What is a Gene?

A gene is basically an instruction for the human body. Each gene has a specific purpose, and every single function of the human body is coded for by one or more genes.

Genes and DNA

The genes are segments of deoxyribonucleic acid (DNA). DNA is sometimes called the “language” of genetics. The body reads the DNA that makes up the genes and is thus able to grow, develop, and function. DNA even has its own alphabet, which has only 4 letters: A, C, G, and T. Those 4 letters, in different combinations, spell out all the different genes and code for all their different functions. Each letter represents a different base of a molecule called a nucleotide. Each gene is a specific sequence of nucleotides. The A stands for “adenine,” the C stands for “cytosine,” the G stands for “guanine, and the T stands for “thymine.”

DNA exists in the form of a double helix. A double helix resembles something like a twisted ladder. If the ladder was “untwisted” and laid out flat, it would look something like this:

On top is one strand of DNA. On the bottom is its complementary strand. A strand of DNA and its complementary strand are composed of nucleotide base pairs. The base pairs always match up according to these two rules: 1) adenine is always with thymine, and 2) cytosine is always with guanine. So, if only one strand was available, a person would know what the complementary strand would look like since DNA always follows its base-pairing rules.

Genes and Chromosomes

The genes are found on structures called chromosomes. Most cells in the human body have 23 pairs of chromosomes, or 46 altogether. Each pair of chromosomes is unique in that it carries its own specific genes. A gene for eye color may be found on one pair, while a gene for height may be on another. Since the chromosomes are in pairs, the genes are as well. Each gene in a pair has the same overall function, but there might be different versions of that gene on each chromosome. Different versions of a gene are called alleles . For example, the picture below depicts a chromosome pair which carries a gene pair with different alleles.

The genes on both chromosomes have the exact same function: to determine this person's eye color. However, there are two different alleles present. The allele on the chromosome on the left codes for brown eyes. The allele on the chromosome on the right codes for blue eyes. There is only “one gene” (the gene for eye color) present, but there are two different alleles. With every gene pair, there will be two alleles. Those alleles may be identical (e.g. both for brown eyes), or they may be different, as in this example.

The genes on both chromosomes have the exact same function: to determine this person's eye color. However, there are two different alleles present. The allele on the chromosome on the left codes for brown eyes. The allele on the chromosome on the right codes for blue eyes. There is only “one gene” (the gene for eye color) present, but there are two different alleles. With every gene pair, there will be two alleles. Those alleles may be identical (e.g. both for brown eyes), or they may be different, as in this example.

Dominant and Recessive Genes

In the example above, there are two alleles for eye color. One allele tells the body that the eyes should be brown, and the other allele tells the body that the eyes should be blue. Which allele does the body pay attention to? The answer to that question depends on which allele is dominant. A dominant allele is one which is expressed even if only one copy is present; it “dominates” its partner allele. A recessive allele, on the other hand, requires two copies to be present in order to be expressed. A recessive allele is not expressed if it is partnered with a dominant allele. In our example, the allele for brown eyes is dominant, and the allele for blue eyes is recessive. The dominant allele, the one for brown eyes, is the one that will be expressed; the individual will have brown eyes. A person would also have brown eyes if the two alleles were identical and both coded for brown eyes. For this person to have blue eyes, both of the alleles would have to code for blue eyes.

Homozygous vs. Heterozygous

In the example of eye color above, the two alleles for the gene are different from one another. One is for brown eyes, the other is for blue. When a person has two different alleles for a trait, it is said that they are heterozygous for that trait. Heterozygous simply means having two non-identical alleles at a given locus (i.e. location on a chromosome). If a person has two identical alleles for a particular trait, that person is said to be homozygous. A person can be homozygous dominant or homozygous recessive. A person who is homozygous dominant for a certain trait has two dominant alleles, like two alleles for brown eyes. A person who is homozygous recessive for a certain trait has two recessive alleles, like two alleles for blue eyes.