What is genetics?
Genetics is the study of heredity. Genes, made of DNA, are the instruction manuals for making proteins, which combine in many ways to create all the parts of our bodies. Genes are arranged in a defined order in packaging units called chromosomes. Proteins also help to create unique body features called traits. Some traits are visible, like eye color. Others are not, like the ability to fight certain diseases. Scientists who study genetics are called geneticists.
A genome is all of the genetic information or hereditary material in an organism. The genomes of individuals within a species are more similar to each other than they are to the genomes of individuals of other species.
Does everybody have the same genes?
Yes and no. While the human genome is mostly the same in all people, slight differences exist. This genetic variation, spread across many genes, makes up about one-tenth of a percent of each person's DNA. Yet these small differences are enough to create people with different appearances and different health. These differences are often inherited, so the more closely related two people are, the more similar their DNA is likely to be.
GENETIC VARIATION
What is genetic variation?
Simply speaking, variation is difference. Genetic variation is a difference in DNA. The “letters” of DNA are molecules called nucleotides: adenine, cytosine, guanine and thymine (A,C,G,T) strung together in long chains called sequences. The occasional single-letter differences that distinguish DNA among people are called single-nucleotide polymorphisms (SNPs, pronounced "snips").
Does everybody have the same number of genes?
Usually, but not always. A key type of genetic variation is “copy number” variation, which has been observed in identical twins that otherwise have identical DNA. Typically, everyone has two copies of each gene, one inherited from each parent. But scientists are learning that these DNA regions can carry anywhere from zero to more than a dozen copies of a gene. In some cases, researchers have found, one twin's DNA differs from the other's by having different numbers of copies of the same gene. This can affect the gene’s activity level, which can affect traits.
Are there other types of genetic variation?
Yes. Another type of genetic variation that goes beyond differences in DNA sequence is epigenetics. This type of variation arises from chemical tags that attach to DNA and affect how it gets read. The chemical tags, called epigenetic markings, act as switches that control how genes can be read.
Is genetic variation related to health and disease?
Yes. Minor differences in DNA sequence have an effect on health and disease risk. Because parents pass on their genes to offspring, some diseases tend to cluster in families, similar to other inherited traits such as some aspects of physical appearance. In addition, epigenetic markings have been associated with some types of disease, such as cancer. However, it is important to recognize that many factors other than genes, including diet, exercise and environmental exposures, also contribute to health and disease.
What is a haplotype?
Just as a SNP is a single-nucleotide change, a haplotype is a set of genetic changes-- usually a group of SNPs that tend to be inherited together. Think of a haplotype as a “gene neighborhood.”
What can haplotypes tell us?
Scientists use haplotype information to compare the genes of people affected by a disease with the genes of unaffected people. For example, this approach has revealed a genetic variation that substantially increases the risk of age-related macular degeneration, the leading cause of severe vision loss in older people. The information may be helpful in the future for disease screening and for the development of new therapies.
What about other diseases?
In October 2005, an international scientific team published a catalog of human haplotypes, nicknamed the HapMap. Researchers are looking through the catalog in an effort to identify gene neighborhoods that are linked to susceptibility to many common diseases, including asthma, diabetes, cancer and heart disease. The information may also help predict gene-related individual reactions to medicines and environmental chemicals.
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