In the tenth generation of the plus selection series there appeared two
individuals, a male and a female, of considerably higher grade than any
previously recorded in this series. They are not included in Table 10
because we have been and still are in doubt as to their exact nature and
think it best to give a separate account of them. If entered in Table 10
one would appear as a 5½ individual born of 3⅞ parents (mean grade), the
other as a 5¾ individual born of 3¾ parents (mean grade). The nearest
individuals in grade to these two produced by the same group of parents
are of grade 4½, but some 4⅛ parents of the same generation produced two
offspring of grade 5. (See Table 10.) Because of the marked advance in
grade of these individuals beyond the ordinary range of variation in the
series we called them “mutants,” without wishing then or now to commit
ourselves to any particular theory as to their nature or origin. We
have used the term and now use it as one of convenience merely. The two
“mutant” individuals had the same father and their mothers were sisters.
Their pedigree for two generations is as follows:
Mutants. Parents. Grandparents.
{ ♀ 2956, +3¾ } { ♀ 1939, +3¾
♂ 4763, +5½ { ♀ 2957, +3½ } { ♂ 1817, +3¾
{
♀ 5153, +5¾ { { ♀ 1162, +3½
{ ♂ 2963, +4 { ♂ 1810, +3¾
The mutant male was mated with the mutant female and also with other
females of the plus series, with the results shown in Table 51. In every
case the young fall into two distinct groups, one of which varies about
the general mean of the plus series (approximately 3¾), while the other
varies about the father’s grade as a mean (approximately 5½).
The mutant female had 16 young, 6 in the lower group, mean 3.87, and 10
in the upper group, mean 5.60. (See Table 51, lowest row.) The other
females had in all 114 young almost equally divided between the two
groups, 58 in the lower group, mean 3.73, and 56 in the upper group,
mean 5.45. This result indicates clearly (what the sequel also confirms)
that the male mutant transmitted in half his gametes the high grade of
pigmentation which he himself manifested, while in the other half of his
gametes he transmitted the ordinary condition of the plus race at that
time. In other words his “mutant” character behaved as a dominant unit in
relation to the ordinary condition of the plus race.
It is evident that the female mutant was of similar constitution. This
being the case, we should expect three-fourths of the offspring of the
two mutants to be in the upper group. In reality 10 of their 16 young
were of this sort.
The male mutant was mated also with females of the minus series with
the results indicated in Table 52. Again, the offspring fall into
two distinct groups, a lower and an upper. The lower group should be
comparable with the result obtained in F₁ when the plus and minus races
are crossed with each other. (Compare Table 50.) Such it proves to be. It
includes 35 individuals of mean grade -0.49 and standard deviation 0.77.
Series 2 of Table 50 is nearly contemporaneous with this experiment. The
F₁ offspring in that series were of mean grade -1 and standard deviation
0.60.
The upper group of offspring (Table 52) result, we may suppose, from a
mutant gamete (grade about 5½) united with a narrow series gamete (grade
about -2). This group includes 31 individuals varying closely about
grade 4½, and with a standard deviation of only 0.31. The lower average
grade of this group (4.43) compared with the similar group of Table 51,
which had a mean of 5.47, shows the influence of the minus-series gamete
upon the heterozygote in lowering its grade by about 1. Whether the
plus-series gametes have any effect upon the grade of the heterozygotes
recorded in the upper group of Table 51 is not certain, because a
_homozygous_ group of mutants has not yet been established. It may be
observed, however, that one individual in the upper group of Table 51
was of grade 6 (colored all over), and it is possible that homozygous
“mutants,” when obtained, will approximate that grade, as most wild
rats do. Further, a comparison of Tables 51 and 53 shows that mutant
heterozygotes formed by crosses with the plus series are of slightly
_lower_ mean grade than the offspring of the two mutants, among which
should occur both homozygous and heterozygous mutants. It seems probable,
therefore, that homozygous mutants will be found to be of somewhat higher
grade than heterozygous ones.
The question early suggested itself to our minds, will these “mutants”
prove to be mutants in the sense of De Vries? Will they prove to be
more stable than the modifications ordinarily secured by selection in
our experiments? To test this matter, we have raised two additional
generations of offspring from the two mutants and have bred a second
generation of offspring from each of the four groups of F₁ offspring
recorded in Tables 51 and 52, derived from matings with the plus and
minus races respectively.
The F₂ descendants of the two original mutants proved very similar to the
F₁ descendants. (See Table 53.) They fall as before into two groups, an
upper and a lower. The former includes 30 individuals of mean grade 5.52,
the latter 2 of mean grade 3.37. As the parents of this generation were
taken wholly from the _upper_ group of offspring of generation F₁, and as
theoretically that group should contain 2 heterozygous individuals to one
which is homozygous for the “mutant” character, it is to be expected that
in F₂ more than three-fourths of the offspring will fall in the upper
group. For any pair, one member of which is homozygous for the mutant
character, should produce only offspring falling in the upper group; and
offspring falling in the _lower_ group should be produced only by pairs
_both_ members of which are heterozygous.
The upper group in F₂ should contain a larger proportion of homozygous
mutants than in F₁, and since the parents of F₃ were chosen from
this upper group of F₂ offspring, it is not surprising that the 11
F₃ offspring recorded up to this time all fall in the upper group.
The mean of this upper group is remarkably constant through the three
generations, and the variability of the group as measured by its
standard deviation is also low, namely, 0.19. This indicates that the
mutant character is a strongly dominant unit in relation to the ordinary
condition of the plus series.
Table 54 shows the character of the F₂ offspring of the original male
mutant mated with females of the plus series. The lower group parents,
those into which the mutant character did not presumably enter at all,
produced 59 offspring recorded in the first part of Table 54. Their mean
grade is 3.78 and their standard deviation 0.33. These are very close to
the constants of the general plus series, which for generation 10 were
3.73 and 0.36, respectively.
The second division of Table 54 shows the character of the young produced
by the F₁ parents of the upper group (Table 51). Such parents are
supposed to have received a “mutant” gamete from their father, grade
about 5.50, and a plus-series gamete from their mother, grade about 3.75.
If they produce gametes of these same two sorts, their offspring should
also fall into two corresponding groups; in fact they do. There are 11
offspring of mean grade 3.86 and 79 offspring of mean grade 5.50. As in
the previous generation, the two groups do not approach each other in
grade. The mean and standard deviation of the lower group of offspring
are similar to those of the plus race. The mean of the upper group is
about the same as that of their parents (upper group of offspring, Table
51), namely, 5.50, as compared with 5.45; their standard deviation
is somewhat lower, namely, 0.15, as compared with 0.23. This result
indicates that the “mutant” character and the hooded character of the
plus series segregate from each other in a simple way without modifying
each other appreciably. It seems possible that they contain the _same
modifiers_ (if modifiers are present) and differ merely by the main unit
which we called the hooded character in the early part of this paper.
Each contains a different condition of that main unit. Consequently
there is no increase of variability in F₂ when these two conditions are
intercrossed. This we should expect to happen, if they differed by more
than a single factor.
A very different result is obtained from the cross between the mutant
and narrow races. Although F₁ from that cross was quite variable (see
Table 52), F₂ is still more variable (see Table 55). The lower group
F₁ individuals, which resembled F₁ between the plus and minus races,
produced 61 young (first division of Table 55), which resemble F₂ between
the plus and minus races. They range in grade from -2 to +3¼, mean +0.58,
standard deviation 1.17. In the two series of crosses between the plus
and minus races (Table 50) the means were +0.24 and +0.72, respectively,
and the standard deviations 1.01 and 0.87. This indicates, as did the
cross with the plus series, that the “lower group” gametes produced by
the original mutant male did not differ materially from gametes produced
by the ordinary plus race from which the mutant sprang.
The second division of Table 55 shows the character of the F₂ young
produced by the upper group of F₁ offspring recorded in Table 52. It
consists of two groups, a lower and an upper. The lower represents the
extracted minus race, the upper represents the extracted dominants or
mutants, whether homozygous or heterozygous. The former group has an
average of +0.75 and a standard deviation of 1.03, which values are
close to the corresponding constants of Series 2, Table 50, the latest
of the plus-minus crosses, in which the mean was +0.72 and the standard
deviation 0.87.
The upper group offspring of Table 55, second division, the homozygous
and heterozygous mutants, number 68; they have a mean grade of 4.77 as
compared with 4.43 in F₁, which consisted exclusively of heterozygotes.
This shows the extracted homozygotes to be of higher grade than the
heterozygotes. The highest grade mutant among the 31 F₁ young, all of
which were heterozygotes, was of grade 5, but among the 68 F₂ young are
16 of higher grade than 5. We expect one-third of these 68 individuals to
be homozygotes. Now all of the F₂ mutants from the cross of mutant with
plus race (Table 54) were of grade 5 or higher, only 2 in 79 being as low
as 5, and 13 of the 79 being of grade 5¾, a grade not attained at all in
F₂ from the mutant-minus cross (Table 55). This result shows us that the
cross with the minus race does affect permanently the mutant character,
lowering its grade even in homozygous mutants extracted from the cross.
It also increases the variability of the mutants, for the standard
deviation of the mutant group in Table 55 is 0.44, whereas in Table 54
(mutant-plus F₂), in a like number of individuals, it was 0.15, or only
about one-third as great.
That the variability of the mutants is unaffected by a cross with the
plus race, but that it is increased by a cross with the minus race, and
that, further, the mean of the mutants is affected little or none by a
cross with the plus race, but that it is lowered by a cross with the
minus race—these several facts are all conformable with the hypothesis
that _the change in variability due either to crossing or to selection
results from modifying factors_ which, as they are independent of the
main factor concerned, are probably transmitted in a different part or
component of the germ-cell than that factor. For if the mutant and the
plus race are alike as regards the modifiers, but differ only in the main
factor, then no change in variability should result from intercrossing
them, but only alternative conditions as regards the main factor. This
is the observed result. But if the mutant and the minus race differ not
only in the main factor, but also in modifiers which are independent of
it, then, when they are crossed, we may expect that through independent
segregation of main factor and modifiers the extracted minus race will be
raised in grade, while the extracted mutants are lowered, and both will
become more variable. This also is the observed result.
One objection may be offered to this interpretation, namely, that
the increased variability is not delayed until F₂, but is already in
evidence to some extent in F₁. The same thing was observable in the
crosses of the plus and minus series (Table 50). From that table, Series
1, it will be observed that when the plus and minus races had standard
deviations of 0.49 and 0.50, respectively, their F₁ offspring had a
standard deviation of 0.71, an increase by nearly one-half; F₂ showed
a further increase to 1.01. In series 2, Table 50, the uncrossed races
(generation 10) had standard deviations of 0.36 and 0.24; their F₁
offspring had a standard deviation of practically twice this, namely
0.60; F₂ showed a further increase to 0.87.
At the time of the mutant-minus race crosses, the minus race (generation
10) had a standard deviation of 0.24, the plus race of 0.36. F₁ (lower
group) had a standard deviation of 0.77, and F₂ of 1.17. F₁ mutants
(upper group) had a standard deviation of 0.31 which rose in F₂ to 0.44.
These various facts will perhaps be better grasped if presented in
tabular form: