When asked what genetics is, people often answer about DNA, genetically modified organisms, disease, and what makes us what we are. Genetics has to do with all of these points, but they are just aspects, and the bigger picture is much more exciting. In this post, I want to outline the scope of genetics and I am hoping to convince you that genetics is relevant for everybody interested in natural history.
The science of inheritance
The science of genetics is the branch of biology that deals with inheritance. This is immediately relevant to all life, because one aspect of life is reproduction, where offspring are made that share parental characteristics (traits). Genetics helps us to understand every-day observations such as the resemblance between offspring and parents, the fact that characteristics of populations (and species) change over time, and that some traits vary continuously (like body height in humans) while others can take only certain states (like yellow or green colour in Mendel’s peas). Natural history, the study of the living world and its incredible diversity of forms, is underpinned by evolution. Evolution, in turn, is underpinned by the simple mechanics of genetics.
Inheritance is particulate
One foundation of genetics is the particulate nature of inheritance, discovered by Gregor Mendel. Mendel, a 19thcentury Moravian friar, conducted crossing experiments with different kinds of peas. The most-remembered traits are the peas’ colours (green and yellow) and shapes (smooth and shrivelled). It was long known that offspring resemble their parents. Mendel found that when crossing pure-breeding individuals that differed in some respect, like green and yellow peas, one state could be dominant. In this example yellow colour is dominant, resulting in a first offspring generation of only yellow peas. However, after crossing such first-generation yellow peas with one another, he recovered both yellow and green offspring. From this he concluded that inheritance does not just blend together the parental characteristics (green had not disappeared by blending into yellow), but that the hereditary material is like particles that can be combined and re-combined. This realisation was revolutionary, because at the time, people had no idea how inheritance worked. Even famous Darwin, a contemporary of Mendel, believed in inheritance by blending parental characteristics. It took the better part of a century to marry the ideas of Mendel and Darwin.
Today, we know the basis of the particulate nature of inheritance. The hereditary material is DNA, a huge polymer molecule. In most familiar species, individuals carry two DNA sets, one inherited from each parent. When two individuals join to make offspring, each of them passes on only one copy of its DNA. So their offspring end up with two copies again. In both inherited sets of DNA, there are the same genes (parts of DNA that have an effect on some measurable trait). But the two copies of each gene may differ somewhat from each other, just like the gene affecting pea colour where there is the green and the yellow variant.
Genetic and environmental effects
Gene variants in the DNA have a major effect on the traits (including diseases) of individuals in a population. For instance, human body height is highly heritable. Children of tall parents tend to be tall. This suggests that the DNA passed on from parents strongly influences offspring size. But while looking at the parents’ height allows a good estimate of the future height of their children, we cannot know it for certain. This is because the conditions in which we grow up are relevant, too. If children starve, they tend to stay shorter. All influences on a trait not determined by the DNA alone are called environmental in genetics speak. One aspect of genetics is the study of how heritable these traits are, i.e. how good an estimate one can make about the characteristics of an individual by looking at the parents. Furthermore, if a trait is found to be heritable, it is then possible to search for the gene(s) underlying the differences observed between individuals. This is very important for plant and animal breeding.
The genetic material is contained in cells, which multiply by dividing into two. Every time a cell divides, it will have to have made a second copy of its DNA so that both daughter cells contain the same information. This copying process is not perfect (although still rather good) and so, very rarely, errors are introduced in a newly generated DNA copy. These errors happen randomly, and they are called mutations. If a mutation has happened in a gene important to some trait, individuals carrying this mutated DNA may differ from other individuals. This is how mutations introduce genetic variability, which may cause trait variability. Because mutations are rare, it is very unlikely that the same mutation happens twice in a population. Thus, when two individuals are found with the same mutated piece of DNA, it is usually safe to conclude that they share a common ancestor in the past from which they inherited that DNA.
DNA sequences reflect the genetic process
This last point is an important one. The fact that mutations are rare makes it possible to trace lines of ancestry. Comparing all the gene copies in a population, it is possible to say which genes are more similar and thus share a common ancestor in the recent past. As time and the number of generations increases since two genetic lineages had their common ancestor, it becomes increasingly likely that mutations cause their DNA sequences to diverge. The presence of genetic diversity makes it possible for us to analyse the relations between individuals using their DNA. This can be done on very different levels. In humans, we have been conducting genetic paternity test for some decades. Going a step further than looking at family relationships, we can now sequence whole human genomes and compare them to a global reference set, allowing us to find out what proportion of our ancestry came from where in the world. But we can go even further. Because all extant organisms share common ancestors in the past, it is also possible to use DNA as a tool to analyse species relationships. For example, we are more closely related to bonobos and chimps than to other primates, and we are about as distantly related to Drosophila flies as flowering plants are to bryophytes. One final point, ancestry relationships may differ between different parts of the genome, for instance as a result of hybridisation. It has become known in the last decade that Europeans carry in their genomes some genetic variants that were received via hybridisation with Neanderthals (whereas the majority of their genes, like those of all other present-day humans, came more directly from African ancestors).
So, in summary, genetics is the science of inheritance. The hereditary material (the genome) consists of DNA molecules containing genes. These genes keep being passed on through a continuous line of parents and offspring. Gene variants in these genomes partially determine the characteristics of their carriers, but the environment has an effect, too. If an individual inherits two different gene variants from their parents, these variants do not blend. They remain distinct and only one of them is passed on to the individual’s offspring. Genes may change over time, if they are hit by mutation, generating a distinguishable genetic lineages and possibly a novel character state. This process is very rare. Looking at gene variants allows us to infer the relationships between individuals, populations, and species.
There are two related podcasts. One is about genetic ancestry analysis in people (naked genetics). The other one is about analysing the structuring of genetic diversity in people (genetics unzipped).