3) SEX CHROMOSOMES DISORDERS/ABNORMALITY

Aneuploidy is a chromosome abnormality that causes individuals to have an abnormal number of chromosomes. If the zygote (fertilised ovum) has three chromosomes instead of the usual pairs, it is trisomic. If the zygote has an unpaired chromosome, it is monosomic. Aneuploid cells occur as a result of chromosome breakage or non-disjunction errors that happen during meiosis (cell division in organisms that reproduce sexually). Two types of errors occur during non-disjunction: homologous chromosomes or sister chromatids.

In sex chromosomes, non-disjunction results in a number of abnormalities .The Klinefelter syndrome is such a disorder in which males have an extra X chromosome. The genotype for males with this disorder is XXY. People with Klinefelter syndrome may also have more than one extra chromosome resulting in genotypes which include XXYY, XXXY, and XXXXY. Other mutations result in males who have an extra Y chromosome and a genotype of XYY.

 (left diagram. trisomic embryo)

 Based on prison studies, these males were once thought to be taller than average males and overly aggressive. Additional studies however, have found XYY males to be normal. Next, Turner syndrome is a condition that affects females. Females with this syndrome, also called monosomy X, have a genotype of only one X chromosome (XO). Trisomy X females have an additional X chromosome and are also referred to as metafemales (XXX). 

 (left diagram. the chromosomes

and the difference in the body structures between people with and without sex chromosomes look like)

Non-disjunction can occur in autosomal cells as well. Down syndrome is most commonly the result of non-disjunction affecting autosomal chromosome 21. Down syndrome is also referred to as trisomy 21 because of the extra chromosome.