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Our cousin the mouse

Lisa Brooks

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Bethesda, Maryland

Humans and mice have a few things in common: hair, five toes on each foot, and a love of cheese. The publication of a decoded version of the mouse genome, this month, shows that genetically we also have a lot in common: 99% of human genes are also found in mice. What does the recently discovered genetic similarity between humans and mice say about the points of similarity between any two given humans, who are 99.9% similar to each other in terms of DNA?

The comparison allows a glimpse into the history of both genomes. 75 million years have passed since we shared a common ancestor - a species that was a small mammal. One family that evolved from this species became rodents, and later mice, and another became primates and eventually humans.
At that time, arbitrary mutations occurred in the DNA sequences of organisms from these two families. Some mutations had beneficial effects. They made the only ones they appeared in more resistant to diseases, for example. Mutations that had negative effects disappeared from the species. Over time, differences accumulated in the parts of the genomes, where the exact sequence of the DNA is not critical to function.

The genome of each individual consists of long molecules of DNA. Humans have about 3 billion units that make up the DNA sequence; Mice have about 2.6 billion such units. Most genes are pieces of DNA that carry information to make proteins. Since 99% of human genes have counterparts in mice, it is clear that most gene functions have been conserved between the two mammals.

If we have roughly the same genes as mice, why do we look so different? Part of the reason lies in the mutations that have accumulated in the DNA sequences. If we think of mice and humans as distant cousins ​​in one family tree, then all humans are brothers and sisters in that family. We are all descendants of a common set of ancestors who lived in Africa about 10,000 generations ago. The humans in our ancestral population had a certain amount of variation in their DNA sequences. Most of the genetic variation in humans today is derived from this pool of ancestors. As their descendants spread around the world, mutations also occurred that added to the genetic variation. Therefore 99.9% of one person's DNA sequence is identical to another's. The remaining part, 0.1% is a small proportion, but in the total human population there are about 10 million places in the genome where differences between people are common. Only a few hundred thousand variations in the sequence seem to affect the way humans function. Only a few hundred of them may be of major medical importance.

Just as comparing the human and mouse genome sequences provides keys to gene function, so does comparing the DNA sequence among humans. Common human diseases, such as diabetes, cancer, heart disease and depression, are influenced by many genes and environmental factors. Researchers can compare the genomes of people suffering from a disease to those of people who are exempt from the same disease and thus find the genetic differences that contribute to the risk of having the same disease.

While we learn from our genomes how each of us develops into a human being, we must remember that our shared humanity extends beyond our genetic differences.

The author of the article is the head of the genetic diversity program at the National Institute for Human Genome Research at the National Institutes of Health in the United States

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