ROB MORRISON: For decades white mice have been the animals used in science laboratories and scientific research. They've helped us uncover mysteries about how our bodies and minds work. But in recent times a very different research animal has been replacing them. It's the zebra fish. This little fish comes from Asia but it's now popular in aquariums worldwide. Easy to keep, zebra fish produce hundreds of eggs which develop independent of their parents and in only a few days. But what can fish development reveal about human development?

DR MICHAEL LARDELLI: They're still a backboned animal like ourselves, a vertebrate animal, as we say. And that means that by studying them we can find answers to questions which are really relevant to human biology and human disease. For me, the most exciting thing about zebra fish... ..I can't point one thing, it's the whole package. It takes a lot to run a mouse facility. With the fish we can keep them at much higher densities, we don't need to have as many personnel to take care of them, just our research costs become much lower. Now, the scientific reasons why we find this organism very useful are that, firstly, we can do something called 'genetic screening'. That's a very powerful technique that can be used in zebra fish. And it's difficult to do that in mice. The other scientific reason is the accessibility of the developing embryo from the fish. So unlike mice, fish lay their eggs into the external environment. And the embryo is independent of the mother once it's fertilised.

BEN TUCKER: It's a very accessible embryo and it's easy to image it. Given that the embryo is transparent, we can use any number of imaging techniques to see any part of the fish at any point. So we can see all of its organs from a very early stage through to a fairly late stage.

ROB MORRISON: Just how an embryo develops, whether a mouse, a fish or a person, depends on the genes it inherits. Each gene carries a message about how body parts should develop or function. If the genes are normal, so is development, while faulty genes produce a faulty embryo. But what are genes? And where are they in the egg?

DR MICHAEL LARDELLI: Basically it's a single cell and within that cell is a nucleus. And within that nucleus are long molecules of DNA. And these very long molecules we call chromosomes.

ROB MORRISON: In short, the chromosomes, which we inherit from our parents, are a little like strings of beads. Each bead is a gene, responsible for part of our development - behaviour, appearance, physiology, just about everything we are or do. And, surprisingly, humans share many of their genes with mice and even fish.

DR MICHAEL LARDELLI: So if we want to understand what those genes are doing in human disease it's useful for us to understand what they're doing in the embryo.

BEN TUCKER: Normally these genes can turn themselves on and off and we can inject a chemical against a specific gene to make it turn off, so taking out the function of a specific gene.

DR MICHAEL LARDELLI: We can use these labelled cells in living embryos to actually follow what those cells do. They can migrate, they can divide in certain ways. And that great accessibility of the embryo and the ability to do these techniques on these accessible embryos is what makes them very useful for us.

ROB MORRISON: Time-lapse photography condenses the few days of development into a minute, showing how a single fertilised egg divides repeatedly, the masses of cells forming different parts of the body. Labelled cells can be traced, allowing researchers to see how altered genes affect the developing fish. In this laboratory they're especially interested in genes that cause some serious human diseases.

DR MICHAEL LARDELLI: In our laboratory we've been looking at two different human genetic diseases. One of our students has been investigating the gene behind the fragile X syndrome and has found some interesting things recently. And we've been investigating one of the most important genes for inherited Alzheimer's disease and we've been able to find out things about the way that gene works that has not been possible to find out in mice or in any other system.

BEN TUCKER: Fragile X is where the chromosome has a genetic mutation in it, which means that the genes on part of the X chromosome are no longer functional. The loss of this fragile X gene leads to a number of abnormalities in humans, like mental retardation and mildly abnormal facial features.

ROB MORRISON: Once a gene can be detected and its effects understood, researchers have a valuable tool for early diagnosis of diseases and possibly for their treatment. So these little fish seem destined to play an increasing research role as they reveal the mysteries of how we become what we are and help scientists discover how to prevent things from going wrong along the way.