By combining in various ways a comparatively few notes the musician is able to make a large number of distinct harmonies. So nature, by combining in various ways the relatively few factors [alleles] which the Creator supplied to each natural species, may produce a large number of distinct varieties. ("After its Kind" 1927, p. 109)
The idea here is that God created each baramin with a certain number of genetic variants called alleles, and just by mixing and matching those alleles, new species were produced within the baramin. The limit of variation was simply the number of alleles created within a baramin. "Some species have many, consequently their power of variation is large. Some have few, consequently their power of variation is small" (p. 109; note that Nelson's concept of the "kind" is a just very big species or a superspecies). Nelson considered mendelian genetics "the last word on evolution."
Will Tinkle, one of the "Team of Ten" that started the Creation Research Society, also endorsed this idea in his book Heredity: A Study in Science and the Bible (St. Thomas Press, 1967). Tinkle called it the "Theory of Heterozygous Creation," which he introduced with an example of variation in heterozygous peas (a very Mendelian choice).
These simple examples have been chosen to illustrate the potential segregation of genes in the original kinds. The possible extent of such variation is a subject for further research. This variation is limited by the number of latent genes created in the plants and animals but the number of new characters appearing can be greater than the number of latent genes, since a gene by itself could make one character, and another character in combination with another gene. Dominant genes pose a problem, since it is impossible for them to be latent. But dominance is only relative, in that a gene which is dominant to one may be recessive to another gene (Heredity, pp. 89-90).
This model is still quite popular with creationists for what I consider to be mostly psychological or apologetic reasons: (1) It does not require the generation of new attributes or "genetic information" (whatever that is) after creation. (2) All changes can be perceived as degenerative, since the originally created heterozygous condition is broken up and disturbed as species separate out alleles from that original gene pool. (3) If you want to twist the definition of evolution, you can claim that speciation by this mechanism is "not evolution."
I call these psychological or apologetic reasons, since I can't find much of an evidential basis for the "theory of heterozygous creation." A person could hold to this theory because of some personal desire or because it fits a propaganda/apologetic agenda, but a brief familiarization with the facts should be enough to sow serious doubts about "heterozygous creation."
Let's take just one example: horses (family Equidae). There's 8-10 species of living horses (depending on how you count them), including zebras, donkeys, and domestic horses. Most creationists accept the ability to hybridize as evidence that the parent species came from the same baramin (created kind), and in the case of the horses, all possible hybrids are known (and most are fertile - the mule is the exception). As members of a single baramin, the modern horse, zebra, and donkey species descended from a single pair of "horses" on the ark (we'll ignore the fossil horses for this example). [For more on horse hybrids, see the forthcoming CORE Issues volume on "Basic Types of Life" coming this summer.]
Now if the theory of heterozygous creation is correct, we should see roughly four alleles per gene locus and a common set of chromosomes (with some leniency for rare mutations). We should also expect to see a pretty uniform and low level of sequence similarity between the horse species. What do we actually find? A lot of variation, a lot more than we should expect if heterozygous creation were correct.
First of all, the chromosomes are highly variable among horses. Hartmann's mountain zebra has 32 chromosomes (2n), while Przewalski's horse has 66 (2n). We're not talking about minor variations either, since the fundamental number varies from 62 in Hartmann's zebra to 102 in the donkey. Horse chromosomes are highly rearranged, something that is not anticipated or predicted by the theory of heterozygous creation. (See Bowling and Ruvinsky, 2000, The Genetics of the Horse CABI publishing, for more information). This kind of variation implies significant chromosomal rearrangement during the origin of the modern horse species.
Second, sequence variation among horse species is surprisingly high. Even within the domestic horse species, mitochondrial DNA is quite variable (Xu and Arnason 1994). Recently, ancient DNA studies have begun to show that some extinct horses from the New World are actually closely related to our modern horses (e.g., see Weinstock et al. 2005), which makes the horse baramin even bigger and more variable.
These differences are not adequately explained by mendelian genetics. This is a point that some creationists like Jean Lightner and I have been trying to stress. Creationists need to grow beyond this century-old mendelian mechanism and start thinking more creatively. Genomes are not unchanging blobs of DNA. They are fluid structures capable of radical reorganization, and I strongly suspect that these reorganizations can be linked to environmental changes. Mendel did great science in his day, but we've come a long way since then. It's time to let him rest and to see genomes for the variable structures that they are.