Understanding Chromosomes
Human Chromosomes
Chromosomes are thread like molecules that contain all the genetic instructions for a living organism. All living things, from plants to people have chromosomes. Each chromosome is a long tightly packed string of individual genes.
Human beings typically have 22 pairs of chromosomes plus a pair of sex chromosomes. These are passed on from parents to child at the moment of conception. These chromosomes control everything from the child's gender to their eye colour.
The 22 pairs of chromosomes are numbered in order of size. There is an additional pair of sex chromosomes, that determine the child's gender. The sex chromosomes are called X and Y. XX denotes a female and XY a male.
This makes a total of 46 chromosomes in each typical human cell.
Conception
The chromosomes for each person are determined at the point of conception.
The human egg cell and the human sperm cell each contain only 23 chromosomes. When a healthy egg is fertilised by a healthy sperm, the fertilised egg has 23 chromosomes from the mother's egg and 23 chromosomes from the father's sperm, totalling 46 chromosomes. This is the unique blueprint for the individual baby that grows.
Embryonic Growth
The singe fertilsed egg with 46 chromsomes then divides repeatedly to form every cell in the new human body. In typical development, each of these cells with have an identical copy of these 46 chromosomes

Chromosome Defects
Chromosome defects most normally occur because of errors in the processes of cell division involved in conception and embryonic development.
MEOISIS
Meoisis is the process of cell division that creates sperm and eggs. Each sperm or egg should have 23 chromosomes. However if Meoisis does not occur correctly, a sperm or egg can have too many or too few chromosomes.
Therefore if the faulty sperm or egg goes on to form an embryo, that embryo will inherit an incorrect number of chromosomes.
MITOSIS
Mitosis is the process of cell division that occurs in embryonic development. If the cell divides incorrectly at some point in the process, subsequent cells will have the incorrect number of chromosomes.