Chromosomes

All the genes in a cell nucleus are contained in chromosomes. Every species and consequently every individual of that species has the same number of chromosomes.

Each cell nucleus normally contains two copies of each chromosome in what researchers call a diploid set of chromosomes. Sperm cells and egg cells, on the other hand, only contain a half (haploid) set of chromosomes before fertilization so that the parental hereditary material is recombined in sexual reproduction.

Each human being therefore has 23 different chromosomes in the haploid set of germ cells, which are then merged into a complete set of 46 chromosomes when fertilized. Chromosome 21 is present three times in certain genetic defects such as trisomy 21, which is also known as Down's syndrome.