3. Human DNA and Genetic Diversity

Human DNA and Genetic Diversity

Human DNA is typically organized into 23 pairs of chromosomes. Each chromosome contains many genes. A gene is a segment of DNA that codes for a protein that contributes to a trait. The cartoon at right represents human chromosome 22.  This chromosome has 49 million base pairs of DNA.  The words listed next to the cartoon indicate which sections of the chromosome code for each trait or protein. An individual’s complete set of genes (including all of their chromosomes) is referred to as their genome.

Some genes are the same across all humans, but others have different versions.  This is important because small differences in the DNA of a gene can give rise to a different trait.  We refer to different versions of genes as alleles.  For example, two people would both have a gene for pigments in the irises of their eyes, but one might have an allele for making green pigments and another brown pigments.

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 in Figure 1,  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 8 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.