Comparative genomics

Comparative genomics is the study of similarities and differences in genome structure and organisation among different organisms. 

For the human genome, two types of comparison are generally useful:

The human and mouse X chromosomes are essentially patchworks of the same eight blocks of genes. The different patterns have emerged through chromosome rearrangements occurring separately in the mouse and human evolutionary lineages (arrows indicate where the orientation of a particular block differs in the two species). During evolution, the other chromosomes have been mixed up, so there is generally no direct correspondence between mouse and human chromosomes. The X chromosome is exceptional because females have two copies and males only one. The genes are therefore subject to dosage control in the different sexes and only function properly on the X chromosome.

Comparisons with the genomes of selected model organisms

Such organisms include the fruit fly (Drosophila melanogaster), a nematode worm (Caenorhabditis elegans) and even baker's yeast (Saccharomyces cerevisiae). These organisms were chosen as models because they are easy to study in the laboratory and a great deal is now known about them at the molecular level. Where the function of a newly discovered human gene is unknown, it is often helpful to consult information from these other organisms - most human genes have counterparts in other animals, even if they are only distantly related to us.

Comparisons to the genomes of other vertebrates

Genome maps are available for a number of mammals and other vertebrates, including many domesticated farm and companion animals. As might be expected, most human genes have direct counterparts in these species, and the sequences are very similar. Gene order is also highly conserved, a phenomenon known as synteny.

The similarities between mammalian genomes can be exploited in a variety of ways. For example, if the order of the genes is the same, the gene map of one species can be used to help find genes in a related species with a map that is poorly developed. Sequence comparisons can also help to identify important other pieces of DNA that control gene expression.

The differences between mammalian genomes are also important. While most genes are conserved among mammals, the species themselves are distinct in many ways. What is it that actually makes us human? This question is not only of general interest but also has useful medical applications. For example, which mammals do not get cancer? Which mammals cannot catch acquired immune deficiency syndrome (AIDS)? What is it about their immune systems and bodily make-up that makes them more resilient than us? In this way, understanding how genes have evolved in different mammals may facilitate the development of better drugs and therapies for humans.