Mr & Ms Mus
have white fur, and desire offspring with black fur. Both
are heterozygous (Aa) for an allele (a) at a locus
that when homozygous (aa) results in offspring with black
fur. The allelic difference is detectable with an ASO test. The parents seek
genetic counseling. Ova from Ms Mus are extracted and fertilized
with sperm from Mr Mus in vitro. several early-stage blastulae
(blastocysts) are extracted from her and maintained in
cell culture. A single cell
from each embryo is extracted and its DNA analyzed with
the ASO test. Three-quarters of the embryos are expected
to be either homozygous AA (top) or heterozygous Aa
(bottom); they are discarded. Approximately one-quarter of the
embryos are homozygous aa (middle) and will have black
fur. One of these is re-implanted in Ms Mus and allowed to develop
to term.
No "genetic engineering" has been employed.
The zygotes were produced by in vitro fertilization, a
common procedure. The ASO screen identified
naturally-occurring variation, and gave the parents a choice at a
very early stage of development. In humans, Pre-implantation
Genetic Diagnosis is currently available for a number of
serious, single-locus genetic diseases. Diagnosis is made at the ~100 cell blastula
stage, while the cells are still pluripotent (able to
differentiate into any tissue type).
See the 1997 movie GATACCA for a discussion of the implications.
