Please explain why having at least one dominant gene in each pair would make the organism stronger/better.

Successful genetic characteristics, such as those that are the result of adaptation to the environment, are generally dominant so they are passed to more offspring, ensuring survival of the species.

Well yes, but why would it make the organism stronger/better than the dominant-gene-holding parent? I.e., please explain why being homozygous makes an organism weak.

Also, why is it then that in the organic gardening community, there is the assumption that "heirloom"/"pure" breeds (of plants, chooks, whatever) are more resilient and less disease-prone than modern hybrids? Is this just a falsity that is used to justify the snobbery of being into "heirloom" plants and being against this form of genetic modification, or is there something more to it?

Not all hybrids are "more vigorous" than their homozygous counter-parts (it is quite common for hybrids to be very un-vigorous in that they cannot reporduce, this is the case with mules, the offspring of a horse and donkey). Partly, it depends on how you define "hybrid" and how you define "species." Usually species is defined as individuals that can produce viable offspring with one another. A true hybrid is a cross between two species and is therefore an evolutionary dead-end (as the aforementioned mule and most of the plants used in today's agriculture). So usually when people talk about "hybrid vigor" they're not talking about a real hybrid, but rather two categories of individuals who are in the same species, such as humans with blue or brown eyes. For a recessive trait, such as blue eyes to be consistent in a population, only blue-eyed people can produce offspring together. This results in a smaller gene pool which can then lead to inbreeding depression. My eyes might be a pretty blue, but I can't walk right and my teeth are really messed up because the few faults that each of my (small) pool of ancestors possessed has been magnified through in-breeding. So when someone with brown eyes and new genes comes in and makes babies with some blue-eyed people, those off-spring tend to be healthier because now they have different genes that can allow them to walk correctly and have good teeth. So much for the superiority of the Aryan race (I chose blue eyes because I actually happen to have blue eyes). As to your question on heirloom breeds vs. modern hybrids: the problem with the hybrids is (like I hinted at above) they cannot reproduce on their own (so they cannot evolve to changing conditions). They have been developed from a fixed line of plants and are essentially clones. When you fill thousands of acres with genetically identical individuals, you are setting yourself up for major disaster through disease or simply changing environmental conditions. The heirloom breeds actually contain large amounts of genetic diversity within themselves and so are more capable to deal with changing conditions. The bottom line is: the more diversity you have in your genome, the more flexibility you have in dealing with novel situations (the more likely you are to have the gene that is suited to the given situation).

I agree that Obama is awesome mostly because of his unique experiences and not necessarily because of any innate genetic qualities. But thank goodness he's president so we can better educate the masses on topics such as hybrid vigor!

I vaguely recall the problem with pure line breeding isn't necessarily the straight transfer of genes, but minor genetic flaws/weaknesses being bred back on themselves and thus increasing in concentration/frequency, like the tendency for purebred dogs to have higher numbers of hip dysplasia cases. Hybrid breeding reduces the chance that defective genes will express themselves because the organism could have other genes possibly canceling them out, instead of doubling up.

As far as heirloom tomatoes go, I just prefer them because they're tasty. :9

Rosie, here's how I like to think about the issue of dominant/recessive genes and hybrid vigor: Imagine that you have two parents (human or otherwise) that are genetically dissimilar (their ancestors haven't had any contact for many, many generations). Their genomes will each be a mix of dominant and recessive genes. The easiest way to think about a recessive gene is that it's a damaged form of a dominant gene. It was damaged through some form of mutation (there are lots of 'natural' ways for this to happen) and it's less effective at making whatever it's supposed to make than the undamaged, 'dominant' gene. Over the course of time, genes get damaged; the process of sex helps to purge those bad genes from populations. So the recessive genes that one parent has were generated more or less randomly over time (not just in their lifetime, but also over the lifetime of their ancestors), as were the recessives in the other parent. So chances are, the set of recessives in each parent are non-overlapping, meaning that most of the recessives in one parent do not exist in the other. When these parents mate and produce a kid, the kid gets half of it's genetic material from one parent (one copy of every gene), and half from the other. Chances are that the recessive, damaged genes of one parent will be 'covered up' or masked by the undamaged, dominant genes of the other parent, resulting in a genome composed of very few 'double recessives', or homozygous-recessives. That explains the phenomenon known as 'hybrid vigor'. It's those 'double recessives' that usually produce a disease. That's why closely related people shouldn't have kids, because their recessives have a better than average chance of matching up, producing a lot of disease if the kid is unlucky enough to get the bad copy from both parents. This is all super-simplistic, but I hope it helps get the basic point across! Maybe this is why lots of people are so turned on by people with accents different than their own: something deep in their genome is driving them to mate with very unrelated people, and the accent is a marker for that? This may also explain why interracial people tend to be more objectively beautiful.
To answer your plant question: as far as i understand 'heirloom' varieties are open-pollinated, old, stable populations of plants. That means that there is a lot of genetic variation in the line that gets mixed every generation. That would be good for disease resistance. Other, hybrid varieties likely have much smaller population sizes and are highly inbred, which can cause genetic 'weakness'. Again, this is a simplistic answer, there are benefits to these hybrids too. Think about purebred dog and cat lines; many of them have common genetic problems. Mutts are much more fit in general (genetically). Hope this helps!

Just to add a little to the 'Biology is awesome' comment. The degree of 'hybridism' does indeed depend on the level you look at, i.e population, race (not talking about the human interpretation of race, but the biological concept), species etc. But to be clear, even those who support the biological species concept will agree that many species can hybridize and form fertile offspring. Most, if not all of the research currently being done on the issue of speciation is wholly dependent on that fact. There are many examples of this; different species of fruit flies can interbreed (as can some other insects), as can some species of fish, as well as amphibians, reptiles (like birds), etc. This process is rampant in plants, as they seem to be able to absorb more genetic oddities than animals. In fact, the phenomenon of allopolyploidization (which is essentially the melding of two separate genomes, each from a different species, within an single organism while keeping those genomes from interacting with one another) is a major driving force in the speciation of all modern flowering plants. Hybridization, therefore, is far from an evolutionary dead-end, it's a very common mechanism for evolution in plants (although probably less so in animals). Not trying to be a dick, just trying to clear things up! Also, modern crops are a totally different beast. They're engineered to be sterile so the evil bastards at Monsanto and the like can keep making money. Their sterility is not a byproduct of being hybrids.

The author of this column may be interested in "outbreeding depression." Sometimes when two genetically distinct populations mix the progeny are less fit than either parent population because they form an intermediate type not well suited to either environment (check out: Lynch 1991, Gharrett et al. 1999, Luijten et al. 2002, Marr et al. 2002).

While I am familiar with the fact that hybridization between species of plants can lead to fertile offspring, I didn't mention it here because whenever this topic has come up in ecology seminars I've participated in, it usually results in a huge debate over what exactly a "species" or "hybrid" really is. I have never seen the concept of heterosis (hybrid vigor) refer to the crossing of species, but rather the crossing of phenotypes or breeds within a species. This was the main point I was trying to make. And I think the literature would still support my claim that the vast majority of inter-species crosses result in infertile hybrids. You may notice the fertile hybrids more because they can reproduce, but these are actually very rare. Of course, this whole topic is getting a lot of attention and debate these days given that we usually base ESA and other conservation recommendations based on the number of individuals of a species present in a given area. So some people want to err on the side of caution and call something a new species if it's a viable hybrid of two other species (so there would then be few individuals of 3 species present), while others would say there is just one species present (in high numbers) because the two varieties formerly considered species were able to produce a viable offspring. So my question is: how do you reconcile supporting the biological species concept and the hypothesis that two different species can produce a viable offspring? I don't study speciation myself, so I haven't really had to think about this in any formal way, but I have struggled answering this question myself and would be curious to know how you think about it, helix, since it seems you are more involved in this area than I am.

try not to confuse "Dominant" genes/traits with "fittest." Dominant doesn't mean better or even better adapted. it just means loudest phenotype.
I suspect that vigor is one of those traits heirloom plants are bred for. Not for nifty spots or strange compulsive behavior or adorable pocket-sized.
Prez Obama is awesome because he supports SCIENCE!!! he's not a creationist, armageddonist, environmental and educational abdicating fool likt those last pricks.

Biology is awesome is awesome!

You raise some really good points, BiA. As you have seen, the concept of 'species' is still very contentious. Species have traditionally been defined (and are largely still defined) based solely on morphology. Someone goes out into the field and collects something that looks different than anything else described, and then they usually describe it as a different species. This has lots of problems (like male and female mallards being originally described as different species and the fact that species people care about for one reason or another are often subject to excessive 'splitting'), but given the difficulty and expense associated with sequencing (to infer species based on phylogeny) and the huge burden of actually observing that newly described organism to see who it can successfully mate with, we are forced to use morphology as a first-pass for describing new species. This turns out to be 'right' the vast majority of the time when other methods are used to check for relatedness to other organisms. So how does this get folded into the biological species concept (BSA)? The BSA is a nice way to think about species, but it's almost always applied post-hoc. It's very rare that you can actually test it with most organisms; or even if you can, it places a huge burden on the researcher and it's rarely worth the effort. Taxonomists know that 'species' is a pretty loose moniker. Imagine what it would take to test the BSA on beetles (over 350,000 described species), let alone species of fish or mammals. Even if you got two species together that don't normally see each other, who knows if you could get them 'comfortable' enough to give mating a chance. And then testing the offspring for fertility...
As far as what this has to do with hybrids: you're absolutely right, more closely related organisms are much more likely to be able to produce fertile offspring. But depending on the taxonomy, that can mean crossing species boundaries might be easy to do. Or it might be impossible. All taxonomy is trying to do is place a hypothesis on what happened during the evolutionary history of a group of taxa, so there's a fair amount of subjectivity involved in calling groups genera, species, subspecies, races, etc. It's a very gray spectrum. There are groups of taxonomists that favor getting rid of the 'species' term altogether.
As far as the visibility and importance of hybrids in real ecology go, you're right that there is a bias in seeing the ones that persist. But they're still an important force in speciation. In particular, exotic plants commonly invade a new habitat, hybridize with native plants (that are somewhat closely related), and effectively drive the native plant to extinction. Furthermore, even if hybrids are sterile, they may be long lived and have a significant impact on the ecosystem. As I mentioned before, more than half of all flowering plants are polyploid, and a significant number of those were probably generated by the successful crossing of two different species. Hybrid contact zones are also common in both plants and animals, where the range boundaries of two species touch each other. There can be mating between species in that zone and in many cases even if the hybrid is less fit than the parents, this can lead to gene flow between the species (the hybrid back-crosses with one of the more pure species, transferring genes between species). This has implications on inbreeding and outbreeding depression, as well as species descriptions and conservation efforts.
Basically, the world is much more like a big melting pot than we're taught in high school and even college bio. Nature doesn't really care what names we give things. And that's usually a good thing for biodiversity! I hope this helps at least a little to answer some of your questions.
And NzG is absolutely right. There are plenty of genetically dominant diseases/conditions out there (like polydactyly - extra fingers or toes). And that's not even bringing epistasis (the interactions between different genes) into it.
Yes, I'm psyched that Obama seems to be a big supporter of science and has said he'll increase the NIH and NSF budgets, but I'll have to take a wait-and-see attitude; the economy is going to make that a tough sell, and Bu*h pledged to double those budgets too...never happened after the Iraq clusterfuck.

"I.e., please explain why being homozygous makes an organism weak. "

They are all the same. When you have genetically different parents, the variation in the offspring is much greater, and the less fit ones do not survive or are simply not noticed in the background. So you only notice the exceptional ones.

The racial mixing of Obama is no different than any other African-American, although the point in time when the mixing occurred is different.

Race is not all that meaningful in biological terms. Geographic variation is really fairly continuous. Traits such as skin color, hair texture, and eye shape occur in all possible combinations. The combinations that we put names to are just social and historical accidents. Obama is sometimes called "black" but never "white" although that'd be as accurate biologically, which illustrates how it's really a social construction.

Just for clarification: a recessive gene is not a "damaged version" of a dominant gene, and is not necessarily less effective at doing or making whatever it is supposed to do or make. The terms dominant and recessive refer only to which allele (version of the same gene) dominates when it comes to phenotype. There are plenty of examples of non-adaptive dominant traits. For example, dwarfism (achondroplasia), Huntington's disease, and neurofibromatosis are all dominant traits. There are also many examples of recessive traits that do not affect the fitness of an individual, such as eye colors other than brown, a lack of dimples, or a normal hairline (as opposed to a widow's peak). Additionally, a normal thumb is an advantageous recessive trait, compared to the dominant clubbed thumb allele.

I <3 geeks

I didn't have a chance to read through everything, but I'd like to respond to "helix" and the "damaged" gene idea and "biology is awesome" and the statement that only two blue eyed people can make a blue eyed baby.
"Helix", I don't like the term "damaged." I don't think it's completely accurate. Of course when you mix two truly damaged genes the end result is often a malformation or disease in the offspring (as in downs, polydactyly, etc). However, blue/green eyes, curly hair, red hair (the list could go on endlessly) are recessive. And I don't consider these traits "damaged."

But before I get too PC with my answer (I can see myself trying to come up with more positive words than damaged and we don't need to go there), let me address "biology is awesome." A blue eyed person can mate with a brown eyed person and have a blue eyed baby because the brown eyed person could have a recessive gene for blue eyes. Just wanted to throw that out there.

The comment by "Big Dork" shows that he is, as claimed, a big dork. He says that -- "Successful genetic characteristics, such as those that are the result of adaptation to the environment, are generally dominant so they are passed to more offspring, ensuring survival of the species."

This is absolute hooey. He fails to understand the most basic things about genetic variation. Variations are inherently random. The ones that cause death of the organism are necessarily fatal (duh) and not successful.

All other variations are randomly dominant or recessive, and whether they are successful over time or not depends not on this but on whether the creature that bears them survives to breed or not. This is how regressive genetic variations (sickle cell anemia, myopia, a genetic predisposition towards diabetes) become common in a population.

Since this has come up a few times, I'll clarify: As I said in my first post, I was over-simplifying my explanation to try to make it shorter (I obviously failed there!) and to explain dominant and recessive **in the context of hybrid vigor**. In that context, we are talking about recessives that are are maladaptive. I gave credence to the fact that dominant traits can also be maladaptive in my example of polydactyly and gave a nod to the complexities associated with epistasis. Certainly not all recessively expressed genes are damaged. It's just an easy way to think about it in the context of the original article. It's like when we teach that there are only four bases in DNA or that there is only one genetic code in humans, it's a simplification because it's right most of the time and the subtle complexities don't matter in the context at hand. Don't get me wrong though, I'm glad there are those out there to point out the problems with this, but this is a comment section, not a textbook (no matter how long my posts seem to always get). And just so I can show that I can be one of those people too, Downs syndrome is not caused by a dominant or recessive gene, it's a chromosomal abnormality (the presence of an extra 21st chromosome, or piece of it). Cheers!

Thanks for the correction! :)