Pre-implantation 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.


Figure modified after Griffith et al. 2000; all text material 2013 by Steven M. Carr