Human DNA and Genetic Diversity
Small differences in DNA among humans can give rise to differences in traits. A gene is a segment of DNA that codes for a specific trait. For example, a person with blood type A has a gene that codes for the production of the blood surface protein A. A person lacking this gene would not produce A blood protein. On each of our 23 chromosomes, we have many different genes, each coding for different proteins and resulting in a variety of traits. An individual’s complete set of genes is referred to as their genome.
DNA is used to study the origin of human characteristics, variation among human beings and the relatedness between humans and other species. From 1990-2003, scientists worked to sequence the human genome, as part of the Human Genome Project (https:// www.genome.gov/12011238/an-overview-of-the-human-genome-project/). The project, led by the National Institute of Health, was based on collaborative research, with contributions by scientists from China, France, Germany, Japan, Spain, the U.K. and the U.S., among others. After collecting DNA samples from many individuals, researchers sequenced the DNA of a subset of individuals and patched it together to create a mosaic of the human genome.
This complete human genome is about 3 billion base pairs long and is freely available for public and research use. The identities of the individuals whose genes were sequenced was not revealed to the researchers working on the project, and participants do not know if their DNA is the DNA that was sequenced. Within this human genome, there are about 22,300 protein-coding genes. Researchers are actively working to figure out what many of these genes do, as well as how the human genome might differ in populations not included in the sample. A “map” of chromosome 22, one of the shortest human chromosomes, is shown at above, with some genes of interest marked.
Human beings share almost all of their DNA. Thus, although we often focus on differences, you and the person sitting beside you have almost identical DNA sequences. Most variation between individuals’ DNA comes in the form of single nucleotide polymorphisms (often referred to as “SNPS”). This refers to a single nucleotide difference between two people’s DNA (for example, base pair A, instead of C). About 1 base pair out of every 1,000 will be different between two individuals. Human genetic variation seems to be continuous- it is very difficult to identify genetic distinctions among groups of people. Research results suggest that although a few traits can be mapped to specific to geographic regions (like arsenic resistance in isolated populations in South America, for example), variation in most traits is distributed throughout the global human population. In other words, humans are pretty much one big, interbreeding group, with few genetic characteristics that could distinguish any “groups.” (See https://www.ncbi.nlm.nih.gov/books/NBK20363/)
Where does Human Genetic Variation Come From?
Genetic variation refers to differences in genomes across members of a population or species. Genetic variation arises in a population in several ways. Mutations (uncorrected replication errors) that occur during the division of sex cells can result in alterations of gene activity or protein function that are passed down to the offspring. Genetic recombination (the re-arranging of genes during sexual reproduction) and gene flow (the introduction of genes from other populations) also increase genetic variation.
Given our population size- over 7 billion humans living on the planet today (see popclock for the most up to date numbers)- we have relatively low genetic variation. This may be because the human species is relatively new, that human populations were historically very small, or that small groups of humans moved out of Africa to populate the entire world. The Human Genome Project and subsequent research into human genetic variation has allowed scientists to learn much more about human diversity, but many questions remain unanswered.