Gene transfer in fertilisation. Computer artwork of sperm cells (at centre) approaching a human egg (top right) with one sperm fertilising it. Overlaid are graphs showing the genetic sequence of DNA (deoxyribonucleic acid). These graphs plot the sequence of the four bases which make up the inherited code in DNA: guanine (G), cytosine (C), thymine (T) and adenine (A). This genetic code in the form of genes is transferred from a sperm into the egg at fertilisation so that the resulting embryo shares half of its genes from the father and half from the mother. Only one sperm can penetrate the egg and when this occurs, the egg membrane forms a barrier to other sperm.