Gene

What is a gene?

Genes are segments of DNA that give your body the instructions for a specific characteristic (trait) or process. DNA is the molecules that make up those instructions. It’s packaged into your cells in chromosomes. If DNA is like a library and your chromosomes are books, your genes are like specific chapters in those books that describe you and how your body works.

When you hear something like “it’s in your genes,” most people think of having your mom’s smile or your dad’s eyes. But genes have many jobs beyond just the traits you can see. Your genes also give instructions for how your various body parts work and interact with each other. For example, genes write instructions not just for your hair color, eye color and height, but also how your muscles help you move and how your immune system responds to germs.

While experts don’t know the exact number, it’s estimated that humans have about 20,000 genes. This counts the genes that give your cells instructions (code) for making proteins. There are also non-coding sections of your DNA that have other jobs. There’s some debate about whether these should be considered genes.

Genes & alleles

Your chromosomes come in pairs, and so do your genes. You get one copy from your biological mother and one from your biological father. So, your two sets of genes aren’t an exact match. What’s more, when cells are dividing, chromosomes can swap parts (crossing over), so each of your chromosomes ends up being a unique mix of both of your parents’ genes — and completely unique to you. This is one of the reasons why most people don’t look exactly like either of their parents or even their siblings.

Different versions of a gene are called alleles. For instance, in a chapter about eye color, one allele might have the sentence for brown eyes and one might have the sentence for blue eyes. The alleles written in your DNA are called your genotype. The trait you actually end up having (in this example, brown eyes or blue eyes) is your phenotype.

How do genes work?

Genes give your body instructions to make proteins that create every part of your body and tell each part how to work. They tell your cell’s machinery:

• What amino acids (individual parts of proteins) to use to make a specific protein
• Where in the DNA to start making the protein
• Where in the DNA to stop making the protein

Genes can be turned on or off or send different instructions. This depends on where in your body they are or what’s happening in your body (for instance, if you’re injured or sick). Your environment can also influence what your genes do (called epigenetics).

Each gene is made up of a string of DNA. DNA has its own language it uses to tell your cells what protein to make. mRNA copies (transcribes) the instructions from the DNA for each gene. It brings the instructions out of the nucleus to organ-like structures (ribosomes) to read (translate). Each grouping of three letters of the DNA’s language (called a codon) tells the ribosome to add a specific amino acid. It can also tell the ribosome to start or stop making the protein. In this way, the ribosome speaks DNA’s language.

There are 20 amino acids that can string together in nearly endless combinations. That means our genes can code for an almost infinite number of traits and functions through proteins (scientists estimate there are over 100,000 types of proteins in humans).

Types of gene interactions (patterns of inheritance)

Genes and their alleles can interact with each other in different ways. These interactions are known as patterns of inheritance. If your cells have two different alleles for a given gene, what trait you’ll actually end up with depends on these interactions.

Mendelian inheritance

Mendelian genes have a dominant and a recessive allele. If you have one of each, or two dominant alleles, you’ll have the dominant trait. You’ll only have the recessive trait if you inherit a recessive gene from both of your parents. Dominant and recessive alleles are probably what most of us think of when we think of genetics — though, most human genes aren’t this simple to figure out.

Mendelian inheritance is named after Gregor Mendel, who noticed it when he did experiments with pea plants. An example of a Mendelian trait in humans is the type of earwax you have. Wet earwax is dominant and dry earwax is recessive.

Incomplete dominance

Sometimes, you might show a mix of the dominant and recessive alleles if you have one of each. This is called incomplete dominance. Curly hair patterns are often used as an example of incomplete dominance. If one parent has curly hair and another straight hair, you might end up with wavy hair.

Codominance

Codominant genes are neither dominant nor recessive to each other. For instance, A and B blood types are codominant. If you get an A from one parent and a B from another, your blood type will be AB. (But both A and B blood types are dominant to O blood type).

Polygenic inheritance

Polygenic traits are controlled by more than one gene, like mixing different amounts of ingredients in a recipe. They’re usually traits with many possible variations, like skin color or height.

Epistasis

Epistasis is when one gene can influence the function (expression) of different gene. A very simple example would be a gene that works like an on/off switch for another. Imagine a traffic light that has a green/red gene set to “green.” You’ll only see the light as green if a second, on/off gene is set to “on.” If the second gene is set to “off,” you won’t see the color at all. Epistatic genes are often polygenic as well — any number of genes influencing others determines the final result.

Some of the genes that determine eye color are epistatic. One gene (OCA2) codes for either brown or blue eyes. Another (HERC2) helps OCA2 produce the pigment that makes eyes brown. Even if your OCA2 genes determine that your eyes are brown, you’ll have blue eyes if your HERC2 gene isn’t working properly.

X-linked genes

X-linked genes are genes that appear on the X chromosome. Diseases caused by alleles of X-linked genes more commonly affect males because they only have one X chromosome. Females have two, so the other allele can protect them from having symptoms of the condition.

Pleiotropy

Genes that code for more than one trait are called pleiotropic. For instance, in people with albinism, a change in both alleles of a single gene leads to a lack of pigment in your hair, eyes and skin.

What’s the structure of genes?

Genes are made up of sections of DNA base pairs (molecules that carry DNA’s instructions). Each set of three base pairs codes for a specific amino acid used to make a protein. They also have trios of base pairs that say where the gene starts and stops. Genes are located on chromosomes in the nucleus of most cells in your body.

Gene changes and disease

Certain alleles that you inherit from your parents can cause a variety of health conditions. Examples include cystic fibrosis, sickle cell disease and Huntington’s disease. Examples of X-linked genetic conditions include hemophilia, red-green color blindness and Duchenne muscular dystrophy.

You can also get genetic diseases as you get older. For instance, changes in tumor suppressor genes, DNA repair genes and genes that control cell division (proto-oncogenes) can cause cancer or make you more likely to get cancer. In some cases, you can also inherit these changes. Examples include Li-Fraumeni syndrome, BRCA gene changes and Lynch syndrome.