Genetic Diagnosis (PGD)
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) produces 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 in
vitro: several early-stage blastocyst embryos 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 1 in 4 embryos is homozygous aa
and will have black fur. One of these is re-implanted in Ms Mus
and allowed to develop to term.
Note that no "genetic engineering"
has been employed. The embryos were produced naturally, by in
vitro fertilization. The genetic screen identified
naturally-occurring variation among them 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 6 ~ 10 cell blastula
stage, while the cells are still pluripotent (able to
develop into any tissue type).
See the 1997 movie GATACCA for a discussion of the implications.
modified after ©2000 Griffith et al.; all text material
©2013 by Steven M. Carr