by James Morris
Illustration by Talia Niederman
We are all, in a way, familiar with genetics. We know that children resemble their parents. We know that there are sometimes uncanny similarities among distant family members. And not a day goes by without some mention of genetics in the news – a gene is implicated in a disease; DNA testing is used to solve a crime; another genome is sequenced.
Yet we might struggle with certain details. What is a genome and why do we care about its sequence? What are genes and how do they relate to traits we see all around us? Why do some traits get passed on – brown eyes, red hair, high blood pressure – but not always, and sometimes in seemingly random ways?
All of your genetic material – your DNA – is contained in the nucleus of your cells. Cells are small, and the nucleus is even smaller. And yet it is within the nucleus that your genetic material is contained. It’s like the genie in Aladdin, who has “phenomenal cosmic powers” but “an itty bitty living space.” Your genetic material has enormous biological powers, but is packaged in the tiny space of a nucleus of a cell.
Today, we have a good understanding of genetics and powerful new tools to edit DNA, like CRISPR. Furthermore, we are learning that our experiences can not only affect us, but also be inherited by epigenetic mechanisms. In this way, the genie is truly out of the bottle.
Wish #1: We recognize that we are all mutants.
You inherit your genetic material from your biological parents. As a result, you have two copies of your genetic material in most of your cells. Each of these copies is a genome. So almost all of your cells have two genomes.
There are a few exceptions, like eggs and sperm, that have one copy, not two. This is because eggs and sperm are produced by a form of cell division (meiosis) that reduces the number of copies from two to one. Then, when an egg and sperm combine to form an embryo, the resulting cell has two copies again.
It’s this form of cell division that explains why you are genetically different from your siblings, even with the same parents. In the process of meiosis, the two copies recombine, and they recombine in a different way every time it occurs. As a result, all of the eggs or sperm from an individual are unique.
They are unique not just from one another, but also from all of the eggs and sperm in the world. Profoundly, they are different from all of the eggs and sperm that have ever existed in the entire history of life.
Then, when a sperm fuses with an egg, they create a unique embryo – also unique in the history of life. You are one-of-a-kind, a genetic instance.
The embryo is a cell that divides over and over to make the trillions of cells in your body. This type of cell division (mitosis) produces exact copies of your genetic material (except for rare mutations). The genomes in each of your cells are therefore more or less the same. This is why you can spit in a vial to obtain a sample of DNA. Your spit contains cheek cells, and each cell has two genomes, just like the ones in the other cells in your body.
DNA is composed of subunits (bases) repeated over and over. It’s the order of the bases that carries genetic information. So, when we “sequence a genome,” we are determining the order of the bases along a DNA molecule. In 2003, the human genome was completely sequenced.
When we say “the” human genome, we have to be a bit careful because, in fact, there is no such thing as the human genome. Humans, like all organisms, harbor lots of genetic differences (mutations). There are as many different human genomes as there are different people on the planet. In a way, then, we are all mutants.
Wish #2: We understand that we all have the same set of genes.
What then is a gene? Genes are stretches of DNA that carry out some function in a cell. Many genes code for proteins, which give a cell its structure and carry out much of the work of the cell.
All of us have the same set of genes. We may have different forms of these genes (alleles) due to mutations from one person to the next. For example, there are forms of genes that can predispose us to cancer, such as certain alleles of the BRCA1 and BRCA2 that increase the risk of breast and ovarian cancers.
Who has the BRCA1 and BRCA2 genes? The answer is that we all do – men and women, people with increased risk and people with decreased risk of cancer. The two genes are in fact essential genes; without them, we would not be alive. However, some people have particular mutations in these genes that make cancer more likely than in others; others don’t.
Wish #3: We won’t say “there is a gene for” common traits.
We have Gregor Mendel to thank for giving us an understanding of how these genes are inherited from one generation to the next. At the same time, the emphasis that we place on his studies sometimes has the unintended effect of reinforcing common misconceptions about inheritance.
Mendel worked with pea plants, focusing on traits like whether the pea is yellow or green, or round or wrinkled. By following one trait at a time and carefully counting the offspring of each cross, he was able to see patterns that up to that point were unclear.
He determined that there are factors responsible for the traits he studied. It’s these factors that today we call “genes.” He also realized that peas contain two copies of each gene, just like us. Different forms of these genes result in different traits, like yellow or green seeds.
Understanding Mendel’s studies helps us understand what genes are and how they behave. However, the traits that Mendel studied are much simpler than the ones we see all around us. Mendel studied traits that are influenced by a single gene. One form of this gene results in yellow seeds, and another in green seeds.
However, most if not all of the traits we are familiar with are not single-gene traits. Consider hair color, eye color, skin color, height, or weight – none of these results from variation in a single gene. Instead, they are influenced by many genes. Human height, for example, results from variation in thousands of genes.
But that’s not all. The environment also influences common traits. Human height, for example, is influenced by nutrition. As a result, it doesn’t make sense to talk about “a gene for” height because height is the result of variation in many genes interacting with the environment. The same is true for common traits we see all around us.
So, we shouldn’t say things like “there is a gene for” height, or for that matter high blood pressure, intelligence, or sexual orientation because, in fact, there isn’t.
© James Morris and Science Whys, 2017