You have approximately 30 trillion cells in your body, and each one has a copy of your DNA. DNA also makes you unique among the 108 billion people who ever lived. It is not responsible for every trait you have.

For example, think of how identical twins tend to have differentiating physical and features, especially as they age. Still, the development of traits in almost all other life on Earth relies heavily on DNA.

DNA contains several important components, but one of the most important is the gene. Variations of genes are called alleles. A wild-type allele is one that is more common in a population of a species and is considered a “normal allele,” while uncommon alleles are considered mutations.

During sexual reproduction, offspring inherit half of their DNA from each parent. For each gene, they have one allele from each parent. Sometimes they are the same allele, which means that a gene is homozygous. If they are different alleles, meaning that the gene is heterozygous, one of them may be dominant.

In that case, the dominant trait will be the one expressed in the offspring’s phenotype, or outward characteristics. Recessive alleles must be homozygous in order for their trait to appear in the individual’s phenotype.

With the exception of some unicellular organisms, DNA is typically stored in the nucleus. Most of the time, DNA coils extremely tightly around scaffolding proteins called histones until it forms a ribbonlike structure called a chromosome.

Genes are lengths of the DNA double helix contained in chromosomes, and they vary greatly in size. When the double helix is flattened, it resembles a ladder; each rung is composed of two bonded molecules called nucleotides.

The four nucleotide bases in DNA are adenine (A), thymine (T), guanine (G) and cytosine (C). A and T only bond with each other and G and C only bond with each other. A bonded with T or G bonded with C are called base pairs. A human’s single gene may contain several hundred base pairs or more than 2 million base pairs.

Despite the fact that during most stages of the cell cycle, chromosomes are too small to see even with the highest-powered microscope, human chromosomes each contain between 20,000 to 25,000 genes.

Humans all share more than 99 percent of their genes. In other words, all of the genetic variability that makes one person different from everyone else happens in less than 1 percent of the human genome. The rest is identical.

Gregor Mendel was a 19th-century Austrian monk and botanist. He is commonly known as the “father of genetics” for the magnitude of his inferences about heredity.

Mendel experimented with pea plants in the garden of his abbey. He observed several traits that seemed to be inherited. By breeding plants with specific phenotypes and then crossbreeding their offspring, Mendel discovered that something lay beneath the surface – what is known today as genotype.

Mendel observed that if he bred plants with yellow seeds with plants with green seeds, the first generation of offspring all had yellow seeds.

If he crossbred those offspring with each other, though, the second generation of offspring always had the same result: 75 percent of them had yellow seeds, but 25 percent of them had green seeds, even though the generation before had all been plants with yellow seeds.

Repeated iterations of this crossbreeding experiment yielded the same results again and again: 75 percent were yellow and 25 percent were green. Mendel theorized that plants with two alleles for yellow had a phenotype of yellow seeds, and so did plants with two alleles where only one was yellow.

The only proportion of the offspring that were not yellow was the one-quarter with two green alleles. Without a dominant yellow allele to mask the green alleles, the seeds were green.

Mendel intuited that the trait for yellow seeds was dominant over the trait for green ones. It followed that these offspring had one allele (the term “allele” was coined after Mendel’s death) for yellow and one for green, although this was purely theoretical for Mendel; he used mainly probability math to explain the offspring ratios because he lacked any scientific equipment or knowledge of DNA.

Punnett squares are a useful way to represent Mendelian inheritance. The visual representation makes it easier to understand how recessive alleles can be masked by dominant traits. For assistance working with Punnett squares, see the link in the Resources section.

Punnett squares are more complicated in cases of incomplete dominance. This is when one allele is only partially dominant over the other allele.

For example, a snapdragon with one allele for white petals and another allele for red petals has pink petals. Neither the red allele nor the white allele are dominant, so they are both partially expressed.

In cases of co-dominance, two alleles are dominant at the same time. An example is the human AB blood type.

There are three potential alleles for blood groups: A, B and O. A and B are dominant and cause an A or B protein (respectively) to bind to red blood cells, while the O allele is recessive and causes no protein to bind. The A or B blood types occur from AA, AO, BB or BO allele pairings, respectively. The O type is from OO.

When someone has the AB blood type, their alleles are co-dominant because their blood cells have both A and B proteins bound to them.

Some human examples of recessive traits are earlobes that are attached to your head, or the ability to curl your tongue. Recessive alleles often lead to reduced function or loss of function. Albinism, for example, is an inherited condition in which the body produces very little melanin. Melanin is a molecule that provides pigment in skin, hair and eyes.

Blue eyes are another example of a recessive trait with reduced melanin. Blue eyes have very low levels of melanin in the iris and stroma. The blue appearance comes from the refraction of light through the eye. Eye color is controlled by more than one gene, but brown eyes are determined by a single allele on one gene, since it is dominant, and that is all it takes.

Since people with blue eyes must have two blue-eye alleles (recessive alleles are expressed in lowercase letters, which are bb in this case), it seems more likely for the majority of any given population to have brown eyes. This is true in most parts the world, but in some countries, blue eyes are most common.

This is especially true in Scandinavian and northern European countries; while approximately 16 percent of the United States and Spain have blue eyes, 89 percent of both Finland and Estonia have blue eyes.

How is it possible for a recessive phenotype to be more common than a dominant phenotype? It depends on the trait, and there are many environmental factors.

The majority of the people in Finland, for example, are Caucasian and blue-eyed, and even a small number of brown-eyed people having children with blue-eyed partners and having brown-eyed offspring will not significantly change the balance in the population.