I think that Alphas and Betas are unmodified (except for subliminal conditioning), but are sorted by intellectual capability pre-conception. Certainly Deltas and Epsilons are stunted artificially
'Oh, stuff like magnesium salts, and alcohol for keeping the Deltas
and Epsilons small and backward, and calcium carbonate for bones, and
all that sort of thing.'
Here's where the early stages of the process are outlined
Still leaning against the incubators he gave them, while the pencils
scurried illegibly across the pages, a brief description of the modern
fertilizing process; spoke first, of course, of its surgical
introduction--'the operation undergone voluntarily for the good of
Society, not to mention the fact that it carries a bonus amounting to
six months' salary'; continued with some account of the technique for
preserving the excised ovary alive and actively developing; passed on
to a consideration of optimum temperature, salinity, viscosity;
referred to the liquor in which the detached and ripened eggs were
kept; and, leading his charges to the work tables, actually showed
them how this liquor was drawn off from the test-tubes; how it was let
out drop by drop on to the specially warmed slides of the microscopes;
how the eggs which it contained were inspected for abnormalities,
counted and transferred to a porous receptacle; how (and he now took
them to watch the operation) this receptacle was immersed in a warm
bouillon containing free-swimming spermatozoa--at a minimum
concentration of one hundred thousand per cubic centimetre, he
insisted; and how, after ten minutes, the container was lifted out of
the liquor and its contents re-examined; how, if any of the eggs
remained unfertilized, it was again immersed, and, if necessary, yet
again; how the fertilized ova went back to the incubators; where the
Alphas and Betas remained until definitely bottled; while the Gammas,
Deltas and Epsilons were brought out again, after only thirty-six
hours, to undergo Bokanovsky's Process.
The Bokanovsky process is cloning.
Another discussion, here
'Reducing the number of revolutions per minute,' Mr. Foster
explained. 'The surrogate goes round slower; therefore passes through
the lung at longer intervals; therefore gives the embryo less oxygen.
Nothing like oxygen-shortage for keeping an embryo below par.' Again
he rubbed his hands.
'But why do you want to keep the embryo below par?' asked an ingenuous
student.
'Ass!' said the Director, breaking a long silence. 'Hasn't it occurred
to you that an Epsilon embryo must have an Epsilon environment as well
as an Epsilon heredity?'
It evidently hadn't occurred to him. He was covered with confusion.
'The lower the caste,' said Mr. Foster, 'the shorter the oxygen.' The
first organ affected was the brain.
So, an embryo that looks like it will be physically healthy and handsome and very smart gets Alpha conditioning and education, while a physically normal embryo expected to be of normal intelligence gets Beta conditioning.
Poor Bernard Marx is an anomaly - he must have looked like he'd be a handsome Alpha type, and he's a very smart fellow, but something went wrong, making him short and unusual looking. Naturally, people think he looks like someone gave him the Delta treatment (but since the full Delta or Epsilon treatment would have been bad for the brain, that almost certainly didn't happen).
