Define aneuploidy. How is it different from polyploidy ? Describe the individualsm having the following chromosomal abnormalities.

$(a)$ Trisomy of $21^{st}$ chromosome

$(b)$ $XXY$

$(c)$ $XO$

$(a)$ Trisomy of $2 \mathrm{l}^{\text {st }}$ chromosome

$(b)$ $\mathrm{XXY}$

$(c)$ $XO$

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

Difference between aneuploidy and polyploidy :

Aneuploidy	Polyploidy
$(1)$ Chromatids fails to segregate during cell division	$(1)$ Failure of cytokinesis after telophase
$(2)$ There is loss or gain of chromosome from the particular number of homologus pair like $2 n-1,2 n+1,2 n-2,2 n+2$	$(2)$ The entire set of chromosomes is altered like $2 n, 3 n, 4 n$
$(3)$ It can lead to disorders like Down syndrome, Turner syndrome,Klinefelter's syndrome.	$(3)$ It is useful in plant breeding.

$(i)$ Down's Syndrome (Mongolism) : This is a autosomal aneuploidy It is causeddue to presence of an additional copy of the chromosome number $21$ (trisomy of $21$ ) due to non-disjunction of chromosomes during sperm or ova formation. 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.

$(ii)$ Klinefelter's Syndrome : This genetic disorder is trisomy of sex chromosomes is also caused due to the presence of an additional copy of $X$-chromosome. They have $47$ chromosomes., They have karyotype of $XXY$. Such an individual has overall masculine development, however, the feminine development (development of breast, i.e., Gynaecomastia) is also expressed. Such individuals are sterile male.

$(iii)$ Turner's Syndrome : Such females are sterile as ovaries are rudimentary besides other features including lack of other secondary sexual characters. This is a monosomy of sex chromosome,Such a disorder is caused due to the absence of one of the $X$ chromosome, They have $45$ chromosomes. They can be represented as $\mathrm{XO}$.