What do Anna Nicole Smith, Kate Moss and genetics have in common?

Successful breeding — whether you're talking about marigolds or mangos or Molossers — requires an understanding of genetics.

Yet despite all the seminars and the books and the magazine articles, it's astonishing that a good many people in dogs still don't quite understand the basics of how dominant and recessive genes work.

Admittedly, the subject as it is taught in biology class isn't exactly sexy. Genotypes, heterozygosity, alleles ... See? I'm losing you already.

In this age of "The Bachelorette" and "American Idol," perhaps the standard template to explain the basics of genetics — the story of 19th-Century Augustinian monk Gregor Mendel and his pollination experiments with pea plants — doesn't cut it anymore.

Perhaps these theories would be better understood in a popular-culture context ... say, using old-school supermodels.

Genetics is usually explained by citing the work of Augustinian friar Gregor Mendel (above left) who worked with pea plants (above right). But we're going to use plus-size supermodel Anna Nicole Smith (below left) and Kate Moss (below right) to explain his rather dry theories.

Anna Nicole vs Kate

There are two kinds of genes — dominant and recessive. But for our purposes, let's call them the dominant genes Anna Nicole Smith, and the recessive ones Kate Moss.

Genes always travel in pairs. So let's imagine our two genes walk into a dressing room, and let's assume that Annas and Kates can exist in multiples. If that's the case, we can have three possible combinations: two Anna Nicoles, two Kates, or one of each.

In genetics, that's called a "genotype": The actual genes an organism has.

Imagine now that our supermodel genes need to get ready for the runway. There's only one mirror in the dressing room, and the two fight for position to apply their MAC lipstick.

If you look in the mirror as they jostle, what you see will depend on who your two supermodels are. If you have two Anna Nicoles, you'll see at least one of those busty blondes in the reflection. If you have two Kate Mosses, the image in the mirror will be stick-thin, even if you only see one. When the two genes are the same like this, whether they're both Annas or both Kates, that's called "homozygous. "

But if you have one Kate Moss and one Anna Nicole — what the geneticists call "heterozygous" — that's a little trickier. Since Anna Nicole is the more, er, zaftig of the two, she wins, pushing lightweight Kate to the back. So while Kate Moss is present, you wouldn’t know it. Looking in the mirror, all you see is bigger-than-life Anna, even though Kate is right behind her.

This reflection is what is called "phenotype" — it's what you see of the genes expressed in the actual animal.

Dominant genes do what our imaginary Anna has done to Kate — they block them. But just because you don't see Kate in the mirror doesn't mean she isn't in the dressing room.

During reproduction, each parent contributes one of his or her genes to the new life being formed. In other words, one of the models in your dressing room goes to your offspring.

Which model?

That's the unpredictable part.

What Does All This Have To Do With Dogs?

We'll get to that. But first, let's fill out a Punnett square using our supermodels.

We don't have to show you what you get if both parents are all homozygous for Anna: You get all Annas. Same with Kate: Breed a sire that carries two Kates to a dam that has two Kates, and all you get are Kates.

But let's say you breed two heterozygous dogs — that is, each dog has one Anna and one Kate.

In this unorthodox Punnett square, Anna Nicole represents a dominant gene and Kate Moss represents a recessive. Since each parent carries two genes, the two genes of the "sire" are on top, and the two of the "dam" are on the side.

Each of the four squares shows the possible combinations, and the odds of them occurring. 

In the above example, each puppy has a 25% chance of being homozygous Anna; in other words, both genes are Anna, or dominant. (Top left)

Each puppy has a 25% chance of being homozygous Kate; in other words, both genes are Kate, or recessive. (Bottom right)

Using our supermodel metaphor, substitute “black coat” for Anna Nicole and “fawn coat” for Kate Moss. The sire's two genes are across the top, and the dam's are down the side of the Punnett square.

The results? With any given puppy, there is a 25 percent chance he will be homozygous black (two Annas), a 25 percent chance he will be homozygous fawn (two Kates) and a 50 percent chance he will be heterozygous black like his parents but carry the recessive gene for fawn (one Anna and one Kate).

You could also do this for nose color. In Dogues de Bordeaux, Anna Nicole would represent the dominant black nose, and Kate Moss recessive brown (in the case of the Dogue de Bordeaux). Here's how that would play out if you bred two black-masked Dogues that each carried the recessive gene for red mask (which is what Dogue people call the expression of brown pigment in their breed):

So if you are a Dogue de Bordeaux breeder, and you want to breed your red-masked female to a black-masked male, knowing his genotype helps you anticipate whether you will have a litter of all black masks or not. If he is homozygous (two Annas) for a black nose, all your puppies will be black masked, but they will carry the gene for red inherited from their mother and could produce it in the future.

But if the sire is heterozygous (one Anna, one Kate), let's look at the outcome below.

As the above Punnett square shows, if you breed a red-masked Dogue to a black-masked Dogue who carries for brown nose, each puppy has a 50/50 chance of being black- or red-masked.

Knowledge Is Power

Of course, these genetics concepts don't just apply to color. Substitute a genetic condition or conformation fault that is known to be linked to a simple recessive gene, and you now have an ability to avoid it in a breeding program. 

Our supermodel scenario also helps to show how genes aren't "diluted" by outcrossing. That is, a gene itself isn't changed or weakened by outcrossing, any more than you can shove Anna Nicole and Kate Moss into a dressing room and hope Naomi Campbell walks out instead. Annas still stay Annas, and Kates still stay Kates. While outcrossing might reduce the chances of two Kates walking out of your dressing room — and so preventing a trait from being expressed — it can also help "hide" a trait, as that elusive Kate hides behind normal, dominant Annas, sometimes for generations.

Only by figuring out who's a carrier — that is, whether there's a Kate lurking behind Anna in that proverbial dressing room — can you hope to eliminate or breed out an undesirable trait.

It's also important to remember that the percentages predicted by the Punnett square reflect the odds for each puppy, not the entire litter.

In the end, drawing Punnett squares and guessing at genetic outcomes isn't a guarantee, because generics is based on probabilities. But then again, so is Las Vegas, and when it comes to the big picture, the casinos win more than they lose. For a
breeder, understanding the risks of a potential genetic problem can provide more consistent results than just groping in the dark. And on a breed-wide level, a focus on genetics will hopefully lead to the ultimate breakthroughs for our most nagging genetic problems: DNA tests that conclusively identify where the Kates are hiding, and how to avoid them.