The science of genetics is relatively new. Its first steps were made in the 1850s and the term ‘genetics’ was not coined until 1905. It was, however, a new word for an old field of enquiry: inheritance. Since prehistoric times it was well understood that children inherited some of the attributes of their parents. Characteristics such as hair colour, face shape and height are passed on in families, from generation to generation. This applies as much to animals and plants – especially those used in farming – as it does to humans.
The search for the mechanisms of inheritance led to the science of genetics and the theory of evolution, but it did not begin there. The ancient Greek theory was ‘pangenesis’, which proposed that every body part sent information via the semen and menstrual blood to create a tiny person, or homunculus, that grew inside the mother. Charles Darwin himself espoused something like this, saying inherited traits travelled between generations as a swarm of tiny packets called ‘gemmules’.
The term ‘gene’ was coined in 1909 by the Danish botanist Wilhelm Johannsen. Its roots lie in the word ‘genesis’ meaning origin. Charles Darwin and his colleagues in the late 1800s referred to a still-hypothetical ‘genetic’ material that transmitted inherited traits. The study of that process became known as genetics in 1905 (thanks to English biologist William Bateson), and soon after Johannsen introduced the concept of the gene.
Johannsen had no idea what form genes took. His term simply meant a unit of inheritance: the genes inherited from the parent carry the instructions required to build the body of a child. The term is also used to describe particular measurable characteristics, so there is a gene for hair type, eye colour, etc. However, today we know that genetic material is a code-carrying molecule of DNA, so a section of DNA can also be described as a gene. Matching this chemical definition of genes with the anatomical one is a key goal of genetic research.
The core activity of genetics is to identify genes among the DNA held in cells, and figure out their function.
Perhaps surprisingly, the founding figure of genetics was a German-speaking monk, living in the northern reaches of the Austro-Hungarian Empire in the mid-19th century. Gregor Mendel’s work, carried out in the cloistered garden of the Abbey of St Thomas in Brno (now a Czech city), was completely ignored from its publication in 1866 to the start of the 20th century, but nevertheless it contained the basic tenets of genetics that still apply today.
Mendel (1822–84) made his discoveries through experiments breeding pea plants in his garden. He had no knowledge of DNA, referred little to cell biology and, instead of the term ‘gene’, used the word ‘factor’. However, Mendel was able to glean some universal rules of genetics from the way the different characteristics of the pea plants were passed from generation to generation. These fundamental rules are the foundations of the core inheritance process, which is called Mendelian genetics in his honour.
Gregor Mendel made his discoveries by diligently controlling which pea plants were allowed to breed with which others. He was aided in this endeavour by the fact that peas can self-cross, meaning a plant can use its own pollen to produce seeds.
Mendel identified several inherited traits, such as flower colour or shape and plant height. He worked on all these traits, but taking height as our exemplar, Mendel isolated a tall plant that always produced tall daughter plants when crossed with itself, and a short plant that always produced short offspring. He then cross-pollinated these two plants to produce offspring (seeds) with one tall and one short parent. He found the first generation of offspring grew into tall plants. Next he self-crossed one plant from the new generation. Three quarters of its offspring were tall, a quarter were short. The same thing happened for all the traits he tested. Mendel’s theories of inheritance were deduced from these startlingly consistent results.
Using the results from his many thousands of breeding experiments carried out over several years, Gregor Mendel outlined what he saw as universal truths about inheritance.
Mendel’s ‘Law of Segregation’ said that each plant had two versions of each factor (gene). When it came to making pollen, the paired versions of each factor were always split. Any offspring would inherit only one version from each parent, with the two combining making a new pair. Another rule, the ‘Law of Independent Assortment’, states that every factor moves between generations independently of the others. A third law, the ‘Law of Dominance’ asserts that some types of factor have a hierarchy that leads to dominant ones being expressed in the organism’s outward appearance, while recessive ones remain hidden. Later research would come to qualify the second law, and some regard the third as less significant because it does not apply to all factors, but together these laws have become the foundation stones of classical genetics.
Mendel’s diligent experiments with pea plants gave the first insight into how inherited factors controlled development.
Classical genetics draws a line between our two definitions of a gene: a gene can be understood as a chemical entity – a piece of DNA – or as an inherited trait, anatomical or otherwise. Mendel’s discoveries showed that the two concepts were not interchangeable. To illustrate this, geneticists invented the term ‘phenotype’.
The phenotype is the outwardly expressed end result of the genes that are inherited. It is the tallness of the pea plant, the colour of your hair or the body plan of an insect. It can also relate to animal behaviours (sometimes referred to as the ‘extended phenotype’). There is often a degree of learning involved in behaviours, such as migration, hunting and nest building, but they are nevertheless ultimately inherited from the parents. Mendel’s master stroke was to figure out the link between the phenotype and the way genetic material is transferred. That genetic material has been given another name: the ‘genotype’.
An organism’s genotype could also be simply described as its genetic make-up. It is a description of the various genes inherited from its parents. As Gregor Mendel discovered, all organisms get one version of each gene from each of their parents, and so the genotype is made up of these pairs.
A particular genotype does not automatically lead to a related phenotype. In fact, the same phenotype – for example, the tallness of a pea plant – can result from a set of different genotypes (albeit a small set). The mechanisms at play are twofold. Firstly, the different versions of the gene interact and combine with each other in particular ways – described by the ideas of genetic dominance (see here) and Mendel’s Third Law. Secondly, the environment in which the organism finds itself also has an impact on how it grows and develops, by varying degrees from gene to gene.
This unusual sounding word derives from German and means something like, ‘of one another’. It is however a very useful term in genetics: an allele is one of several possible versions of a gene. So using the example of Mendel’s pea plants, the gene for plant height has two alleles: tall or short. Another example is eye colour: blue, green, grey, brown and hazel are best described as alleles of the same gene.
A genotype contains two alleles for each gene. If the alleles are identical, then it is described as homozygous – in other words, when it comes to dividing the alleles up to make the sex cells that are used in producing the next generation (pollen, sperm, eggs etc), each cell will definitely contain the same allele. When a genotype contains two differing alleles it is described as heterozygous. As a result, half the sex cells will have one allele, and half the other. Nevertheless, homozygous and heterozygous genotypes can still produce the same phenotype, thanks to an additional complication known as dominance (see here).
These guinea pigs all have the same gene that controls hair colour; but each has inherited a different version, or allele, of that gene.
A genome is the total genetic material used by an organism. The concept of our own human genome has become familiar ever since the launch of the Human Genome Project in 1990. That effort to map all of the genetic material used to make a human body was completed in 2003, although the work continues, figuring out how that material is divided up into somewhere between 20,000 and 25,000 genes.
Other organisms with mapped genomes include the E. coli baterium, the worm Caenorhabditis elegans and the fruit fly. The amount of genetic material – the DNA – in each organism varies, as do the number of genes.
The ‘gene pool’ is another familiar term but one with a rather different meaning. It refers to the enire set of genes, including their many different alleles, that exists throughout a population of organisms. The gene pool represents the genetic variation wihthin a group of organisms.
As well as studying the genes of an individual, geneticists also seek to understand the behaviours of genes shared by a community, population or entire species.
In general terminology, a ‘hybrid’ is an organism that is the product of a cross between two distinct breeds. In terms of biology, and genetics in particular, however, a hybrid is an organism that has a heterogametic genotype. Put more simply, the organism has inherited two differing alleles, or versions of a gene, from its parents.
Gregor Mendel’s success with solving the puzzle of inheritance involved repeated hybridizations. Although he had no idea of how it was happening, Mendel correctly surmised that his crosses of pea plants were creating hybrids – pea plants that had inherited two different versions of a gene.
It was this breakthrough that allowed him to discover that one allele is not always equal to another. Some are dominant over others, and it is this interplay of alleles that shows how a particular genotype leads to a corresponding phenotype.
This żubroń is a hybrid of domestic cattle and European bison.
Genetic dominance is the overriding feature that explains the results of Mendel’s hybrid experiments (see here). Returning to the example of a tall pea plant crossed with a short one, the tall parent has the genotype TT, with T being the tall allele. The short parent’s genotype is tt, with t being the short allele.
The next generation of plants all receive a T from one parent and a t from the other. They all have a genotype of Tt. The T allele is dominant over the t, and so all Tt genotypes lead to a tall phenotype. Next, Mendel crossed a Tt plant with itself. This led to four genotypes, all equally likely: TT, Tt, tT and tt. Any genotype with the dominant T allele results in a tall phenotype, while only the tt genotype produces the short phenotype. When explained like that, the 3:1 ratio of tall to short discovered by Mendel makes perfect sense. It also highlights the incredible leap of imagination Mendel made to figure it all out.
Genetic dominance is not integral to an allele; it is a phenomenon that arises when two alleles meet. One allele may be dominant over the other, in which case this second allele is termed ‘recessive’ to the dominant one. However, it is possible that the recessive allele in this particular genotype is itself dominant over a third allele. For example, the dark hair allele is dominant over the blonde allele, which in turn is dominant over red hair.
Some traits, such as red hair, are entirely recessive but have little effect on an organism’s chance of survival. Other examples, such as albinism (opposite), can be more troublesome. In all cases, recessive phenotypes are infrequent in the population because they require a homozygous, double-recessive genotype. Many genotypes can contain a recessive allele that remains hidden by a dominant partner. Only when two heterozygous parents, or ‘carriers’, produce offspring is there potential for the recessive phenotype to appear – seemingly out of the blue.
The idea of genetic dominance is compelling and simple to understand. However, it is seldom this simple when genotypes are expressed as phenotypes, since there are two other options: codominance and incomplete dominance. These occur when neither of the alleles in a genotype is dominant over the other. The result is that both are expressed in the phenotype in some way, but the difference is a little nuanced.
When a genotype is codominant, both alleles are fully expressed in different parts of the organism. An example would be a red flower and a white flower producing offspring that had flowers covered in red and white blotches. The distinct effects of both alleles are seen. If the genotype is showing incomplete dominance, however, the result is a completely new phenotype that is a blend of the effect of the two alleles. In this situation, a red and a white flower (of a different species to the previous example) would produce offspring with completely pink flowers.
The black and white blotches of dairy cattle are a product of codominance, where two genes are expressed in patches.