I’m sure that most of you have heard of genes and DNA, maybe in the context of getting traits from your parents or maybe only in the classroom. Some of you may have even sequenced your own DNA through Ancestry or 23andMe!
Genes are located within DNA and they act as instructions (recipe cards) to make molecules called proteins. All people have the same genes (like baking a cake, you always need milk, eggs and flour) with very small variations in how the gene, or recipe, is written. These differences are determined by the arrangement of the small molecules (nitrogen bases) that make up DNA (deoxyribonucleic acid), and they contribute to the individual characteristics among humans (or whether we have a strawberry cake or a chocolate cake).
DNA is physically composed of these small molecules called nitrogen bases. There are four nitrogen bases: Adenosine (A), Guanine (G), Thymine (T), and Cytosine (C). Think of them as the letters that write the recipe. Every person has two copies of DNA and hence, two copies of every single gene. Each copy of the gene is called an allele. You receive one copy of DNA from your mother and one copy from your father. The copy of DNA you receive from each of your parents is completely random, which is one reason why siblings aren’t identical. The letters, or nitrogen bases, can be the same or different between the two copies of the gene.
If the alleles, or gene copies, are the same, the person is homozygous for that gene.
If the alleles, or gene copies, are different, the person is heterozygous for that gene.
Whether or not the copies of the gene match becomes really important if there is a mutation. A mutation changes the recipe, or the gene, so that the protein made is incorrect and nonfunctional. Mutations can occur in multiple ways:
→ Some of the letters could be deleted so the recipe can not be read fully and only parts of the protein can be made
→ Some letters could be inserted so there is an extra ingredient listed and the protein comes out in a different shape
→ Some letters could be replaced by other letters so the recipe is read incorrectly and the wrong protein is made
Mutations can result in genetic diseases. Some diseases will only show if the person is homozygous for the mutation, meaning both copies of the gene have the same mutation. In this case, the disease is said to follow an autosomal recessive pattern. When a disease is autosomal recessive, you will only show symptoms if you receive a copy of DNA from your mother that has the mutation and a copy of DNA from your father that has the mutation.
Other diseases can show even if the person is heterozygous for the mutation, meaning only one copy of the gene has the mutation. These diseases follow an autosomal dominant pattern. Now you will show symptoms even if you receive only one copy of DNA with the mutation, from your mother or from your father. With autosomal dominant diseases, if you are homozygous for the mutation (have the mutation on both copies of the gene), the symptoms are usually worse and can sometimes be lethal before birth.
Other traits beyond diseases can be inherited in an autosomal dominant or recessive fashion. Think of hair color. Have you ever heard the saying that you can only have a red-headed baby if there is a red head on both sides of the family? That is because red hair color is an autosomal recessive trait, meaning you have to inherit the red hair allele from your father and your mother to have red hair yourself.
The study of genes is broad. A current hot topic is genetic testing. Genetic testing can be performed in order to personalize medicine. For example, many cancer treatments are now based on the specific genetic mutation(s) that caused the tumor. By having drugs target exactly what went wrong in the cells, the horrible side effects currently seen with chemotherapies can be minimized and treatment can be more effective. With the field of genetics becoming so impactful in medicine, we hope that this post will help you gain a better understanding of some of the fundamental principles.
Thanks for reading and make sure to check back next week for the launch of our PROTEINS introductory post! In the meantime, let us know if you have any questions, comments or feedback and don’t forget to follow/ like us!
Written by Annah
Illustrated by Rhea
Disclaimer: We are not medical professionals. This post is not meant to be used as a diagnostic tool or as medical advice. Further, the opinions in this post are our opinions and in no way reflect the opinions of our mentors or Medical University of South Carolina.