Let's remind ourselves what a gene actually is. Most cells in your body have a nucleus, which holds 23 pairs of chromosomes. Each chromosome is made of DNA, a long molecule shaped a bit like a twisted rope ladder. The steps of the ladder are the letters of the genetic code. A section of DNA that codes for a particular thing, like eye colour, is called a gene. It takes about 30 000 genes to make a human being. Every nucleus contains a complete set, packaged up into 23 pairs of chromosomes. The only exceptions are egg and sperm cells, which only have one from each pair of chromosomes (see meiosis)