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Genetics primer
By Mayo Clinic Staff in DNA

Genetics primer
Genetics primer

Genetics can be confusing. Add in the fact that researchers are continually discovering new information about genes and the role they play in everything from skin color to how much you like sweets — and, well, keeping it all straight can be overwhelming for anyone.

If you find yourself scratching your head and wondering how genes work, it may help to brush up on some of the basics.

Your body's blueprint

Genes are made of DNA. Genes provide instructions to make proteins. Proteins make cells. And cells make you.

Cells are the basic building blocks of all living things. Your body is made of trillions of cells. They provide structure for your body, take in nutrients from food, convert those nutrients into energy and carry out specialized functions. Almost every cell has a nucleus that contains your genes.

All of your genes together are called your genome — a complete set of instructions to build your individual body and keep it working.

The genes in your cells are made of DNA

Deoxyribonucleic acid (DNA) is a special code of instructions that makes up a gene and tells it what to do. The code is made from small chemicals called bases. The bases come in four types: A, C, T and G.

A gene is a section of DNA made from a specific combination of bases. Every human has about 20,000 genes and 3 billion bases. Human genes vary in size from a few hundred to more than 2 million DNA bases.

A chromosome is a tightly wound coil of DNA

In the middle of each cell, the DNA molecule is packaged into threadlike structures called chromosomes. Chromosomes hold genes and genes are made of DNA. Most people have 46 chromosomes or 23 pairs in each cell. When you were born, you inherited one set of 23 chromosomes from your mother and the other set of 23 from your father.

Variation makes you unique

You share 99.9 percent of your DNA with any other person. The differences between people come from slight variations in 0.1 percent of genes. Variations are found throughout the genome, on all 46 human chromosomes. Genome variations include polymorphisms and mutations.

A polymorphism is a common variation

A polymorphism is a DNA sequence change (variation) that's common in the population. Sequencing simply means the exact order of the bases in a strand of DNA. Polymorphisms are responsible for many of the normal differences between people like eye color, hair color and blood type.

SNPs can identify certain traits and diseases

Single nucleotide polymorphisms (SNPs) — pronounced "snips" — are the most common type of genetic variation among people. SNPs appear normally throughout a person's DNA and account for about 90 percent of variation in the human genome. The word "single" refers to the fact that this variant involves a single base pair. Any one of the four DNA bases can be substituted for another — for example, an A instead of a T or a T instead of a C.

SNPs are most often found in the DNA between genes and have no effect on health or development. Doctors look for SNPs in DNA as markers that help predict a person's risk of certain diseases or response to some drugs, and track the inheritance of disease within families. Popular direct-to-consumer genetic tests use SNPs to provide information on athleticism, ancestry, physical traits like eye color and the body's response to certain foods.

Some changes in DNA aren't common

If a change in a DNA sequence happens in less than 1 percent of people, then the genetic variation is called a pathogenic variant or mutation. If 99.9 percent of people have a G base in a specific DNA sequence and 0.1 percent have a C base, it's called a mutation.

  1. Hereditary mutations. These may be inherited from a parent and are present throughout a person's life in almost every cell in the body. These mutations can be passed on to children.
  2. Acquired mutations. People are not born with an acquired mutation. Instead, the change happens at some point during a person's life. These mutations are only in some cells, and not in every cell in the body. The changes can be caused by environmental factors like ultraviolet radiation from the sun. Or the mutation can occur if an error is made as DNA copies itself during cell division. Acquired mutations in somatic cells — cells in the body other than sperm and egg cells — can't be passed on to children.

You can't change your genetics, but your genes are constantly changing

Your genes are turned on and off throughout your life because of a process called gene regulation. Each cell turns on (expresses) only some of its genes. The rest of the genes are turned off (repressed).

The epigenome is made of chemical compounds and proteins that attach to DNA and determine when genes are switched on or off. Epigenetic compounds do not change your DNA sequence. Instead, they affect the way cells use the DNA's instructions. When a gene is turned off, it doesn't provide your body the instructions for making specific proteins.

Epigenomics is the study of how environmental and lifestyle factors alter how genes behave. Factors like diet, smoking, exposure to toxins, physical activity and stress can affect the epigenome and impact your health.

Your lifestyle decisions can impact your epigenome

Until recently, scientists thought that diseases were caused mainly by changes in DNA sequence, bacteria and viruses, or environmental factors. Now, researchers know that changes in the epigenome can cause disease. For example, epigenetic changes can turn off a gene that normally protects you from diseases, like certain cancers — making you more susceptible to developing cancer later in life. Some of these changes are even passed from one generation to the next.

Epigenomics is helping researchers and doctors better understand the human body and improve human health. It's not always clear exactly which outside exposure affects a person's epigenome and the long-term effects. But as scientists learn more about the genome, it's becoming increasingly clear that your day-to-day actions and lifestyle decisions can determine how your genes are expressed — impacting your health.

© 1998-2019 Mayo Foundation for Medical Education and Research (MFMER). All rights reserved.

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