When most people think of genetics, they think of a modern, high-tech science, with men in lab coats doing strange things to cells. What they don't realise is that the science of genetics was invented by a 19th Century monk who enjoyed a spot of gardening.
Gregor Mendel was born in 1822 in what is now the Czech Republic, the son of poor peasant farmers. Although he did well at school, his parents could not afford to send him to university, so he went instead to the Augustinian monastery at Brunn1. After studying there for some time, he moved to the University of Vienna in Austria to study science and mathematics. After failing his exams for a teaching degree, Mendel returned to the monastery where he became Abbot and spent the rest of his life.
The monastery at Brunn was blessed with large and beautiful gardens, and Mendel was a keen gardener. It was during his work in the garden that he began to take a close interest in garden peas. He noticed that peas had certain characteristics that seemed to be passed from generation to generation. For example, plants with peas that were green had offspring with green peas, while those with yellow peas produced yellow offspring. Over seven years, Mendel carried out an enormous number of experiments with these plants, studying characteristics such as height, seed shape, seed colour and flower colour. Despite knowing nothing about DNA2 or the biochemistry of inheritance, Mendel developed his two 'Laws of Heredity', which remain the basis of modern genetics.
Big Plants and Little Plants
Mendel's experiments relied on studying pairs of characteristics that seemed to be 'either-or' in the plants. For example, the garden had tall pea plants and short pea plants, but no in-between ones. So, Mendel decided to cross a tall plant with a short plant and measure the result. To his surprise, all the offspring were tall, rather than the intermediate size that might have been expected. Continuing the experiment, he crossed the new tall plants with each other. In the next generation, three-quarters of the plants were tall, but one-quarter were short. In summary:
These results were repeated with whatever pair of characteristics Mendel chose. Yellow seeds crossed with green seeds produced all yellow seeds. If the new yellow seeds were crossed with each other, three-quarters were yellow and one-quarter were green.
From these simple experiments, Mendel theorized that these characteristics must be inherited as 'particles' of some sort - what we now know as 'genes'. Each plant had two genes for each characteristic. If the gene for tallness is shown with a capital T and the gene for 'shortness' by a lower-case t, then each plant could be either TT (pure-bred tall), tt (pure-bred short) or Tt. These Tt plants were tall because the T gene is 'dominant' to the t gene, which is referred to as 'recessive'3:
In Mendel's original experiment, each offspring plant must have inherited a T gene from the tall parent and a t gene from the short parent - no other combinations are possible. At this stage in his experiments, however, Mendel did not know for certain that each offspring inherited one gene from each parent. It was confirmed when Mendel crossed the new Tt plants, and this happened:
It can be seen that three-quarters of the plants will be tall, having either TT or Tt genes, while the remaining quarter have inherited the 'recessive' short gene from both parents and will therefore be short. This is precisely what happened in the real experiment, and the separation of the pairs of genes during reproduction became Mendel's First Law.
Having covered the concepts of dominance and recessivity, this seems like a good time for a quick...
Allele - different versions of a gene that can be found at a given 'locus' (qv). So, in the example above, T (tall) and t (short) are alleles of the gene for 'tallness'. Some genes may have only two possible alleles and some may have a wider variety, although any individual will still only have two.
Gametes - sex cells: eggs (ova) in women, sperm in men. Each gamete contains only one of each pair of genes. When two gametes combine to form a new organism, the pairs are brought together.
Gene - a sequence of DNA that encodes a particular protein4. Humans, like Mendel's pea plants, have two copies of each gene. As we have seen, these 'copies' need not be identical (cf 'allele').
Genotype - the combination of alleles found in a particular organism.
Heterozygous - having two different alleles for a gene (such as the Tt plants discussed above).
Homozygous - having two copies of the same allele for a gene (such as the TT or tt plants).
Locus (plural: 'loci') - a region of DNA in which a particular gene is located.
Phenotype - the physical features of an organism that are determined by the genotype.
Try This At Home
To see Mendel's first law in action, try rolling your tongue (ie, curling your tongue into a tube). Then ask your parents, siblings, children, aunts, uncles, cousins and grandparents if they can do it. Tongue rolling is an action controlled by a single gene with two alleles. Those who can do it have at least one dominant 'R' gene, while those who can't have two recessive 'r' genes.
Big Yellow Plants and Little Green Plants
In Mendel's second set of experiments, he looked at two pairs of characteristics together. For example, he took tall plants that produced yellow seeds (both dominant characteristics) and crossed them with short plants producing green seeds (both recessive characteristics). Using 'Y' to represent the 'yellow' allele and 'y' to represent the 'green' allele, the results looked something like this:
As might have been predicted from the results of the first set of experiments, all the offspring were tall with yellow seeds. When these offspring are crossed, things start to get a little complicated. It is perhaps easier to start with the possible gametes that each parent can produce - remembering that only one of each pair of genes will be passed to each gamete. So, a parent that is (Tt Yy) can produce four different gametes:
Then, when two (Tt Yy) parents mate, there are 16 possible combinations in the offspring:
|TY||TT YY||TT Yy||Tt YY||Tt Yy|
|Ty||TT yY||TT yy||Tt yY||Tt yy|
|tY||tT YY||tT Yy||tt YY||tt Yy|
|ty||Tt Yy||tT yy||tt yY||tt yy|
But, because of dominance and recessivity, these 16 genotypes give four phenotypes, in varying proportions:
|Tall, yellow seeds||9/16||TT YY
|Tall, green seeds||3/16||TT yy
|Short, yellow seeds||3/16||tt YY
|Short, green seeds||1/16||tt yy|
Again, these proportions correspond to what Mendel saw in real life when he bred the plants5. From this, he made another important deduction - characteristics are not necessarily linked. In a parent that is (Tt Yy), the T gene can be passed on with either the Y or y genes. This 'independent segregation' is Mendel's Second Law of Heredity.
The Strongest Link
In developing his Second Law, Mendel definitely struck lucky. Characteristics are only inherited independently if they are on different chromosomes (ie, different strands of DNA). Different genes on the same chromosome are said to be 'linked' and tend to be inherited together. Pea plants have seven chromosomes and, in a very fortunate coincidence, the seven characteristics that Mendel chose to study are found one on each chromosome.
So, in Summary...
Leaving aside all the 'T's and 'y's, Mendel's Laws of Heredity can be summed up thus:
Members of each pair of alleles separate when gametes are formed.
Separation of each pair of alleles is independent of other pairs.
As in all areas of scientific knowledge, understanding increases over time, and subsequent discoveries - such as the unravelling of DNA or the mechanism of protein synthesis - have added layers of complexity to Mendel's original discoveries. This does not alter the fact that Mendel's work still underpins the entire modern discipline of genetics. Indeed, his work was so far ahead of its time that, after it was published in 1865, it was more or less forgotten about until it was confirmed 35 years later.
Mendel's work allowed us to understand the concept of inheritance, leading to breakthroughs in fields as diverse as agriculture and medicine. Mendel's discoveries also meshed nicely with the theory of evolution. The recent sequencing of whole genomes promises a further round of medical and technological advances. We've come a long way since a poor Abbot in an Austrian monastery first noticed that some of his pea plants were taller then others.
1 Then part of Austria, it is now the Czech city of Brno.
2 Deoxyribonucleic acid.
3 The terms 'dominant' and 'recessive' were coined by Mendel and are still used today. It was many, many years before the actual mechanisms of dominance and recessivity started to be understood.
4 Yep, that's literally all 'gene' means, which is why talk of a 'gene for intelligence' or a 'gene for homosexuality' is meaningless.
5 With hindsight, his proportions were perhaps a little too perfect, and it has been suggested that he 'tidied' his data slightly. This should not, however, detract from the importance of his work or the brilliance of his deductions.