Professor Judith Rapoport introduces copy number variants, which are deletions and insertions in chromosomes.

Chromosomes can vary in a lot of different ways across people and within people, and everyone knows about certain types of copy number variants. For example, people with Down syndrome have three chromosome 21s. That is one very big copy number variant. But the chromosome is made up of repeats of different structures within the chromosome, and even deletions of parts of the chromosome, and there are all sorts of diseases in pediatrics, typically, where you could be missing a little piece of the chromosome. The kind of copy number variants that we have been studying, which are just changes in material in the chromosome that are too small to see with a microscope but can be seen in new ways that have been developed to scan all of your genes the entire chromosome. We’re not just looking for copy number changes in parts of the chromosome, but we are looking for parts that are very rare, because there is a lot of normal variation, just like people are varied in terms of how tall they are and what color their eyes are. There is a lot of healthy variation between people in terms of these very same variants. So, when we find a copy number variant in, say, a group of patients as we have and are still doing, we first have to look it up on tables to make sure it hasn’t been reported before in normal subjects. This is such a new field that the database for the normal variation is growing all the time. So this is a work in progress, because we’ll find something that we think is rare, and then over the next year it is found more than we thought it would be in normal populations.

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