Describe chromosomal disorders and provide examples.

(N/A) Chromosomal disorders are caused by the absence,excess,or abnormal arrangement of one or more chromosomes.
Failure of segregation of chromatids during the cell division cycle results in the gain or loss of a chromosome$(s)$,a condition called aneuploidy.
For example,Down's syndrome results from the gain of an extra copy of chromosome $21$.
Similarly,Turner's syndrome results from the loss of an $X$-chromosome in human females.
Failure of cytokinesis after the telophase stage of cell division results in an increase in a whole set of chromosomes in an organism,a phenomenon known as polyploidy. This condition is often seen in plants.
The total number of chromosomes in a normal human cell is $46$ ($23$ pairs). Out of these,$22$ pairs are autosomes and one pair consists of sex chromosomes.
Sometimes,though rarely,an additional copy of a chromosome may be included in an individual,or an individual may lack one chromosome from any pair. These situations are known as trisomy or monosomy,respectively. Such conditions lead to very serious consequences for the individual.
Down's syndrome,Turner's syndrome,and Klinefelter's syndrome are common examples of chromosomal disorders.
Down's syndrome is caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$). This disorder was first described by Langdon Down $(1866)$. The affected individual is short-statured with a small round head,furrowed tongue,and partially open mouth. The palm is broad with a characteristic palm crease. Physical,psychomotor,and mental development is retarded.