Introduction to Genetics

Your Cells Contain Your DNA

Your body is made of trillions of building blocks called cells, the same way a house is built from bricks. There are many types of cells: brain cells, heart cells, skin cells, and more!

Almost every cell has DNA, or deoxyribonucleic acid. Most cells have the same DNA but each type of cell "reads" different parts of it. DNA tells the cell how to develop and function.

Most DNA is stored inside a compartment in the cell called the nucleus, similar to an egg yolk inside an egg. There is also some DNA housed outside the nucleus, in structures called mitochondria. All of your DNA together is called your genome.

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DNA is Packaged in Chromosomes

Altogether, the DNA inside a single human cell is about six feet long. In order to fit inside a cell, DNA is coiled up tightly and divided into sections called chromosomes.

You inherit one set of 23 chromosomes from your dad and one set of 23 chromosomes from your mom — for a total of 23 pairs of chromosomes.

Organisms vary greatly in their number of chromosomes. For example, cucumbers have 7 pairs, chimpanzees have 24 pairs, and giant salamanders have 30 pairs.

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The Sex Chromosomes Determine Genetic Sex

The 23rd pair of chromosomes, called the sex chromosomes, are different from the other 22 pairs. There are two kinds of sex chromosomes, the X chromosome and the Y chromosome. Having two X chromosomes (XX) makes you genetically female. Having an X chromosome and a Y chromosome (XY) makes you genetically male.

How does that work? You inherited one of your mom's two X chromosomes. And you inherited either your dad's X or Y chromosome. If you inherited his X chromosome, then you have XX chromosomes and are genetically female. If you inherited his Y chromosome, then you have XY chromosomes and are genetically male. Sometimes, a person's genetic sex, the sex assigned at birth, and/or the deeply-held sense of gender are not all the same.

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The DNA Cookbook

If all your DNA (your genome) is a cookbook, then your genes are the recipes. Genes are sections of DNA that contain instructions for making proteins and RNAs. Human DNA contains around 20,000 protein-coding genes and likely thousands of RNA genes.

Proteins do most of the work in the cell. They carry out tasks necessary for building and maintaining the cell, like transporting oxygen, detecting invading bacteria, forming the structure of your hair, and countless other jobs.

RNAs are molecules similar to DNA. They help build proteins and regulate when and where different proteins are made.

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How Do Genes Code for Proteins?

The process of building a protein using the DNA code happens in two steps called transcription and translation. During transcription, the cell creates a temporary copy of the gene called messenger RNA. During translation (as shown), the cell follows the gene instructions contained in the messenger RNA to link together a chain of small molecules, called amino acids.

There are 20 different amino acids, each with unique properties. The order in which they are combined determines the properties of the protein. A vast array of proteins are possible, and they can move, interact with other proteins, and change shape to do their jobs.

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Not All DNA Codes for Proteins

Only a small part of your DNA contains the genes that code for proteins.

Some DNA contains regulatory elements, which are stretches of DNA that regulate how and when the cell should "read" different genes.

Some DNA codes for different kinds of RNAs that play a variety of roles in the cell. And some DNA has no known function.

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The DNA Structure Is a Double Helix

DNA is shaped like a long twisted ladder, or a double helix. Each rung of the ladder is made of two molecules called bases, forming a base pair. There are four types of DNA bases: adenine (A), thymine (T), guanine (G), and cytosine (C). The bases always pair up according to these rules:

• A pairs with T
• C pairs with G

The two vertical sides of the ladder are made of alternating sugar (S) and phosphate (P) molecules. The combination of a sugar, a phosphate, and a base is called a nucleotide.

These nucleotides are connected together into long DNA sequences. We have about 3 billion nucleotides in our genome.

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DNA Differences Make Us Unique

It's been estimated that our DNA sequences are about 99.5% identical. But there are parts of our DNA that vary from person to person.

These DNA differences are called genetic variants. Some genetic variants have no effect. Others influence traits and health — like your hair color, weight, and risk of certain diseases.

A genetic variant can influence your traits by changing how a protein gets built, which changes how that protein does its job. For example, certain genetic variants cause proteins inside the hair follicles to produce more red pigment molecules — leading to red hair.

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What Are Genetic Variants?

The most common type of genetic variant is when people have different base pairs (DNA letters) at one particular place in their DNA. For example, at the same place in a gene, one person might have an A (adenine), while another person has a C (cytosine). These differences are called single nucleotide polymorphisms, or SNPs (pronounced "snips").

Other examples of genetic variants include:

• Deletion: base pair(s) have been removed
• Insertion: base pair(s) have been added
• Duplication: an extra copy or copies of a segment of DNA have been added
• Translocation: a segment of DNA has changed location

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How New Genetic Variants Arise

You inherit genetic variants from your parents, but new variants can also appear in your DNA. They can be the result of damage to DNA, or a side effect of normal cell processes like making new copies of DNA.

Usually new genetic variants are corrected by "DNA repair" proteins in the cell. Most uncorrected DNA segments are not harmful. In fact, they typically have no effect at all.

Occasionally, a new genetic variant can lead to a new trait — for example, by changing the function of the protein that that gene codes for.

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New Genetic Variants Can Become More Common Over Time

If a new genetic variant results in a new trait that helps living things survive and reproduce, that variant and trait can become more common over time.

For example, research suggests that most ancient humans were lactose intolerant, meaning they could not digest the sugar lactose in dairy products like cheese, milk, and yogurt, as they grew older.

But several thousands years ago, in some regions of Europe and Africa, humans started domesticating cows, goats, and sheep and depended on the animals' milk for nutrients. New genetic variants appeared by chance that gave these groups of people the ability to digest dairy into adulthood. This helped them adjust to their new diet, and these genetic variants became more common.

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One Trait, Many Genes

Oftentimes people think that a single gene or genetic variant determines a trait. Sometimes that's true, but it's usually more complicated than that. Take eye color, for example: many genes play a role in creating the color pigments in the eye, and genetic variants in any of them can affect your eye color.

This can also be true for diseases. For example, dozens of common genetic variants are linked to developing type 2 diabetes. Alone, each of them only have a tiny impact on risk of developing type 2 diabetes, but together they can add up to an overall higher risk.

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Lifestyle, Environment, and Your Genes

Genetics can tell you a lot about yourself, but it isn't the only factor to consider.

For almost all human traits, factors like lifestyle and environment play a role along with genetics. Your height, for example, depends on the effects of many genetic variants as well as environmental and lifestyle factors like your diet.

This means you can have genetic variants that are linked to being tall, but if you have poor nutrition growing up, you may end up being shorter than other people with those same genetic variants.

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Likelihood and Risk in Genetics

Even when we know multiple factors influence the development of a trait (like genetics, lifestyle, and environment), it's impossible to know for sure how things will turn out for a given person. That's why genetic associations are stated in terms of likelihood or risk.

For example, in one study, researchers found that 30% of male participants with certain genetic variants experienced complete hair loss or balding, while 70% did not.

This means that men with those same genetic variants have about a 30% risk of experiencing complete balding. You could also say that they have a 70% chance of not experiencing complete balding.

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How Scientists "Read" DNA

In the 1990s and early 2000s, scientists mapped the complete sequence of the human genome for the first time. Those data helped scientists learn about how thousands of genes function in the body, and how certain genetic variants impact human traits, health, and disease.

Scientists can "read" DNA with techniques called "sequencing" and "genotyping."

Sequencing determines the exact DNA letters and their order in a stretch of DNA.

Genotyping looks at DNA letters of interest at specific locations. Genotyping is the technology that 23andMe uses.

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Summary

• Your body is made of trillions of cells.
• Almost all of your cells contain a complete copy of your DNA.
• Your DNA are organized into 23 pairs of chromosomes.
• Genes are sections of DNA that contain the instructions for making things like proteins and RNAs.
• Proteins do important jobs in your cells, like providing the structure and function for your cells.
• Genetic variants, or DNA differences, can cause people to have different versions of a protein. Having different proteins can result in having different traits.
• Most of your traits are shaped by many factors, including your genetic variants, your environment, and your lifestyle.

Discovering more about your genetics can be one of the most exciting ways to learn more about yourself. Now that you have learned these basic DNA concepts, you're well on your way to exploring even more about yourself through your DNA!