Describe chromosomal disorder and describe its examples
 

The chromosomal disorders are caused due to absence or excess or abnormal arrangement of one or more chromosomes.

Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome($s$) called aneuploidy.

For example Down's syndrome results in the gain of extra copy of chromosome $21$ .

Similarly, Turner's syndrome results due to loss of an $X$-chromosome in human females.

Failure to cytokinesis after telophase stage of cell division result in an increase in a whole set chromosomes in an organism and this phenomenon is 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 of chromosomes are sex chromosome.

Sometimes, though rarely either an additional copy of a chromosome may be included in an individual or an individual may lack one of any one pair of chromosomes.

These situations are known as trisomy or monosomy of a chromosome respectively.

Such a situation leads to very serious consequences in the individual.

Down's syndrome, Turner's syndrome, Klinefelter's syndrome are common examples of chromosomal disorders.

The cause of this genetic disorder is the presence of an additional copy of the chromosome number $21$ (trisomy of $21$ ).

A The disorder was first described by Langdon Down ($1866$).

The affected individual is short statured with small round head, furrowed tongue and partially open mouth.

Palm is broad with characteristic palm crease.

Physical, psychomotor and mental development is retarded.