In all mammals, including humans, the sex of offspring is decided by the presence or absence of the Y chromosome. Females have two X- chromosomes and no Y chromosome while males have one X chromosome and one Y-chromosome. The sex of the offspring depends on whether the fertilising sperm contains an X chromosome or a Y-chromosome. This is the basic rule of sex determination in humans though there are some variations, for example women with 3X chromosomes or an inactive Y chromosome. Fathers pass on their Y chromosome to their sons and not to their daughters. This makes for a very simple patrilineal inheritance pattern that is the mirror-image of mitochondrial DNA. The Y chromosome is a genetic wreck with only 27 still-active genes. Just one of these genes, called SRY, is responsible for determining sex. All human embryos start off as female until about six weeks after conception when SRY is switched on. This initiates a cascade of structural and hormonal changes in the developing embryo which eventually results in a male foetus and the birth of a baby boy. If SRY remains inactivated for any reason the embryo continues to develop as female, even though it has a Y-chromosome, and the result is the birth of a baby girl. Images of the Y-chromosome (on the left) and the much large X chromosome Genetic variation in the Y chromosome centres on two different kinds of marker system. The so-called SNP system follows slowly mutating sites and divides Y-chromosome into a number of haplogroups, which are themselves divided into different haplotypes. The other system, which is the one we use, follows the fast mutating variation in repeating DNA blocks called VNTRs. We prefer this system for genealogy because it is able to differentiate branches within a family tree where the SNP system cannot. We use 26 Y-chromosome markers which is more than enough for any genealogical project.