or disorder is due to missing, extra, or irregular portions of chromosomal DNA. It can be due to change in structure of one or more than one chromosome. There are many different types of chromosomal abnormalities that can be divided as numerical and structural.

Complete answer:
Chromosomes are stick-like structures present in the middle of each and every cell. In humans almost every cell consists of 23 pairs of chromosomes and a total of 46 in number. During fertilization half of the chromosome comes from the mother and the rest half from the father. First 22 pairs are considered as autosome and 23rd pair is called as sex chromosomes that is X (for female) and Y (for male). Abnormalities in chromosomes mostly occur during cell division.
Chromosome abnormalities may also happen due to one or more of these following-
Errors during division of sex cells (meiosis)
Errors during division of different other cells (mitosis)
Exposure to few substances that may lead to birth defects (teratogens).
The common type of chromosomal abnormality is aneuploidy (an extra or missing chromosome). Aneuploidy mostly shows trisomy (i.e. three copies of a chromosome) and monosomy (single copy of a chromosome).
Few common examples of chromosomal abnormalities are -
Down syndrome- Also known as Mongolism that is due to an additional copy of the chromosome number 21 or trisomy of 21 (trisomy of autosome). Symptoms like short statued with small round head, partially open mouth with furrowed tongue.
Klinefelter's syndrome- It is due to trisomy of sex chromosome i.e. presence of additional copy of X-chromosome resulted karyotype of 47 (XXY). This is common in male that develop female characters like breast development.
Turner syndrome- It is due to monosomy i.e. a disorder caused due to the absence of one of the X chromosomes leads to decrease in chromosome number (45 with XO). It is mainly women who lack secondary sexual characteristics and are sterile.

Note:
Chromosomal abnormalities caused due to absence or abnormal arrangement of one or more chromosomes that leads to either aneuploidy or polyploidy. These include Down syndrome, Edward's syndrome and sex chromosomes are also responsible for disease like Klinefelter's syndrome, Turner's syndrome etc.