MENDEL'S WORK
The task that Mendel set before himself was to gain some clear conception
of the manner in which the definite and fixed varieties found within a
species are related to one another, and he realised at the outset that the
best chance of success lay in working with material of such a nature as to
reduce the problem to its simplest terms. He decided that the plant with
which he was to work must be normally self-fertilising and unlikely to be
crossed through the interference of insects, while at the same time it must
possess definite fixed varieties which bred true to type. In the common pea
(_Pisum sativum_) he found the plant he sought. A hardy annual, prolific,
easily worked, _Pisum_ has a further advantage in that the insects which
normally visit flowers are unable to gather pollen from it and so to bring
about cross fertilisation. At the same time it exists in a number of
strains presenting well-marked and fixed differences. The flowers may be
purple, or red, or white; the plants may be tall or dwarf; the ripe seeds
may be yellow or green, round or wrinkled--such are a few of the characters
in which the various races of peas differ from one another. {18}
In planning his crossing experiments Mendel adopted an attitude which
marked him off sharply from the earlier hybridisers. He realised that their
failure to elucidate any general principle of heredity from the results of
cross fertilisation was due to their not having concentrated upon
particular characters or traced them carefully through a sequence of
generations. That source of failure he was careful to avoid, and throughout
his experiments he crossed plants presenting sharply contrasted characters,
and devoted his efforts to observing the behaviour of these characters in
successive generations. Thus in one series of experiments he concentrated
his attention on the transmission of the characters tallness and dwarfness,
neglecting in so far as these experiments were concerned any other
characters in which the parent plants might differ from one another. For
this purpose he chose two strains of peas, one of about 6 feet in height,
and another of about 1-1/2 feet. Previous testing had shown that each
strain bred true to its peculiar height. These two strains were
artificially crossed[1] with one another, and it was found to make no
difference which was used as the pollen parent and which was used as the
ovule parent. In either case the result was the same. The result of
crossing tall with dwarf was in every case nothing but talls, as tall or
even a little taller than the tall parent. For this reason Mendel termed
tallness the DOMINANT and {19} dwarfness the RECESSIVE character. The next
stage was to collect and sow the seeds of these tall hybrids. Such seeds in
the following year gave rise to a mixed generation consisting of talls and
dwarfs _but no intermediates_. By raising a considerable number of such
plants Mendel was able to establish the fact that the number of talls which
occurred in this generation was almost exactly three times as great as the
number of the dwarfs. As in the previous year, seed were carefully
collected from this, the second hybrid generation, and in every case _the
seeds from each individual plant were harvested separately and separately
sown in the following year_. By this respect for the individuality of the
different plants, however closely they resembled one another, Mendel found
the clue that had eluded the efforts of all his predecessors. The seeds
collected from the dwarf recessives bred true, giving nothing but dwarfs.
And this was true for every dwarf tested. But with the talls it was quite
otherwise. Although indistinguishable in appearance, some of them bred
true, while others behaved like the original tall hybrids, giving a
generation consisting of talls and dwarfs in the proportion of three of
{20} the former to one of the latter. Counting showed that the number of
the talls which gave dwarfs was double that of the talls which bred true.
T x D --------------------- P
|
T(D) --------------------- F_1
|
+-----------+-----------+-------------+
T T(D) T(D) D --- F_2
| +---+----+--+ +--+----+---+ |
T T T(D) T(D) D T T(D) T(D) D D --- F_3
| |
T D --- F_4
If we denote a dwarf plant as D, a true breeding tall plant as T, and a
tall which gives both talls and dwarfs in the ratio 3 : 1 as T(D), the
result of these experiments may be briefly summarised in the foregoing
scheme.[2]
Mendel experimented with other pairs of contrasted characters and found
that in every instance they followed the same scheme of inheritance. Thus
coloured flowers were dominant to white, in the ripe seeds yellow was
dominant to green, and round shape was dominant to wrinkled, and so on. In
every case where the inheritance of an alternative pair of characters was
concerned the effect of the cross in successive generations was to produce
three and only three different sorts of individuals, viz. dominants which
bred true, dominants which gave both dominant and recessive offspring in
the ratio 3 : 1, and recessives which always bred true. Having determined a
general scheme of inheritance which experiment showed to hold good for each
of the seven pairs of alternative characters with which he worked, Mendel
set himself to providing a theoretical interpretation of this scheme which,
as he clearly realised, must be in terms of germ cells. He {21} conceived
of the gametes as bearers of something capable of giving rise to the
characters of the plant, but he regarded any individual gamete as being
able to carry one and one only of any alternative pair of characters. A
given gamete could carry tallness _or_ dwarfness, but not both. The two
were mutually exclusive so far as the gamete was concerned. It must be pure
for one or the other of such a pair, and this conception of the purity of
the gametes is the most essential part of Mendel's theory.
[Illustration: FIG. 1.
Scheme of inheritance in the cross of tall with dwarf pea. Gametes
represented by small and zygotes by larger circles.]
We may now proceed with the help of the accompanying scheme (Fig. 1) to
deduce the results that should flow from Mendel's conception of the nature
of the gametes, and to see how far they are in accordance with the facts.
Since the original tall plant belonged to a strain which bred true, all the
gametes produced by it must bear the tall character. Similarly all the
gametes of the original dwarf plant must bear the dwarf character. A cross
between these two means the union of {22} a gamete containing tallness with
one bearing dwarfness. Owing to the completely dominant nature of the tall
character, such a plant is in appearance indistinguishable from the pure
tall, but it differs markedly from it in the nature of the gametes to which
it gives rise. When the formation of the gametes occurs, the elements
representing dwarfness and tallness SEGREGATE from one another, so that
half of the gametes produced contain the one, and half contain the other of
these two elements. For on hypothesis every gamete must be pure for one or
other of these two characters. And this is true for the ovules as well as
for the pollen grains. Such hybrid F_1 plants, therefore, must produce a
series of ovules consisting of those bearing tallness and those bearing
dwarfness, and must produce them in equal numbers. And similarly for the
pollen grains. We may now calculate what should happen when such a series
of pollen grains meets such a series of ovules, _i.e._ the nature of the
generation that should be produced when the hybrid is allowed to fertilise
itself. Let us suppose that there are 4x ovules so that 2x are "tall" and
2x are "dwarf." These are brought in contact with a mass of pollen grains
of which half are "tall" and half are "dwarf." It is obvious that a "tall"
ovule has an equal chance of being fertilised by a "tall" or a "dwarf"
pollen grain. Hence of our 2x "tall" ovules, x will be fertilised by "tall"
pollen grains and x will be fertilised by "dwarf" pollen grains. The former
must give rise to tall {23} plants, and since the dwarf character has been
entirely eliminated from them they must in the future breed true. The
latter must also give rise to tall plants, but since they carry also the
recessive dwarf character they must when bred from produce both tails and
dwarfs. Each of the 2x dwarf ovules, again, has an equal chance of being
fertilised by a "tall" or by a "dwarf" pollen grain. Hence x will give rise
to tall plants carrying the recessive dwarf character, while x will produce
plants from which the tall character has been eliminated, _i.e._ to pure
recessive dwarfs. Consequently from the 4x ovules of the self-fertilised
hybrid we ought to obtain 3x tall and x dwarf plants. And of the 3x talls x
should breed true to tallness, while the remaining 2x, having been formed
like the original hybrid by the union of a "tall" and a "dwarf" gamete,
ought to behave like it when bred from and give talls and dwarfs in the
ratio 3 : 1. Now this is precisely the result actually obtained by
experiment (cf. p. 17), and the close accord of the experimental results
with those deduced on the assumption of the purity of the gametes as
enunciated by Mendel affords the strongest of arguments for regarding the
nature of the gametes and their relation to the characters of the zygotes
in the way that he has done.
It is possible to put the theory to a further test. The explanation of the
3 : 1 ratio of dominants and recessives in the F_2 generation is regarded
as due to the F_1 individuals producing equal numbers of gametes bearing
the {24} dominant and recessive elements respectively. If now the F_1 plant
be crossed with the pure recessive, we are bringing together a series of
gametes consisting of equal numbers of dominants and recessives with a
series consisting solely of recessives. We ought from such a cross to
obtain equal numbers of dominant and recessive individuals, and further,
the dominants so produced ought all to give both dominants and recessives
in the ratio 3 : 1 when they themselves are bred from. Both of these
expectations were amply confirmed by experiment, and crossing with the
recessive is now a recognised way of testing whether a plant or animal
bearing a dominant character is a pure dominant, or an impure dominant
which is carrying the recessive character. In the former case the offspring
will be all of the dominant form, while in the latter they will consist on
the average of equal numbers of dominants and recessives.
So far we have been concerned with the results obtained when two
individuals differing in a single pair of characters are crossed together
and with the interpretation of those results. But Mendel also used plants
which differed in more than a single pair of differentiating characters. In
such cases he found that each pair of characters followed the same definite
rule, but that the inheritance of each pair was absolutely independent of
the other. Thus, for example, when a tall plant bearing coloured flowers
was crossed with a dwarf plant {25} bearing white flowers the resulting
hybrid was a tall plant with coloured flowers. For coloured flowers are
dominant to white, and tallness is dominant to dwarfness. In the succeeding
generation there are plants with coloured flowers and plants with white
flowers in the proportion of 3 : 1, and at the same time tall plants and
dwarf plants in the same proportion. Hence the chances that a tall plant
will have coloured flowers are three times as great as its chance of having
white flowers. And this is also true for the dwarf plants. As the result of
this cross, therefore, we should expect an F_2 generation consisting of
four classes, viz. coloured talls, white talls, coloured dwarfs, and white
dwarfs, and we should further expect these four forms to appear in the
ratio of 9 coloured talls, 3 white talls, 3 coloured dwarfs, and 1 white
dwarf. For this is the only ratio which satisfies the conditions that the
talls should be to the dwarfs as 3 : 1, and at the same time the coloured
should be to the whites as 3 : 1. And these are the proportions that Mendel
found to obtain actually in his experiments. Put in a more general form, it
may be stated that when two individuals are crossed which differ in two
pairs of differentiating characters the hybrids (F_1) are all of the same
form, exhibiting the dominant character of each of the two pairs, while the
F_2 generation produced by such hybrids consists on the average of 9
showing both dominants, 3 showing one dominant and one recessive, {26} 3
showing the other dominant and the other recessive, and 1 showing both
recessive characters. And, as Mendel pointed out, the principle may be
extended indefinitely. If, for example, the parents differ in three pair of
characters A, B, and C, respectively dominant to a, b, and c, the F_1
individuals will be all of the form ABC, while the F_2 generation will
consists of 27 ABC, 9 ABc, 9 AbC, 9 aBC, 3 Abc, 3 aBc, 3 abC, and 1 abc.
When individuals differing in a number of alternative characters are
crossed together, the hybrid generation, provided that the original parents
were of pure strains, consists of plants of the same form; but when these
are bred from a redistribution of the various characters occurs. That
redistribution follows the same definite rule for each character, and if
the constitution of the original parents be known, the nature of the F_2
generation, _i.e._ the number of possible forms and the proportions in
which they occur, can be readily calculated. Moreover, as Mendel showed, we
can calculate also the chances of any given form breeding true. To this
point, however, we shall return later.
Of Mendel's experiments with beans it is sufficient to say here that they
corroborated his more ample work with peas. He is also known to have made
experiments with many other plants, and a few of his results are
incidentally given in his series of letters to Naegeli the botanist. To the
breeding and crossing of bees he also devoted much {27} time and attention,
but unhappily the record of these experiments appears to have been lost.
The only other published work that we possess dealing with heredity is a
brief paper on some crossing experiments with the hawkweeds (_Hieracium_),
a genus that he chose for working with because of the enormous number of
forms under which it naturally exists. By crossing together the more
distinct varieties, he evidently hoped to produce some of these numerous
wild forms, and so throw light upon their origin and nature. In this hope
he was disappointed. Owing in part to the great technical difficulties
attending the cross fertilisation of these flowers he succeeded in
obtaining very few hybrids. Moreover, the behaviour of those which he did
obtain was quite contrary to what he had found in the peas. Instead of
giving a variety of forms in the F_2 generation, they bred true and
continued to do so as long as they were kept under observation. More recent
research has shown that this is due to a peculiar form of parthenogenesis
(cf. p. 135), and not to any failure of the characters to separate clearly
from one another in the gametes. Mendel, however, could not have known of
this, and his inability to discover in _Hieracium_ any indication of the
rule which he had found to hold good for both peas and beans must have been
a source of considerable disappointment. Whether for this reason, or owing
to the utter neglect of his work by the scientific world, Mendel gave up
his experimental {28} researches during the latter part of his life. His
closing years were shadowed with ill-health and embittered by a controversy
with the Government on a question of the rights of his monastery. He died
of Bright's disease in 1884.
_Note._--Shortly after the discovery of Mendel's paper a need was felt
for terms of a general nature to express the constitution of
individuals in respect of inherited characters, and Bateson accordingly
proposed the words homozygote and heterozygote. An individual is said
to be homozygous for a given character when it has been formed by two
gametes each bearing the character, and all the gametes of a homozygote
bear the character in respect of which it is homozygous. When, however,
the zygote is formed by two gametes of which one bears the given
character while the other does not, it is said to be heterozygous for
the character in question, and only half the gametes produced by such a
heterozygote bear the character. An individual may be homozygous for
one or more characters, and at the same time may be heterozygous for
others.
* * * * *
{29}