Infertility affects about one in 6 couples and about half of cases are thought to have a genetic cause. Some cases can be treated successfully by in vitro fertilisation but this is expensive, not always available without high cost to the individual, and involves hormone treatments and minor surgery. In men one major genetic cause of infertility is loss of genes from the male Y chromosome. We are trying to understand what some of these and other genes involved do in fertile people with the aim of better understanding what goes wrong in those with fertility problems. To do this we use a range of different types of experiments. Some are designed to work out how chromosomes are assembled during the early stages of making sperm and eggs. Some of these experiments are test tube based. Others experiments can only be done by making changes in the mouse gene equivalent to the human gene we are interested in. Then we have a so-called mouse model and by studying these animals we can make predictions about the effects expected in humans. To reduce the use of animals in this work we are also trying to develop methods based on growing cells from mouse sperm and egg-making tissues in test tubes.