Genetics Principles: From DNA to Inheritance
Let’s talk about genes. Not the “jeans” that you wear, but “genes,” the stuff you inherit from your biological parents. What are these genes, though? And how do you actually inherit them? And is everything inherited? Also why do some people look so much like one of their parents and not at all like their other parent? Let’s start with the basics of genetics principles.
Each cell in every person’s body contains deoxyribonucleic acid, or DNA for short. DNA is an example of a polymer, or a large molecule that is made up of repeating units. Within each cell there is approximately 6 feet of DNA, but it is wound really tightly around proteins so that it can fit into each and every cell. Most cells (except for the cells involved in reproduction) have two copies of DNA, because each birth parent contributes one copy of DNA to their biological offspring.
Within the very large molecule of DNA, a small percentage of it (approximately 1%) tells the cell to make certain proteins. Once the cell makes these proteins, the particular actions of the proteins cause each person to develop certain traits, or characteristics that make them unique. The regions of the DNA that encode for the proteins that are responsible for these traits are called “genes,” and the field of science that studies genes is called “genetics.” The process and the rules by which genes are inherited are called “genetics principles.”
What kind of traits are we talking about? Well, all kinds, to be honest, but we will group the traits into three main categories:
- Physical characteristics, such as hair color, eye color, height, or weight;
- Non-physical characteristics, such as somebody’s intelligence or their impulsivity; and
- Future propensities, such as likelihood of somebody developing cancer, heart disease, or Alzheimer’s disease.
Because each person has two copies of DNA, the parts of the DNA that encode for each gene (called the “genotype”) have to work together to decide what traits the person will have. The trait that is ultimately expressed is called the “phenotype.”
What does it mean that two copies of the DNA work together? It turns out that there are a number of possible genetics principles that the two copies of the DNA can follow. We will briefly review five of the most common genetics principles, shown below.
5 Common Genetics Principles
1. The gene follows dominant/recessive inheritance patterns
In these kinds of cases, there are two possible types of genes, one of which is “dominant” over the other type, which is called “recessive.” One example of a gene that follows this kind of inheritance is the gene for freckles. If both copies of the DNA contain a gene that says, “Give this person freckles,” then you will have freckles. What if you have only one copy of the gene that says “Give this person freckles,” and one copy of the gene that says, “Definitely do not give this person freckles?” Here also, because the freckles gene is dominant, you will still have freckles. Only if both copies of the DNA have the gene that says, “Absolutely no freckles” – only then will you end up without any freckles.
If a gene is recessive, by contrast, then genetics principles state that in order to express the trait that is controlled by that gene, you need to have two copies of the gene. One example of this kind of trait is left-handedness: in order for someone to be left-handed, they need both copies of the gene to say, “Make this person left-handed.” For genes that are recessive, people can be “carriers,” which means that they can be carrying one copy of the recessive gene even though it isn’t expressed in any trait. Two right-handed people can have a left-handed child, if both of them are carrying the recessive gene for being left-handed.
2. Multiple types of genes can be expressed simultaneously
This kind of inheritance occurs in blood typing. As a reminder, people can have four different blood types: A, B, AB, and O. People with blood type A either have two copies of genes that say, “Produce antigen A on our cells,” or one copy that says, “Make antigen A” and one copy that says nothing.
Similarly, people with blood type B either have two copies of the genes for the B antigen, or one copy of the gene for the B antigen and one copy of the gene that says nothing. What happens if one copy of the gene says, “Make antigen A,” and the other copy says, “Make antigen B?” Well in that case, the genetics principles dictate that the person makes both antigens that appear on the outside of their cells, and their phenotype (the trait that we can observe) is that they have the blood type of AB.
3. One trait is encoded by multiple genes
One example of a trait that fits into this genetic pattern is a person’s height. This means that multiple genes found on multiple parts of the DNA are responsible for coding for the proteins that control how tall someone will be, and for each one of those genes, a person has two copies (one from each biological parent) that can be the same or different. What is the result of this kind of inheritance? It means that there is a wide range of possible heights that people can have, rather than certain defined heights.
In other words, whereas having freckles or not having freckles is a binary choice (you either have them or you don’t), the measurement of someone’s height has a much wider range of possible answers. This is why two tall parents usually have tall children, but the children’s height is only rarely exactly the same as the height of their parents.
4. Traits are only partially genetic
One example of a trait that has this kind of inheritance pattern is intelligence. Researchers think that the intelligence of each person is based partially on genetics, i.e., what genes they have inherited from their parents, although there is not a clear agreement on what percentage is actually genetic, nor have researchers been able to identify specific genes that are responsible. Rather, they think that multiple genes are each partially responsible for how intelligent somebody is, and that overall, genetics accounts for between 50-80% of a person’s overall intelligence.
What about the remaining 20-50%, though? Researchers think that this remaining percentage is determined by someone’s environment, i.e., how they spend their time, whether they participate in intellectually stimulating activities, and even what kinds of food they eat! This explains why most of the time intelligent parents also have intelligent children, but it also explains why there is wide variability in how closely a child’s intelligence mirrors the intelligence of their parents.
5. Traits are X-linked
Remember how we said that everyone has two copies of each gene, one from each biological parent? Well that is true for most genes, but not for all of them. More specifically, there is one set of chromosomes (a word that refers to how different parts of the DNA cluster together) for which people don’t necessarily receive two copies of each gene: the X and Y chromosome pair. This chromosome pair generally determines biological sex: if someone has two copies of the X chromosome, they are biologically female, and if they have one X and one Y chromosome, they are biologically male.
This differentiation occurs during reproduction: the egg always contributes an X chromosome, and the sperm contributes either an X chromosome (in which case the embryo has two copies of the X chromosome), or a Y chromosome (in which case the embryo has one X chromosome and one Y chromosome). How does this relate to genetics? Well there are a lot more genes on the X chromosome than on the Y chromosome, which means that people who have one copy of an X chromosome and one of the Y chromosome only receive one copy of all of these genes. This means that if there is a gene found on the X chromosome, the one copy that is found on the gene will determine the person’s trait, or phenotype.
Genetics Principles Example
One example of this is red-green color blindness. This gene is recessive and found on the X chromosome. Genetics principles dictate, therefore, that if someone has two X chromosomes, they both have to have the color-blind gene in order for the person to be color blind. If someone only has one X chromosome, however (and one Y chromosome), then they only need one copy of the gene to be color blind in order for them to have the trait of being color blind. This genetics principle explains why there are many more color blind men (approximately 1 in 12) than color blind women (approximately 1 in 200).
In summary, scientists are learning new things every day about how our DNA determines our unique traits, and what parts of those traits are inherited by our offspring, but the genetics principles that enable inheritance to occur have already explained a lot!
Author: Mindy Levine, PhD