There is a common perception that if you start with something with a desirable trait and "line breed"+select, you can enhance the trait and eventually get whatever you want. It's wrong. There's a limited amount of genetic diversity underlying each trait that exists in the population. You cannot go beyond that without involving some kind of mutagenesis or transgenic approaches (I seriously doubt any has been applied to severums). Not to mention that there are so many other factors that would influence the outcome.
- how many genes are involved in the given trait.
- how many alleles there are for each gene, the the phenotypes conferred by each allele.
- dominance vs. recessive and in rare cases, allele compatibility
- how these genes physically relate to each other (linkage)
- whether the alleles are closely linked to any deleterious alleles (of other genes), or similarly, whether they are autosomal or on sex chromosomes. This affects the allele frequency in a population as well as the segregation of independent traits, among other things.
We have two questions at hand:
1) The origin of the "red spotted + gold background" trait. There are at least three possibilities:
(A) A combination of existing alleles from the same species. This is consistent with the "line-breeding" hypothesis, but it's difficult to explain the sterility. Inbred depression is common but how many inbred dog lines are sterile?
(B) A combination of genes from different species. This is consistent with the "hybrid" hypothesis favored by Ken Davis and others (i.e., regular gold sev crossed with another red-spotted species, such as H. efasciatus). This could also explain the sterility phenotype as F1s from interspecific crosses, while sometimes viable, are often sterile.
(C) A mutation(s) is involved. If this is the case, a loss-of-function mutation of a single gene (e.g., one that's localized on a male sex chromosome, see below) could be responsible for the phenotype. Ectopic over-expression of a gene is a straightforward alternative.
2) The apparent observation that all red severums are males. Perhaps the key question is - how is sex determined in severums? Again, let's look at the possibilities (all three below do happen in fish).
A) Sex is determined genetically. Pure and simple - like us. If this is the case, it's most likely that the "red gene" is on the male sex chromosome. It also follows that if you cross a red male to a gold female, all males in the F1 generation are red, and all females are gold.
B) Sex is determined by the environmental conditions under which fertilization and/or embryo development take place. If this is the case, it could be that the expression of the "red gene" is favored under the conditions that produce males only. It follows that in any single red male x gold female cross, all F1s are either male reds, or female golds.
C) Sex is determined by post-embryonic development. A classic example is the Midas experiment done by R.C. Francis and G.W. Barlow (PNAS, Vol. 90, pp. 10673-10675), which showed that the fish that grow fastest BECOME males, and the fish that grow slower BECOME females. In other words, each fish has the potential to become male or female, but sex is determined by their growth. If this is the case for severums, it follows that in a red male x gold female cross, the bigger fish in the F1 are red males, and the smaller ones are gold females.
I don't know how many people in the world truly know what red severums are, and reading internet articles/posts will never make me one of them. Just want to point out the potential alternatives. If only I can find some evidence that can be reproduced by independent sources!
