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Almost every cell in your body holds a copy of the same set of instructions, written in a code called DNA. If the DNA from just one of your cells were stretched out, it would be about two metres long. Along it lie roughly 20,000 genes, short sections of DNA carrying the instructions that build and run the body.

You receive half of your DNA from your mother and half from your father, which is why children often look like their parents without being exactly like either of them. In fact, any two people on Earth share about 99.9 percent of their DNA. The tiny difference that remains, together with the environment in which a person grows up, helps to explain why each of us is unique.

Reading all of a person's DNA, known as the genome, was once a huge task. The Human Genome Project, an international effort that was completed in 2003, took about thirteen years and cost around three billion dollars. Today, a whole genome can be read in a few days for a tiny part of that cost.

As a result, genetic tests have become part of everyday medicine. Some tests look for a single change in a gene that is known to cause a disease. Others estimate a person's risk of developing a condition later in life. Women carrying certain changes in two particular genes have a much higher risk of breast cancer, and those who know about it can be checked more often by their doctors. In Israel, couples who are planning to have children are offered tests that show whether both partners carry the same genetic condition, which could be passed on to a child.

Genes, however, are not destiny. For most common illnesses, such as heart disease, many genes are involved, each with a small effect, and the way people live, including what they eat and how much they exercise, matters a great deal. A test showing a higher risk is not a diagnosis, and a test showing a lower risk is not a promise.

Genetic information also raises difficult ethical questions. Who should be allowed to see your results: your doctor, your employer, an insurance company? Because you share DNA with your relatives, a test that you take may also reveal information about your parents, brothers and sisters, who never agreed to be tested. And some people would simply prefer not to know about a disease that cannot yet be treated. "There is no single right answer," says Noam Peretz, who helps families understand their test results. "Our job is to make sure that each person who takes a test understands what the results can and cannot tell them."

Almost every cell in your body holds a copy of the same set of instructions, written in a code called DNA. If the DNA from just one of your cells were stretched out, it would be about two metres long. Along it lie roughly 20,000 genes, short sections of DNA carrying the instructions that build and run the body.

You receive half of your DNA from your mother and half from your father, which is why children often look like their parents without being exactly like either of them. In fact, any two people on Earth share about 99.9 percent of their DNA. The tiny difference that remains, together with the environment in which a person grows up, helps to explain why each of us is unique.

Reading all of a person's DNA, known as the genome, was once a huge task. The Human Genome Project, an international effort that was completed in 2003, took about thirteen years and cost around three billion dollars. Today, a whole genome can be read in a few days for a tiny part of that cost.

As a result, genetic tests have become part of everyday medicine. Some tests look for a single change in a gene that is known to cause a disease. Others estimate a person's risk of developing a condition later in life. Women carrying certain changes in two particular genes have a much higher risk of breast cancer, and those who know about it can be checked more often by their doctors. In Israel, couples who are planning to have children are offered tests that show whether both partners carry the same genetic condition, which could be passed on to a child.

Genes, however, are not destiny. For most common illnesses, such as heart disease, many genes are involved, each with a small effect, and the way people live, including what they eat and how much they exercise, matters a great deal. A test showing a higher risk is not a diagnosis, and a test showing a lower risk is not a promise.

Genetic information also raises difficult ethical questions. Who should be allowed to see your results: your doctor, your employer, an insurance company? Because you share DNA with your relatives, a test that you take may also reveal information about your parents, brothers and sisters, who never agreed to be tested. And some people would simply prefer not to know about a disease that cannot yet be treated. "There is no single right answer," says Noam Peretz, who helps families understand their test results. "Our job is to make sure that each person who takes a test understands what the results can and cannot tell them."

Almost every cell in your body holds a copy of the same set of instructions, written in a code called DNA. If the DNA from just one of your cells were stretched out, it would be about two metres long. Along it lie roughly 20,000 genes, short sections of DNA carrying the instructions that build and run the body.

You receive half of your DNA from your mother and half from your father, which is why children often look like their parents without being exactly like either of them. In fact, any two people on Earth share about 99.9 percent of their DNA. The tiny difference that remains, together with the environment in which a person grows up, helps to explain why each of us is unique.

Reading all of a person's DNA, known as the genome, was once a huge task. The Human Genome Project, an international effort that was completed in 2003, took about thirteen years and cost around three billion dollars. Today, a whole genome can be read in a few days for a tiny part of that cost.

As a result, genetic tests have become part of everyday medicine. Some tests look for a single change in a gene that is known to cause a disease. Others estimate a person's risk of developing a condition later in life. Women carrying certain changes in two particular genes have a much higher risk of breast cancer, and those who know about it can be checked more often by their doctors. In Israel, couples who are planning to have children are offered tests that show whether both partners carry the same genetic condition, which could be passed on to a child.

Genes, however, are not destiny. For most common illnesses, such as heart disease, many genes are involved, each with a small effect, and the way people live, including what they eat and how much they exercise, matters a great deal. A test showing a higher risk is not a diagnosis, and a test showing a lower risk is not a promise.

Genetic information also raises difficult ethical questions. Who should be allowed to see your results: your doctor, your employer, an insurance company? Because you share DNA with your relatives, a test that you take may also reveal information about your parents, brothers and sisters, who never agreed to be tested. And some people would simply prefer not to know about a disease that cannot yet be treated. "There is no single right answer," says Noam Peretz, who helps families understand their test results. "Our job is to make sure that each person who takes a test understands what the results can and cannot tell them."