So now the question arises, what is the origin of genetic – that is, inheritable-variation? How do genotypes change? As a simple (and not completely incorrect) analogy, we can think of an organism’s genotype as a book. This book is also known as its genome (not to worry if this seems too simple, we will add needed complexities as we go along). An organism’s genome is no ordinary book. For simplicity we can think of it as a single unbroken string of characters. In humans, this string is approximately 3.2 billion characters (or letters) long (~3,200,000,000). In case you are wondering, a character corresponds to a base pair within a DNA molecule, which we will consider in detail in Chapter 7. Within this string of characters there are regions of what look like words and sentences, that is, regions that look like they have meaning. There are also long regions that appear to be meaningless. To continue our analogy, a few critical changes to the words in a sentence can change the meaning of a story, sometimes subtly, sometimes dramatically, and sometimes a change will lead to a story that makes no sense at all.
At this point we will define the meaningful regions (the words and sentences) as corresponding to genes and the other sequences as intragenic regions, that is, spaces between genes. We estimate that humans have ~25,000 genes (we will return to a molecular level discussion of genes and how they work in Chapters 7 through 9). As we continue to learn more about the molecular biology of organisms, our understanding of both genes and intragenic regions becomes increasingly sophisticated. The end result is that regions that appear meaningless can influence the meaning of the genome. Many regions of the genome are unique, they occur only once within the string of characters. Others are repeated, sometimes hundreds to thousands of times. When we compare the genotypes of individuals of the same type of organism, we find that they differ at a number of places. For example, over ~55,000,000 variations have been found between human genomes and more are likely to be identified. When present within a population of organisms, these genotypic differences are known as polymorphisms, from the Latin meaning multiple forms. Polymorphisms are the basis for DNA-based forensic identification tests. One thing to note, however, is that only a small number of these variations are present within any one individual, and considering the size of the human genome, most people differ from one another less than 1 to 4 letters out of every 1000. That amounts to between 3 to 12 million letter differences between two unrelated individuals. Most of these differences are single characters, but there can be changes that involve moving regions from one place to another, or the deletion or duplication of specific regions. In sexually reproducing organisms, like humans, there are typically two copies of this book in each cell of the body, one derived from each of the organism’s parents - organisms with two genomic “books” are known as diploid. When a sexual organism reproduces, it produces reproductive cells, known as gametes: sometimes these are the same size. When gametes differ in size the smaller one is known as a sperm and the larger is known as an egg. Each gamete contains one copy of its own unique version of the genomic book and is said to be haploid. This haploid genome is produced through a complex process known as meiosis that leads to the significant shuffling between the organism’s original parental genomes. When the haploid sperm and haploid egg cells fuse a new and unique (diploid) organism is formed with its own unique pair of genomic books.