young married couple with heart hands

Uniquely Human

The spiral, ladder-shaped molecule of DNA is a familiar sight. The sides are alternating phosphate and sugar molecules while the rungs consist of four amino acids known as nitrogenous base pairs; adenine bonds with thymine, and cytosine with guanine. The practically limitless sequencing of these base pairs not only results in the variety of plants and animals that exist in the world, it allows for variation within each species.

Long threads of DNA containing millions of base pairs are tightly coiled to form chromosomes. Humans have forty-six chromosomes, or more accurately twenty-three separate pairs, and we receive a complete set of genetic material from our parents at conception 

DNA diagram

Image credit: US National Library of Medicine

Almost every type of cell in the body contains a complete set of an individual’s DNA. As these cells grow and divide billions of times each day, the genetic material within the nucleus first has to make an identical copy of itself in a process known as DNA replication.

DNA located in the nucleus of a cell

How DNA Creates Diversity

The variation of traits within a species is caused by the activity of genes. Eye color, hair color, and blood type (though red and white blood cells don’t have a nucleus containing DNA) are examples of inherited traits that vary from person to person when a series of base pairs called a gene provides instructions for protein synthesis. Genes are simply a series of nitrogenous base pairs on a strand of DNA used as a template to build protein molecules that influence growth in muscle, bone, brain, skin, and hair cells.

If we receive a complete set of DNA from each parent, what causes a particular gene to be activated when every genetic trait has two possible outcomes?

Genes vary in length from hundreds of base pairs to hundreds of thousands. Since our DNA is from both of our parents, every genetic trait has two possible expressions depending on whether a gene is dominant or recessive. In this context, genes are referred to as alleles. Simply put, when a dominant allele is present for a specific trait, that particular gene will manufacture proteins that express the dominant trait. Conversely, if a dominant allele for a trait is missing and the individual instead has two copies of the recessive allele, that gene will make proteins that express the recessive trait.

But it’s sometimes more complicated than that. Whether an allele is dominant or recessive can also depend on the particular genetic trait being expressed. Most genetic traits require the activity of more than one allele, so the allele that codes for a protein that helps express that particular genetic trait would be dominant and the one that is less effective or could interfere with protein synthesis might be recessive.