We are complex in many ways, yet, often, people think about genetics in an over-simplistic manner: “one gene, one trait”. But reality is more complicated. In this blog we will go over common misconceptions that people usually have about genetics, explain why these misconceptions are wrong, and describe what scientists think about the biology now.
Going back to Mendel
“What is the gene for language?” It’s a question that geneticists often get asked. A big misconception about genetics is that for every trait, there is a single causal gene. It oversimplifies the relationship between traits and genetics down to one gene, one trait. This misconception stems from how we are first introduced to genetics in school, where we learn about Gregor Mendel and his experiments. Let’s have a look at why that leads to misunderstandings about how traits are inherited.
We need to go all the way back to the 19th century and introduce you to Mendel – a monk and biologist who conducted several experiments with peas and established the basic principles of genetics long before scientists knew that genes were even a thing.
Mendel’s experiments consisted of breeding different kinds of peas (watch this video for a refresher). For instance, in one of his experiments, he bred yellow-coloured peas with green-coloured peas. The first generation of cross-bred peas were always yellow, but in the second generation, the ratio was 3 yellow peas for every 1 green pea. Therefore, he concluded that traits (here, seed colour) are controlled by “factors” (genetic variants) that are transmitted from one generation to the next. And that one of the genetic variants is “dominant” over the other, so if the pea has the yellow-coloured genetic variant (i.e. dominant), it will always develop the trait.
One gene, one trait
Mendel’s experiments were right for some particular cases in genetics: monogenic traits. In a monogenic trait, one genetic variant in one gene changes the function of the gene and this causes the trait. If the trait is dominant, you display the trait when one copy of your gene has the variant. If the trait is recessive, you display the trait when both copies of the gene have the variant. You can check examples of these patterns following the link: https://medlineplus.gov/genetics/understanding/inheritance/inheritancepatterns/
An example of a dominant monogenic disorder is childhood apraxia of speech. Children with this very rare condition have difficulties with making the sequences of mouth and face movements that we use for articulating speech. Their speech can be very hard to understand for people around them. In 2001, researchers studied a large family where several members had this speech disorder and found they all had a variant in the FOXP2 gene. FOXP2 was the first gene linked with speech and language: a major scientific breakthrough.
But here’s the twist: recently, scientists found up to 30 more genes associated with childhood apraxia of speech. If a child carries a disease-variant in any one of these 30 genes, it will likely result in speech problems. Therefore, childhood apraxia of speech is a monogenic condition, as people carrying one causal variant will develop the speech disorder. But in different people, different genes can cause the same monogenic condition.
We are complex, and so is our DNA
Most traits are not monogenic and are rather caused by more than one genetic change acting together in combination. These are commonly called polygenic, complex, or multifactorial traits. Here, the combined effects of genetic variants in multiple genes and their interaction with the environment explain a person’s tendency to develop that trait. A lot of small genetic effects add up and you can have a little bit more or a little bit less, instead of all-or-nothing in a monogenic condition.
An example of a polygenic trait is dyslexia, a trait in which people have unexpected difficulties learning to read and/or spell despite having adequate opportunity and education. A study published in 2022 (analysing more than a million people) identified 42 separate positions in our DNA that are associated with the predisposition to dyslexia, each of which increases the likelihood of being dyslexic by a very small amount. Similarly, at school we are taught about traits that supposedly follow a straightforward inheritance pattern, but in reality are polygenic, such as eye colour, hair colour, whether you get dimples or not, attached earlobes, rolling your tongue, being left-handed, and many more.
Beyond the genes
When most people think about genetics, they think of “the genes”. And when people think of a genetic cause, they think of a mutation (i.e. a change in DNA) that is in a “gene”. But it is not always the case. We have about 20,000 genes in our DNA; however, they only make up 2 percent of our entire DNA code! What about the other 98 percent?
For a while, scientists thought this was “junk”, not used. Geneticists call it “noncoding DNA”, and we now know that it is essential for the correct functioning of our cells, providing them with an instruction manual for how and when genes themselves are switched on and off.
Interestingly, most of the genetic variants associated with polygenic traits are in this noncoding DNA. So, as you can imagine, it is usually not so straightforward to identify which gene is affected by a change in the DNA and how this ultimately affects a trait.
We are complex, and so is our DNA.
Writer: Lucia de Hoyos
Editor: Jitse Amelink, Else Eising
Translation Dutch: Anniek Corporaal
Translation German: Carmen Ramoser
Recommended readings
- “Understanding Genetics.” MedlinePlus, medlineplus.gov/genetics/understanding. Accessed 27 May 2024.
- Zimmer, Carl. “Seven Big Misconceptions About Heredity.” Medium, 23 May 2019, carlzimmer.medium.com/seven-big-misconceptions-about-heredity-42c94ba80365. Accessed 27 May 2024.
- Rutherford, Adam. “Rethink Everything We Know About Genes and Identity Politics.” TEDx Glasgow, 2018, https://www.youtube.com/watch?v=f55-ltaieV0&t=9s. Accessed 27 May 2024.
