Fatal Inheritance by Catherine Shaw

in Brünn, and had all the necessary time and leisure to do as he pleased in the monastery garden, and the approval of his superior as well.
    ‘In order to carry out numerical observations, Mendel repeated his experiments on many hundreds of plants, crossing the same type with the same type, for each of the many possibilities, again and again, observing and counting the results. What had been believed at first – that crossing a plant with green peas and a plant with yellow peas could produce a plant with either green or yellow peas (regardless of which parent was green and which yellow), and that it was merely a matter of chance – turned out to be false. In fact, crossing purebred strains yielded very clear and observable results. Considering just the property of the peas being green or yellow, he saw over a great number of experiments that crossing two purebred green plants could only produce green plants, and crossing two purebred yellow plants could only produce yellow, whereas crossing a green and a yellow invariably produced a green.
    ‘He then proceeded to mate purebred plants with those of the second generation of mixings, and now he observed something new. Mating a purebred green with a green that was actually of green and yellow parentage produced only green offspring, but mating a purebred yellow with a green of mixed parentage produced green exactly half the time, and yellow exactly half the time. As for the mating of two mixed plants, the result was a green offspring three-quarters of the time, and a yellow offspring one quarter. Can you guess the rule? It is a purely mathematical question by now!’
    I stopped and leant my elbows on the railing of a bridge, ruminating over the pattern he was describing. He stopped next to me, and extracting a crumpled envelope from his pocket together with a stub of pencil, he drew the following diagram.
     

    ‘Look!’ he exclaimed. ‘This is Mendel’s stroke of genius, and the picture explains it all! Each organ, whether stamen or pistil, must contain two
markers
of some kind, which can be either green or yellow. From the stamens only one of the two can pass to the offspring, and from the pistils again only one, and these are selected by random chance! In a purebred green plant, both markers will be green for the stamens and the pistils, so, if they are crossed, they can only pass green markers to the offspring, and for purebred yellow it will be the same. But when you cross the plants – then what happens to a plant with one green and one yellow marker?’
    ‘Well,’ I said, ‘you claimed that crossing a green and a yellow purebred always creates a green, so I am obliged to conclude that a plant with one green and one yellow marker must be green.’
    ‘Logically spoken!’ he cried and again looked at me with an undue measure of admiration. ‘Mendel called these markers
alleles
, and the fact that green and yellow together make green, he described by saying that the green allele is dominant and the yellow recessive. Thus, of the four possible father-mother combinations green-green, green-yellow, yellow-green and yellow-yellow, only the last one will actually show yellow peas – so only one quarter of the offspring, on average, will be yellow! But the mixed plants will contain alleles that are totally invisible when observing the plants themselves, but that will affect a certain number of the offspring, according to the laws of probability. Generalise this theory to all possible physical traits, and you have the entire theory of their heredity and the manner in which they are passed down through the generations.’
    ‘Is that mystery not contained in the nature of the alleles?’ I said. ‘They seem to hold the secret of it, whatever they are.’
    ‘We do not yet know their precise nature,’ he said confidently, ‘but we will. They are some kind of purely physical entity, microscopically small and carried within each living creature. When we have