Biology 1866

Experiments on Plant Hybridization

Gregor Mendel

Traits pass down as discrete hidden units that sort, hide, and reappear by simple ratios.

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In depth · the introduction

By breeding peas and carefully counting the offspring, a monk discovered that traits are passed on as tiny separate packets — what we now call genes.

The big idea

People used to assume that a child's traits were just a blend of the parents' — mix a tall plant with a short one and you'd expect a medium one. Mendel patiently bred thousands of pea plants and found something different. Cross a tall pea with a short pea, and every plant in the next generation is tall. The shortness hasn't blended away or vanished — it's hiding.

Let those plants breed, and shortness pops back up in the following generation, in almost exactly one plant out of four. That neat ratio gave the secret away. Each plant must carry two hidden instructions for height, one from each parent, and pass just one of them on. Some instructions are “loud” (dominant) and some are “quiet” (recessive) — and a quiet one can lie hidden for a generation, then reappear unchanged.

How it came about

Gregor Mendel was a monk and teacher at the monastery in Brünn (now Brno). In the garden there, over about eight years, he grew and hand-pollinated tens of thousands of pea plants, tracking single traits and tallying the results like an accountant. He presented his findings to the local natural-history society in 1865 and published them in 1866.

And then — almost nothing. The work was too mathematical for the biologists of the day and sat unread for 34 years. Only in 1900 did three botanists, working separately, stumble on the same rules and dig up Mendel's forgotten paper. He had been dead for sixteen years.

Why it mattered

Mendel discovered the basic rules of inheritance — that traits travel in discrete units that don't blend but shuffle and reappear by predictable odds. His pea-counting, ignored in his lifetime, became the foundation of genetics and everything that grew from it, from crop breeding to the prediction of inherited disease.

A way to picture it

Imagine each parent holds two cards for a trait, and deals just one — at random — to the child. A “tall” card is bold and shows through whenever it's present; a “short” card is shy and only shows if the child happens to get two of them. Deal cards from two Tt parents and, on average, three children in four show tall, one in four short. Lay out all the combinations and you have a Punnett square — try it below.

Where it sits

Darwin had shown that life evolves by natural selection, but he lacked a mechanism of inheritance — and the common idea that offspring simply blend their parents would have diluted any new variation away. Mendel's particulate factors were exactly the missing piece: they pass on whole, so variation persists. Decades later the two ideas were fused into the “modern synthesis,” and Mendel's abstract factor finally got a body in the DNA double helix.

The original document

Original source text

Introduction & the choice of Pisum

Gregor Mendel · Versuche über Pflanzen-Hybriden · read 1865, published 1866 · trans. Druery & Bateson (1901)

Experience of artificial fertilisation, such as is effected with ornamental plants in order to obtain new variations in colour, has led to the experiments which will here be discussed. The striking regularity with which the same hybrid forms always reappeared whenever fertilisation took place between the same species induced further experiments to be undertaken, the object of which was to follow up the developments of the hybrids in their progeny.

Those who survey the work done in this department will arrive at the conviction that among all the numerous experiments made, not one has been carried out to such an extent and in such a way as to make it possible to determine the number of different forms under which the offspring of hybrids appear, or to arrange these forms with certainty according to their separate generations, or definitely to ascertain their statistical relations.

The selection of the plant group for experiments of this kind must be made with all possible care if it be desired to avoid from the outset every risk of questionable results. The experimental plants must necessarily possess constant differentiating characters, and their hybrids must be protected from the influence of all foreign pollen during the flowering period. The genus Pisum was found to possess the necessary qualifications.

Dominant and recessive characters

The characters which were selected for experiment relate to the form of the ripe seed (round or wrinkled); the colour of the seed albumen (yellow or green); the length of the stem (tall or dwarf); and four others — seven differentiating characters in all, each appearing in two clear, non-blending forms.

Those characters which are transmitted entire, or almost unchanged in the hybridisation, and therefore in themselves constitute the characters of the hybrid, are termed the dominant, and those which become latent in the process recessive.

The expression “recessive” has been chosen because the characters thereby designated withdraw or entirely disappear in the hybrids, but nevertheless reappear unchanged in their progeny. Transitional forms were not observed in any experiment.

The first and second generations: 3 : 1

In the first generation from the crossing, every hybrid plant displays the dominant character, and the recessive form escapes observation completely. It is, moreover, perfectly immaterial whether the dominant character belongs to the seed-bearer or to the pollen-parent; the form of the hybrid is identical in both cases.

In this generation there reappear, together with the dominant characters, also the recessive ones with their peculiarities fully developed, and this occurs in the definitely expressed average proportion of three to one, so that among each four plants of this generation three display the dominant character and one the recessive.

Of 8023 seeds from one experiment, 6022 were yellow and 2001 green — a ratio of 3.01 to 1. In another, of 7324 seeds, 5474 were round and 1850 wrinkled — 2.96 to 1. The whole of the experiments yielded the average ratio 2.98 to 1.

The 2 : 1 : 1 resolution

Those forms which in the next generation again breed true to the dominant character are revealed to be of two kinds. The dominant offspring are not uniform: one third of them breed true, while two thirds behave as hybrids — themselves splitting 3 : 1 in the generation that follows.

The relation of three to one, in which the distribution of the dominant and recessive characters results in the first generation, resolves itself therefore in all experiments into the proportion of 2 : 1 : 1 if at the same time the dominant character be distinguished according to its significance as a hybrid-character or as a parental one.

The law of combination

Pea hybrids form germinal and pollen cells which, in their composition, correspond in equal numbers to all the constant forms resulting from the combination of the characters united in fertilisation. The behaviour of each pair of differentiating characters in hybrid union is independent of the other differences in the two original parental stocks.

It is now clear why, in a hybrid plant, the differentiating characters can re-emerge in their parental purity: each germ cell and each pollen cell carries only one of the two. The constancy of the offspring, and the definite numerical ratios in which the forms appear, follow of necessity.

Read at the meetings of February 8 and March 8, 1865