Cells are the basic building blocks of all living things. There are trillions of cells in the human body, almost all of which contain a chemical molecule that carries hereditary material. This chemical molecule is called deoxyribonucleic acid, or DNA. The DNA molecule is usually found in a cell’s nucleus, which acts as the command center for the cell, telling the cell when to grow, divide, or die. Information contained in the DNA molecule tells cells how they should function. This information is written in a language that cells can understand. The alphabet for this language contains only four letters: A, G, C, and T. The letters stand for four chemicals contained in the DNA molecule: adenine (A), guanine (G), cytosine (C), and thymine (T).

Human DNA contains approximately 6 billion of these letters, called nitrogen bases. Th e order of the bases in the DNA molecule determines an organism’s traits, in the same way that different orders of letters make up different words and different sequences of words make up different sentences. More than 99% of this sequence is exactly the same in all humans. The differences in the DNA sequence are what make each human unique.

During reproduction, DNA is passed down from parent to child. Parts of the DNA molecule contain sequences of bases that tell the cell how and when to make proteins, and which proteins to make. Much of the work that needs to be done in order to keep the human body functioning correctly is carried out by proteins. The parts of the DNA sequence that contain the instructions on how and when to make a protein are called genes. Some genes carry a code to tell a cell how to make a particular protein. Other genes tell the cell when to make them.

In humans, genes can range in size from a few hundred to more than 2 million nitrogen bases. Scientists estimate that humans have between 20,000 and 25,000 genes in their genomes. A genome is the collection of all of the genes in an organism. All cells that contain DNA contain a complete genome, but some genes are switched “on” while others are switched “off” in certain cells. The genes that are active contain all the information the cell needs to make the specific proteins needed for that cell, organ, or tissue to survive and function the way it should.

Every person has two copies of most genes in their genome. One copy of the gene comes from their mother, and the other one comes from their father. The two copies of the genes are not exactly the same. They contain small changes in the sequence of DNA bases. These two different versions of the same gene are called alleles. The small differences in the alleles cause offspring to resemble their parents without looking exactly the same as either parent.

The DNA molecule is very long and thin. Almost all cells in the human body contain a piece of DNA that is approximately one meter long. A cell’s nucleus, however, is usually only about six micrometers across. Trying to stuff this much thin, and breakable, genetic material into the cell’s nucleus would be difficult. Instead, the DNA molecule and the genes it contains are wrapped around special proteins called histones, like a piece of string wrapped around a spool. The DNA molecule is too long to be wrapped around only one histone, though, so it is wrapped around several, making what looks like a string of beads. This string of beads is condensed into compact structures called chromosomes.

Chromosomes are made up of thousands of genes. Humans have 46 chromosomes, in 23 pairs. During a special type of cell division, called meiosis, the chromosome number from each parent is divided in half so there are 23 chromosomes in the mother’s egg and 23 chromosomes in the father’s sperm cells. When egg and sperm meet during fertilization, the 23 chromosomes from the mother and 23 chromosomes from the father combine, making 46 chromosomes for the resulting embryo. Because chromosomes come in pairs, so do genes. The two different copies of the genes are alleles.

The alleles that are passed down from parent to child can be dominant, recessive, or a combination of the two. Dominant traits can mask (cover up) the appearance of a recessive trait in the offspring. In order for a recessive genetic trait to be expressed in the next generation, both parents must pass on the gene for that trait. A dominant genetic trait, on the other hand, will be expressed if either mom or dad, or both, pass on the allele for the trait. Take the trait for dimples, for example. To have dimples is a dominant trait. To not have dimples is a recessive trait. On paper, geneticists show that a trait is dominant by using uppercase letters. They might use the letter “D” to represent dimples, for example. Recessive traits are indicated by lowercase letters (d). Therefore, if a child has no dimples, both parents must carry the no-dimple allele and pass those alleles on to the child. This makes the child homozygous for this trait because both alleles are the same (dd). But what if mom has dimples and dad does not? Will the child automatically have dimples? It depends. If dad does not have dimples, then he must be homozygous for the no-dimple trait because the no-dimples trait is recessive. It is possible that mom is homozygous for the dominant dimples trait. If this is the case, all of the couple’s children will have dimples because mom’s dominant allele will mask dad’s allele and allow the trait to be expressed.

There is another possibility, though. Mom could be heterozygous for the dimple trait. Heterozygous means that mom has two different alleles for the dimple trait—one dominant and one recessive (Dd). Mom can pass on either the dominant or the recessive gene to her child. If she passes on the dominant one (and dad passes on one of his recessive alleles), the child will have dimples and be heterozygous for the dimple trait. If she passes on the recessive allele to the child, the child will not have dimples and be homozygous for the recessive no-dimple trait.

An organism’s outward, physical appearance is called its phenotype. Dominant traits, such as dimples or a bent pinkie, for example, will show up in someone’s phenotype. Recessive traits show up only if the person is homozygous for the recessive trait. Phenotype is determined by the person’s DNA code, which makes up an organism’s genotype. Even if a person does not show a genetic trait, such as a curved thumb or a straight hairline, it does not mean that an allele for these traits does not exist in that person’s genotype. If they are heterozygous for a recessive trait, the trait will be masked by a dominant allele and will not show up in their phenotype. However, they can still pass that allele down to the next generation, where it may or may not be expressed. This explains how recessive traits continue to be passed on. It also explains why some people are said to look more like a grandparent or a great aunt or uncle than they look like their parents.