The Story of a Loaf of BreadWood, T. B. (Thomas Barlow)
Science
The Story of a Loaf of Bread
Wood, T. B. (Thomas Barlow)
Bread; Wheat
Translating this into words, out of every 16 plants in the second
generation there were four long eared plants, three beardless and one
bearded; eight plants with ears of intermediate length, six beardless
and two bearded; and four short eared plants, three beardless and one
bearded. The illustration shows all these types. The experiment has
been repeated several times and the same proportions were invariably
obtained. The result, too, was independent of the way the cross was
made. Seed was collected separately from large numbers of single
plants of each type. The seed from each plant was sown by itself
in a row, so that its progeny could be separately observed. It was
found that all the plants of the second generation possessing ears of
intermediate length produced in the third generation plants with long
ears, short ears, and medium ears in the proportion of 1 : 1 : 2, the
same proportion in fact as in the second generation. Short eared plants
produced only short eared offspring, long eared plants only long eared
offspring. Bearded plants produced only bearded offspring. Beardless
plants, however, produced in some cases only beardless offspring, in
other cases both beardless and bearded offspring in the proportion of
three of the former to one of the latter. Out of every three beardless
plants only one was found to breed true, whilst two gave a mixed
progeny. It appears therefore that in the second generation some of the
types which occur breed true, whilst others do not. Some of the true
breeding individuals can be picked out at sight, for instance, those
with long or short bearded ears. Some of those which will not breed
true can also be recognised by inspection, for instance, all the plants
with ears of intermediate length. In other cases it is only possible to
pick out the individual plants which breed true by growing their seed
and observing how it behaves. If it produces progeny all of which are
like the plant from which the seed was obtained, that plant is a fixed
type and will breed true continuously in the future. The final result
of the experiment was to obtain in three years from the time the cross
was made, four fixed types which subsequent experience has shown breed
true continuously, a long eared bearded type, a short eared beardless
type, a long eared beardless type and a short eared bearded type. Of
these the second two are exactly like the two parental varieties, but
the first two are new, each combining one character from each parent.
These fixed types already existed in the second generation. Mendel’s
discoveries with peas showed how to pick them out. Obviously there is
no need for the years of roguing by which the older hybridisers used
to attempt to fix their desired type. All the types are present in the
second generation. Mendel has shown how the fixed ones may be picked
out.
[Illustration: Fig. 4. _P_, _P_, the two parental types. _F₁_ the first
cross. _F₂_, 1-6, the types found in the second generation]
Public-domain text, read in full here on John Shaqi.
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