Genetic information is contained in a molecule called DNA.

DNA is a long, very tightly twisted molecular strand. If the DNA strands of a single cell were opened, they would be almost two meters long.

Each chromosome contains one DNA molecule. Chromosomes are thread-like molecules that carry hereditary information about everything from the individual's height to their eye color. Chromosomes are located in the nuclei of cells, from where they direct cell function.

Chromosomes can be observed with a light microscope, by staining a cell during its division stage. At that point, the chromosomes are packed so tightly that the power of the light microscope is sufficient to view the chromosomes.

Chromosomes can be arranged in pairs based on their properties. These chromosome pairs are identical and therefore known as homologous chromosomes. A human has 23 homologous chromosomes, of which 22 pairs are numbered chromosomes, called autosomes. The final pair consists of the sex chromosomes, X and Y. In other words, humans have 46 chromosomes in each cell - except for gametes, which differ slightly from other cells in terms of their size and structure.

While most cells in our body have two copies of each autosome, sperm and egg cells carry only one copy of each autosome. When a sperm fertilizes an egg, the embryo now contains two copies of each autosome, one from the father and one from the mother. Consequently, each person has two copies (alleles) of every gene carried on an autosome: one inherited from their father and one from their mother.

The X and Y chromosomes are known as the sex chromosomes. Women have two copies of the X chromosome, one from their father and one from their mother. Men have one X chromosome, from their mother, and one Y chromosome, from their father.

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