Understanding Genetics: The Basics

Humans are made up of trillions of cells. Within most cells there is a nucleus where your genetic information resides and provides instructions for growth and development of those cells within the body.

Understanding Genetics - Figure 1Figure 1

Are chromosomes, genes, and DNA the same thing?

Chromosomes are structures that contain all of our DNA, encompassing all of the genetic material including all of the genes and all of the areas between the genes involved with controlling how those genes are turned on and off (and bits of DNA that we don’t completely understand as well). Within the DNA are genes, individual segments of DNA that "spell out" how to make all the different proteins (building blocks) that make up all of our parts. Genes, therefore, are the basic units of heredity. Chromosomes and genes can be thought of similar to a beaded necklace. The string represents the chromosome, and the beads represent the genes strung along the chromosome. Chromosomes and genes both contain genetic material, but they are not the same. 

In humans, we have genes as small as a few base pairs and genes as large as 2 million base pairs. Overall, the human genome is comprised of close to 30,000 genes.

Humans have 23 pairs of chromosomes (46 in total; 23 maternal chromosomes and 23 paternal chromosomes). They come in pairs because we inherit one set from each of our parents. The last set of chromosomes is referred to as the sex chromosomes because they determine whether you are biologically a male vs female. Chromosomes and genes cannot be seen with the naked eye, but under special microscopes chromosomes look like spaghetti noodles floating around the nucleus. The genes located on each chromosome are specific. For instance, the hemoglobin-beta gene is always found in a very specific place on chromosome 11.

Understanding Genetics - Figure 2https://www.genome.gov/genetics-glossary/Karyotype

Genes serve as blueprints for our body. They do not do much of the ‘work’ first-hand in the body, but they provide the instructions for making proteins and other elements needed for humans to function and survive. Most observable characteristics or traits (our phenotype) are shaped by the action of genes through the combination of alleles, versions of our genes, parents pass down (our genotype).

Inside each gene there are a set of important instructions. This set of instructions is your genetic code. Your genetic code consists of an arrangement of four nucleotides abbreviated as four letters: A, T, C, and G.  These four letters are arranged in groups of 3 to “spell” out different instructions required to make very specific proteins that have a specific ‘job’ to do in the body. Cells read these blueprints like we read a book or a webpage.   

As an example, the genetic code within the GP1BA, GP1BB and GP9 genes make a protein complex called glycoprotein (GP) Ib-IX found on the surface of platelets that plays an important role in helping platelets “hold hands” and stick together, helping to form blood clots. If there is a spelling change in the code (for example one of the Ts is switched to an A in just the right place in one of the genes), the protein may be made incorrectly or not at all. If one of these proteins are missing or abnormal, platelets cannot “hold hands” the way they should and don’t work well to plug up holes in blood vessels when bleeding starts. The result is abnormal bleeding, and a low platelet count in a particular type of inherited platelet disorder called Bernard Soulier Syndrome. Most spelling changes that occur (or “variants”) are silent and don’t cause any changes in the proteins at all. Only rarely does a spelling change alter how the protein works or cause it to not be made at all – and this type of variant, a “pathologic variant,” results in genetic disorders.

How can genes cause disease?

Genes provide codes not only for growth and development, but also for what illnesses we may develop. That is because sometimes our genes contain variants (also called mutations) that may predispose to certain health concerns.

A variant (or mutation) is an alteration in the nucleotide sequence of a DNA molecule and the primary cause of diversity among all living things. Some mutations may cause disease, others may provide a beneficial advantage, and some mutations are neutral and do not have any affect. Neutral mutations, also called polymorphisms, contribute to the natural variation in living things.

Where do we get our DNA from?

Our genome is created when a sperm cell unites with an egg. This is called fertilization. Sperm and eggs only contain one pair of all 23 chromosomes. They are referred to as haploid. When they come together during fertilization, the sperm cell and egg share their genetic material and become a diploid cell that is called a zygote. A zygote is simply a fertilized egg. Diploid means the fertilized egg has 23 pairs of chromosomes, essential for human development. The fertilized egg will continue to divide into multiple cells that will go on to form an embryo.

 Understanding Genetics - Figure 3https://biology-igcse.weebly.com/summary12.html


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