Mendel added pollen (male germ cells) from a line with yellow peas to the female part of a flower from a green pea line. In the next generation he got an unexpected result. Instead of all the offspring being intermediate, all the plants in the new generation looked like one of the parents and not the other. They all had yellow peas. This is not at all what would be expected if the ‘blood’ of the two lines was blended into a yellowish-green mixture.
The next step was to self-fertilise these first-generation yellow plants; in other words to expose their eggs to pollen from the same individual. That gave another unforeseen outcome. Both the original colours, yellow and green, reappeared in the next generation. Whatever it was that produced green could still do so, even though it had spent time within a plant with yellow peas. This did not fit at all with the idea that the different properties of each parent were blended together. Inheritance was, his experiment showed, based on particles rather than fluids.
Mendel did more. He added up the numbers of yellow and green peas in each generation. In the first generation (the offspring of the crossed pure lines) all the plants had yellow peas. In the second, obtained by self-fertilising the yellow plants from the first generation, there were always, on the average, three yellows to one green. From this simple result, Mendel deduced the fundamental rule of genetics.
Pea colour was, he thought, controlled by pairs of factors (or genes, as they became known). Each adult plant had two factors for pea colour, but pollen or egg received only one. On fertilisation – when pollen met egg – a new plant with two factors (or genes) was reborn. The colour of the peas was determined by what the plant inherited. In the original pure lines all individuals carried either two ‘yellow’ or two ‘green’ versions of the seed colour gene. As a result, crosses within a pure line gave a new family of plants identical to their parents.
When pollen from one pure line fertilised eggs from a different line new plants were produced with two different factors, one from each parent. In Mendel’s experiment these plants looked yellow although each carried a hidden set of instructions for making green peas. In other words, the effects of the yellow version were concealing those of the green. The factor for yellow is, we say, dominant to that for green, which is recessive.
Plants with both variants make two kinds of pollen or egg. Half carry the instructions for making green peas and half for yellow. There are hence four ways in which pollen and egg can be brought together when two plants of this kind are mated, or a single one self-fertilised. One quarter of fertilisations involve yellow with yellow, one quarter green with green; and two quarters – one half – yellow with green.
Mendel had already shown that yellow with green produces an individual with yellow peas. Yellow with yellow, needless to say, produces plants with yellow peas, and in a plant with two green factors the pea is green. The ratio of colours in this second generation is therefore three yellow to one green. Mendel worked backwards from this ratio to define his basic rule of inheritance.
Mendel made crosses using many different characters – flower colour, plant height and pea shape – and found that the same ratios applied to each. He also tested the inheritance of pairs of characters considered together. For example, plants with yellow and smooth peas were crossed with others with green and wrinkled peas. His law applied again. Patterns of inheritance of colour were not influenced by those for shape. From this he deduced that separate genes (rather than alternative forms of the same one) must be involved for each attribute. Both for distinct forms of the same trait (yellow or green colour, for example) and for quite different ones (such as colour and shape) inheritance was based on the segregation of physical units. Mendel was the first to prove that offspring are not the average of their parents and that genetics is based on differences rather than similarities.
Biologists since his day have delighted in picking over his results (and accusing him of fraud because they may fit his theories too well). They argue about what he thought his factors were, and speculate about why his work was ignored. Whatever lies behind its long obscurity, Mendel’s result was rediscovered by plant breeders in the first year of the twentieth century and was soon found to apply to hundreds of characters in both animals and plants. Mendel had the good luck, or the genius, needed to be right where all his predecessors had been wrong. No science traces its origin to a single individual more directly than does genetics, and Mendel’s work is still the foundation of the whole enormous subject which it has become.
Mendel rescued Darwin from his dilemma. A gene for green pea colour or for white skin, rare though it may be, is not diluted by the presence of many copies of genes for other colours. Instead, it can persist unchanged over the generations and will become more common should it gain an advantage.
Soon after the crucial rules were rediscovered they were used to interpret patterns of human inheritance. It is not possible to carry out breeding experiments on our fellow citizens. They would take too long, for one thing. Instead, biologists must rely on the experiments which are done as humans go about their sexual business. They use family trees or pedigrees – from the French pied de grue, crane’s foot, after a supposed resemblance of the earliest aristocratic pedigrees (which were arranged in concentric circles) to a bird’s toes. Some are fanciful, going back to Adam himself, but geneticists usually have fewer generations to play with, although one or two pedigrees do trace back for hundreds of years.
The first was published in 1903. It showed the inheritance of shortened hands and fingers in a Norwegian village. Such fingers ran in families and showed a clear pattern. The trait never skipped a generation. Anyone with short fingers had a parent, a grandparent and so on with the same thing. If an affected person married someone without the abnormality (as most did), about half their children were affected. If any of their normal children married another person with normal hands the character disappeared from that branch of the family.
The pattern is just what we expect for a dominant character. Only one copy of the damaged DNA (as in the case of yellow pea colour) is needed to show its effects. Most sufferers, coming as they do from a marriage between a normal and an affected parent, have a single copy of the normal and a single copy of the abnormal form, one from either parent. As a result, their own sperm – or eggs – are of two types, half carrying the normal and half the abnormal variant. When they marry, half their children carry a copy of the damaged gene. The chance of any child of a normal and an affected person having short fingers is hence one in two. An unaffected couple never has a child showing the abnormality as neither of them possesses the flawed instruction that makes it.
Other inherited traits do not behave in this simple way. They are recessives. To show the effect, two copies of the inherited factor, one from each parent, are needed. The parents themselves usually each have a single copy and appear quite normal. Most do not know that they are at risk of having an affected child. Sometimes, though, their offspring looks more like a distant relative or an ancestor than it does either parent. Before Mendel, that pattern was inexplicable. Such children were sometimes called ‘throwbacks’. Now we know that they are obeying Mendel’s laws. They have, by chance, inherited two copies of a recessive abnormality while their mother and father each have just one.
In Britain, one child in several thousand is an albino, lacking any pigment in eyes, hair or skin. Elsewhere, the anomaly is more common. In some North American Indians, about one person in a hundred and fifty is an albino. According to the Book of Enoch (one of the apocryphal books of the Bible), Noah himself suffered from the condition. If he did, there is not much sign of the gene in his descendants.
The great majority of albino children are born to parents of normal skin colour. They must each have a single copy of the albino factor matched with another copy of that for full pigmentation. Half the father’s sperm carry the altered gene. Should one of these fertilise one of that half of his partner’s