Define aneuploidy. How is it different from polyploidy? Describe the individuals having the following chromosomal abnormalities:
$(a)$ Trisomy of $21^{st}$ chromosome
$(b)$ $XXY$
$(c)$ $XO$

(A-D) Definition of Aneuploidy: The failure of segregation of chromatids during the cell division cycle results in the gain or loss of a chromosome$(s)$, which is called aneuploidy.
Difference between aneuploidy and polyploidy:

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

$(a)$ Down's Syndrome: This is an autosomal aneuploidy caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$) due to non-disjunction during gamete formation. The affected individual is short-statured with a small round head, furrowed tongue, and partially open mouth. Palm is broad with a characteristic palm crease. Physical, psychomotor, and mental development is retarded.
$(b)$ Klinefelter's Syndrome: This genetic disorder is a trisomy of sex chromosomes caused by the presence of an additional $X$-chromosome ($47$ chromosomes, $XXY$). Such an individual has overall masculine development, but feminine development (e.g., Gynaecomastia) is also expressed. Such individuals are sterile males.
$(c)$ Turner's Syndrome: This is a monosomy of sex chromosomes caused by the absence of one $X$-chromosome ($45$ chromosomes, $XO$). Such females are sterile as ovaries are rudimentary, and they lack other secondary sexual characters.