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Section 9: The Reproductive Cells of the Hybrids

Experiments in Plant Hybridization (1865)
by Gregor Mendel

The Reproductive Cells of the Hybrids

The results of the previously described experiments led to further experiments, the results of which appear fitted to afford some conclusions as regards the composition of the egg and pollen cells of hybrids. An important clue is afforded in Pisum by the circumstance that among the progeny of the hybrids constant forms appear, and that this occurs, too, in respect of all combinations of the associated characters. So far as experience goes, we find it in every case confirmed that constant progeny can only be formed when the egg cells and the fertilizing pollen are of like character, so that both are provided with the material for creating quite similar individuals, as is the case with the normal fertilization of pure species. We must therefore regard it as certain that exactly similar factors must be at work also in the production of the constant forms in the hybrid plants. Since the various constant forms are produced in one plant, or even in one flower of a plant, the conclusion appears logical that in the ovaries of the hybrids there are formed as many sorts of egg cells, and in the anthers as many sorts of pollen cells, as there are possible constant combination forms, and that these egg and pollen cells agree in their internal compositions with those of the separate forms.

In point of fact it is possible to demonstrate theoretically that this hypothesis would fully suffice to account for the development of the hybrids in the separate generations, if we might at the same time assume that the various kinds of egg and pollen cells were formed in the hybrids on the average in equal numbers.

In order to bring these assumptions to an experimental proof, the following experiments were designed. Two forms which were constantly different in the form of the seed and the color of the albumen were united by fertilization.

If the differentiating characters are again indicated as A, B, a, b, we have:

         AB Seed parents,       ab Pollen parents,
         A  form round          a form wrinkled
         B albumen yellow       b albumen green

The artificially fertilized seeds were sown together with several seeds of both original stocks, and the most vigorous examples were chosen for the reciprocal crossing. There were fertilized:

  1. The hybrids with the pollen of AB
  2. The hybrids with the pollen of ab
  3. AB with the pollen of the hybrids.
  4. ab with the pollen of the hybrids.
For each of these 4 experiments the whole of the flowers on 3 plants were fertilized. If the above theory be correct, there must be developed on the hybrids egg and pollen cells of the forms AB,Ab,aB, ab, and there would be combined:
  1. The egg cells AB, Ab, aB, ab with the pollen cells AB.
  2. The egg cells AB, Ab, aB, ab with the pollen cells ab.
  3. The egg cells AB with the pollen cells AB, Ab, aB, and ab.
  4. The egg cells ab with the pollen cells AB, Ab, aB, and ab.
From each of these experiments there could then result only the following forms: If, furthermore, the several forms of the egg and pollen cells of the hybrids were produced on an average in equal numbers, then in each experiment the said 4 combinations should stand in the same ratio to each other. A perfect agreement in the numerical relations was, however, not to be expected since in each fertilization, even in normal cases, some egg cells remain undeveloped or subsequently die, and many even of the well-formed seeds fail to germinate when sown. The above assumption is also limited in so far that while it demands the formation of an equal number of the various sorts of egg and pollen cells, it does not require that this should apply to each separate hybrid with mathematical exactness.

The first and second experiments had primarily the object of proving the composition of the hybrid egg cells, while the third and fourth experiments were to decide that of the pollen cells. As is shown by the above demonstration the first and third experiments and the second and fourth experiments should produce precisely the same combinations, and even in the second year the result should be partially visible in the form and color of the artificially fertilized seed. In the first and third experiments the dominant characters of form and color, A and B, appear in each union, and are also partly constant and partly in hybrid union with the recessive characters a and b, for which reason they must impress their peculiarity upon the whole of the seeds. all seeds should therefore appear round and yellow, if the theory be justified. In the second and fourth experiments, on the other hand, one union is hybrid in form and in color, and consequently the seeds are round and yellow; another is hybrid in form, but constant in the recessive character of color, whence the seeds are round and green; the third is constant in the recessive character of form but hybrid in color, consequently the seeds are wrinkled and yellow; the fourth is constant in both recessive characters, so that the seeds are wrinkled and green. In both these experiments there were consequently four sorts of seed to be expected; namely, round and yellow, round and green, wrinkled and yellow, wrinkled and green.

The crop fulfilled these expectations perfectly. There were obtained in the

In the 2nd Experiment, 31 round and yellow, 26 round and green, 27 wrinkled and yellow, 26 wrinkled and green seeds.

In the 4th Experiment, 24 round and yellow, 25 round and green, 22 wrinkled and yellow, 27 wrinkled and green seeds.

There could scarcely be now any doubt of the success of the experiment; the next generation must afford the final proof. From the seed sown there resulted for the first experiment 90 plants, and for the third 87 plants which fruited: these yielded for the

1st Exp. 3rd Exp.
  20     25 round yellow seeds .....................  AB
  23     19 round yellow and green seeds ............. ABb
  25     22 round and wrinkled yellow seeds .......... AaB
  22     21 round and wrinkled green and yellow seeds.. AaBb
In the second and fourth experiments the round and yellow seeds yielded plants with round and wrinkled yellow and green seeds, AaBb.

From the round green seeds plants resulted with round and wrinkled green seeds, Aab.

The wrinkled yellow seeds gave plants with wrinkled yellow and green seeds, aBb.

From the wrinkled green seeds plants were raised which yielded again only wrinkled and green seeds, ab.

Although in these two experiments likewise some seeds did not germinate, the figures arrived at already in the previous year were not affected thereby, since each kind of seed gave plants which, as regards their seed, were like each other and different from the others. There resulted therefore from the

      2nd. Exp.    4th Exp.
         31          24    plants of the form AaBb
         26          25    plants of the form Aab
         27          22    plants of the form aBb
         26          27    plants of the form ab
In all the experiments, therefore, there appeared all the forms which the proposed theory demands, and they came in nearly equal numbers.

In a further experiment the characters of flower-color and length of stem were experimented upon, and selection was so made that in the third year of the experiment each character ought to appear in half of all the plants if the above theory were correct. A,B,a,b serve again as indicating the various characters.

The form Ab was fertilized with ab, which produced the hybrid Aab. Furthermore, aB was also fertilized with ab, whence the hybrid aBb. In the second year, for further fertilization, the hybrid Aab was used as seed parent, and hybrid aBb as pollen parent.

From the fertilization between the possible egg and pollen cells four combinations should result, namely: From this it is perceived that, according to the above theory, in the third year of the experiment out of all the plants
   half should have violet-red flowers (Aa)  Classes  1, 3
    "    "  "        white flowers     (a)      "     2, 4
    "    "  "        a long axis       (Bb)     "     1, 2
    "    "  "        a short axis      (b)      "     3, 4
From 45 fertilizations of the second year 187 seeds resulted, of which only 166 reached the flowering stage in the third year. Among these the separate classes appeared in the numbers following:
      Class    Flower color         Stem
   1  violet-red    long         47 times
   2  white         long         40    "
   3  violet-red    short        38    "
   4  white         short        41    "
There subsequently appeared The theory adduced is therefore satisfactorily confirmed in this experiment also.

For the characters of form of pod, color of pod, and position of flowe\ rs, experiments were also made on a small scale and results obtained in perfect agreement. All combinations, which were possible through the union of the differentiating characters duly appeared, and in nearly equal numbers.

Experimentally, therefore, the theory is confirmed that the pea hybrids form egg and pollen cells which, in their constitution, represent in equal numbers all constant forms which result from the combination of the characters united in fertilization.

The difference of the forms among the progeny of the hybrids, as well as the respective ratios of the numbers in which they are observed, find a sufficient explanation in the principle above deduced. The simplest case is afforded by the developmental series of each pair of differentiating characters. This series is represented by the expression A+2Aa+a, in which A and a signify the forms with constant differentiating characters, and Aa the hybrid form of both. It includes in 3 different classes 4 individuals. In the formation of these, pollen and egg cells of the form A and a take part on the average equally in the fertilization; hence each form [occurs] twice, since four individuals are formed. There participate consequently in the fertilization

It remains, therefore, purely a matter of chance which of the two sorts of pollen will become united with each separate egg cell. According, however, to the law of probability, it will always happen, on the average of many cases, that each pollen form A and a will unite equally often with each egg cell form A and a, consequently one of the two pollen cells A in the fertilization will meet with the egg cell A and the other with the egg cell a, and so likewise one pollen cell a will unite with an egg cell A, and the other with the egg cell a.
   Pollen cells   A     A   a     a
                  |      \ /      |
                  |       X       |
                  |      / \      |
     Egg cells    A     A   a     a
The result of the fertilization may be made clear by putting the signs for the conjoined egg and pollen cells in the form of fractions, those for the pollen cells above and those for the egg cells below the line. We then have
                 A       A       a       a
               ----- + ----- + ----- + -----
                 A       a       A       a
In the first and fourth term the egg and pollen cells are of like kind, consequently the product of their union must be constant, namely A and a; in the second and third, on the other hand, there again results a union of the two differentiating characters of the stocks, consequently the forms resulting from these fertilizations are identical with those of the hybrid from which they sprang. There occurs accordingly a repeated hybridization. This explains the striking fact that the hybrids are able to produce, besides the two parental forms, offspring which are like themselves;
            A            a
          -----  and   -----
            a            A
both give the same union Aa, since, as already remarked above, it makes no difference in the result of fertilization to which of the two characters the pollen or egg cells belong. We may write then
         A     A     a     a
        --- + --- + --- + ---  =  A + 2Aa + a
         A     a     A     a
This represents the average result of the self-fertilization of the hybrids when two differentiating characters are united in them. In individual flowers and in individual plants, however, the ratios in which the forms of the series are produced may suffer not inconsiderable fluctuations. Apart from the fact that the numbers in which both sorts of egg cells occur in the seed vessels can only be regarded as equal on the average, it remains purely a matter of chance which of the two sorts of pollen may fertilize each separate egg cell. For this reason the separate values must necessarily be subject to fluctuations, and there are even extreme cases possible, as were described earlier in connection with the experiments on the forms of the seed and the color of the albumen. The true ratios of the numbers can only be ascertained by an average deduced from the sum of as many single values as possible; the greater the number the more are merely chance effects eliminated.

The developmental series for hybrids in which two kinds of differentiating characters are united contains among 16 individuals 9 different forms,AB + Ab + aB + ab + 2ABb + 2aBb + 2AaB + 2Aab + 4AaBb. Between the differentiating characters of the original stocks Aa and Bb 4 constant combinations are possible, and consequently the hybrids produce the corresponding 4 forms of egg and pollen cells AB, Ab,aB,ab, and each of these will on the average figure 4 times in the fertilization, since 16 individuals are included in the series. Therefore, the participators in the fertilization are

In the process of fertilization each pollen form unites on an average equally often with each egg cell form, so that each of the 4 pollen cells AB unites once with one of the forms of egg cell AB,Ab aBab. In precisely the same way the rest of the pollen cells of the forms AbaBab unite with all the other egg cells. We obtain therefore
    AB     AB     AB     AB     Ab     Ab     Ab     Ab
   ---- + ---- + ---- + ---- + ---- + ---- + ---- + ----
    AB     Ab     aB     ab     AB     Ab     aB     ab
      aB     aB     aB     aB     ab     ab     ab     ab
   + ---- + ---- + ---- + ---- + ---- + ---- + ---- + ----
      AB     Ab     aB     ab     AB     Ab     aB     ab
or In precisely similar fashion is the developmental series of hybrids exhibited when three kinds of differentiating characters are conjoined in them. The hybrids form 8 various kinds of egg and pollen cells: ABC, ABc, AbC, Abc, aBC, aBc, abC, abc; and each pollen form unites itself again on the average once with each form of egg cell.

The law of combination of different characters which governs the development of the hybrids finds therefore its foundation and explanation in the principle enunciated, that the hybrids produce egg cells and pollen cells which in equal numbers represent all constant forms which result from the combinations of the characters brought together in fertilization.

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