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Some genes cause genetic conditions when they are missing or altered. When scientists look at the chromosomes of someone with a genetic condition, they occasionally see bits of chromosomes missing: an important clue at the start of a gene hunt. Otherwise, they must use 'reference' fragments of DNA to track genes in affected families.

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Researchers can track down genes involved in genetic conditions by looking at DNA from affected families. They study how the condition is passed on from one generation to the next, looking for a piece of DNA that is inherited along with the condition. During the search for the gene involved in Huntington's disease, for example, a DNA fragment from chromosome 4 gave researchers the clue they needed.

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There may be many genes in a small section of a chromosome, just as there are usually several buildings in a street. To decide which is the gene they are looking for, scientists must look at the DNA sequence that makes up each gene. If they are looking for a gene involved in a genetic condition, then they may find differences in the DNA of that gene in an affected person.

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The largest of your chromosomes, chromosome 1, is made up of about 300 million base pairs of DNA code. Even the smallest, chromosome 21, is about 50 million base pairs long. A single gene is usually only hundreds or thousands of base pairs long - just a fraction of the length of the whole chromosome. So gene hunters need to narrow down their search using a 'map' of the chromosome, with known 'landmarks' on it, like finding out which street in the area a building is in.

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Genes affect our susceptibility to several common illnesses, but so do many other things, like our diet, surroundings and lifestyle. It's a bit like betting on a horse race – the horse, rider, course and weather can all affect the outcome in a way that is hard to predict. Many genetic and non-genetic factors affect our health (and personality and appearance), but scientists don't yet know what they all are or how they interact. Ways of finding out include population studies and studies of twins.

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