Abnormal Karyotypes
Down’s Syndrome is a common genetic abnormality referred to as Trisomy 21. Instead of having the complement of 46 chromosomes of 22 homologous pairs plus 2 sex chromosomes, there are 47 chromosomes consisting of an additional Chromosome 21.
The appearance of extra or missing chromosomes arises during meiosis in an event called non-disjunction. After fertilization, a zygote with an improper chromosome complement occurs.