It’s in the genes. Height, sense of humor, or athletic skill are all traits we’ve heard as being in the genes. But what are genes really, and how do they influence who we are, what we look like, and how we function?
What Is A Gene?
What is this thing that can so hugely influence so much of our and other animals’ and plants’ very essence? Most simply, a gene is a specific area of DNA on a chromosome that codes for the production of certain proteins that influence a particular trait (e.g., hair color). However, to understand what a gene is, some understanding of basic genetics is necessary, including what DNA, chromosomes, and traits are. To do that, we’ll take a look at the story of a baby girl named Emma.
Emma started off as a healthy and vibrant infant. However, after six months, she began crying inexplicably. Her parents wrote it off as a baby being a baby. But then, she started showing a yellowish hue. Her hands and feet started feeling cold. She cried more and more often, and more intensely. Her parents began to get worried; this seemed like much more than a baby being a baby. Emma’s pediatrician ran a simple blood test that confirmed Emma suffered from a disease called sickle cell anemia.
”Sickle-cell anemia?!? How do we get rid of it?” her mom asked.
”I’m sorry,” the doctor said. ”This is a genetic disease. It’s not caused by germs. We can only treat the symptoms.”
”Because Emma’s DNA tells every one of her cells to make a protein that causes her red blood cells to not work correctly.”
”Well, fix it!”
”I’m so sorry. DNA just does one thing. It tells the body’s cells which proteins to make. And in Emma’s case, those instructions tell her to make the protein that causes sickle-cell anemia. And since every one of her cells has these instructions, there is not much we can change.”
By looking at Emma’s story, we can better understand what genes actually are.
DNA ; Chromosomes
Deoxyribonucleic Acid (DNA) is a chemical compound found in all eukaryotic cells that tells the cell’s machinery to produce specific proteins. It has a ‘code’ that gets translated into proteins. Every one of Emma’s cells has the exact same DNA as every other cell.
Each one of Emma’s cells has long strands of DNA. The structure by which her DNA is organized is a chromosome. Different organisms have different numbers of chromosomes. For example, humans (including Emma) have 23 pairs of chromosomes (human chromosomes number 1-23), while onions have eight pairs of chromosomes (onion chromosomes number 1-8). Emma got half of each chromosome pair from her mother and the other half from her father. Scientists have figured out which chromosomes code for particular traits. You may have guessed that the areas on chromosomes that code for particular trait-influencing proteins are what we’re talking about here: genes!
A trait is simply a characteristic of an organism. Some of Emma’s traits are easily identifiable and entirely heritable (e.g., hair color and sickle cell anemia), while some are only partially heritable (e.g., body weight or predilection for heart disease). Emma’s genes are the specific sections of specific chromosomes that entirely or partially determine Emma’s heritable traits by coding for the production of those proteins that influence the expression of her traits.
Monogenic and Polygenic Traits
It is estimated that humans have between 20,000 and 25,000 genes in their 23 chromosome pairs. Interestingly, less than 2% of the DNA on chromosomes are part of identified genes. The Human Genome Project is an attempt to map those genes so that their impact can be better understood.
Human chromosome 11 provides a good example to describe the relationship between genes, traits, and chromosomes. It is also where Emma’s sickle cell anemia comes from. It is estimated that human chromosome 11 contains nearly 1,500 genes, coding for the proteins that influence many different traits, including sickle-cell anemia and many of the olfactory (smell) receptor genes in humans.
Sickle-cell anemia is an example of a monogenic trait, a trait that is influenced by a single gene, the HBB (hemoglobin, beta) gene on chromosome 11. This gene usually codes for production of the protein beta-globin. However, Emma’s HBB gene codes for the production of an abnormal version of beta-globin called hemoglobin S, the protein that causes sickle-cell anemia. Different alleles (variants) for the HBB gene can also code for the production of other proteins that cause other disorders.
In contrast, polygenic traits are influenced by multiple genes that are often on entirely different chromosomes. There are many polygenic traits, and some examples of these in humans include height and skin color.
Alleles & Gene Therapy
It is a misnomer to say, ‘Emma has the sickle cell gene.’ All people have the area of chromosome 11 (i.e., ‘the gene’) that codes for the proteins that determine whether or not someone has sickle-cell anemia. However, genes have different variants, called alleles. It would be more accurate to say, ‘Emma has the alleles for sickle-cell anemia.’
One potentially significant method to take advantage of how genes operate – coding for certain protein production – is gene therapy. In gene therapy, viruses that normally do their dirty work by inserting themselves into normal cells’ DNA machinery to hijack some of the cell’s ‘normal’ protein production, have their nasty virus DNA cleared and have desirable DNA (i.e., a gene allele) inserted. The virus with the desirable DNA is injected into target cells, which the now-desirable virus ‘hijacks,’ inducing the production of desirable proteins. Gene therapy is still in its infancy, but is looked to as a potential therapy for all kinds of diseases and genetic disorders, including cancers.
Genes are specific areas of DNA on a chromosome that code for certain protein production that influences particular traits. Those proteins influence a huge number of organisms’ characteristics – from their physical appearance to their susceptibility (or resistance) to disease. Humans have at least 20,000 known genes, and scientists’ understanding of the mechanism of operation is evolving at a rapid rate. One way scientists are leveraging their increased understanding of genes is through the use of gene therapy.