DNA is present in every cell in our body. Each cell has a small compartment, the nucleus, in which the DNA is present. In this nucleus the DNA is rolled in a long strand of X-like structures: the chromosomes. In humans each nucleus contains 46 chromosomes. Each chromosome is present in two-fold, so there are 23 pairs. Men and women have chromosome 1 till 22 in common, the 23th pair consists of two X-chromosomes in women and of a X and a Y-chromosome in men. In contrary to the normal cells in our body, the sex chromosomes have only one chromosome of a pair. So 23 chromosomes in total. At fertilization the 23 chromosomes of the father merge with the 23 of the mother and the cell contains 46 chromosomes again. Each chromosome consists of two parts, these parts are called the chromatides. Each chromatide has a p (short) arm and a q (long) arm. The two chromatides are completely identical and are held together by a structure called the centromere. Each chromatide contains a couple of genetic properties, the genes. Each gene has two or more possible versions. Just think of eyecolor for example. This can be blue, brown or green. Each version is called an allele. If an individual has 2 identical alleles we call this person homozygous. If he/she has two different alleles we call the person heterozygous. The different alleles of a gene are not all equally strong. One allele can be stronger. This allele is dominant over the other one, the recessive allele. In the case of a heterozygous individual the dominant allele determines the version of the gene to be expressed. If the blue allele is dominant over the brown allele, you will get blue eyes. But most genetic properties are determined by a combination of genes. After crossing over a wide variety can occur in a genetic property.