Nicola Powles-Glover reveals how, more than 50 years ago, curiosity about the colour of a mouse led to a breakthrough

Dr Mary F. Lyon is a brilliant geneticist. Her research has focused on the effects of radiation and other agents on genetic changes known as mutations and how they cause diseases. Her fundamental ideas and findings were well ahead of other theories in her field.

Dr Lyon's curiosity, more than 50 years ago, about the unusual colour of the coat of a mouse, led directly to insight into a fundamental genetic mechanism, called X-chromosome inactivation also known as "Lyonisation". This phenomenon has perhaps opened more lines of inquiry and stimulated more work than any recent biological concept and has also led to understanding the underlying events that lead to a number of medical conditions.

You might wonder what the X-chromosome is and why it is so important? It is one of the two sex-determining chromosomes found in all cells in many animal species, including humans, the other is called Y. Females carry two X chromosomes. Males have one X and one Y. To stop duplication of effort, early in embryonic development one of the two X-chromosomes is switched off. It becomes condensed and its genes do not function. X-inactivation ensures that XX females and XY males have the same dose of X-linked gene products. Knowledge of X-chromosome inactivation is important for understanding the birth defects that result from abnormalities or changes in the X-chromosome. For example, mutations can occur in X-chromosome genes at conception and cause conditions like haemophilia and muscular dystrophy. Boys are affected but the females don't have to use both of their X chromosomes so they only use the normal one. Doctors can look for cells with mutated genes and this helps in diagnosis.

Mary Lyon's discovery of X-chromosome inactivation in 1961 was largely based on serendipity. It was sometime before substantial progress was made, because so little was known about mouse genetics at the time. The DNA molecule had also not been discovered yet, so work in this area had not started. The double helix structure was not identified until 1953.

Dr Lyon has worked all of her life for the Medical Research Council having joined the MRC at the Institute of Animal Genetics in Edinburgh in 1948 after qualifying in zoology at Cambridge. The research group later relocated in 1955 to MRC Harwell.

She retired from active research in 1990, but can still be found busily working in her office at Harwell writing and reviewing scientific papers. The MRC has honoured her by naming the research centre for mammalian genetics research at MRC Harwell after her.

The Mary Lyon Centre is an international resource for functional genetics, providing the tools and space needed to generate mouse models of human disease for both MRC scientists at Harwell and the wider research community. This is achieved through the Mary Lyon Centre's expertise in animal production using a number of scientific techniques, which can change the sequence of the DNA.

By getting rid of some of the message or putting extra letters in, these changes can be seen in the resulting mice. The Mary Lyon Centre also acts like a large reference library for research scientists from around the world. They will search to see if any mice exist that have a mutation in the gene they are working on.

If the scientist finds one on the Mary Lyon Centre database, they can ask for the mouse to be sent to them.

This is a far cry from where Dr Lyon came from in 1961. She has contributed to and witnessed the science of genetics evolve throughout her career. Dr Lyon believes there is still much more to learn from the humble mouse.