How am I different from my dog?

14 Jan

Fergus smiles

It is not our differences that divide us. It is our inability to recognize, accept, and celebrate those differences.” –Audre Lorde

My dog, Fergus, is furrier than I am. He detects smells more capably, and with a wetter nose. And, this one’s a biggie, words never come out of his mouth.

But while genes help explain the diversity of life on our planet, they show us to an astonishing degree that all we living things are similar.

Gene for gene, people are said to be more than 99 percent similar, one guy to the next. Related genes in humans and apes are almost as alike. We begin to see more differences when we compare the human genome to the genomes of other animals, like dogs and mice, but not much. The related genes of mice and men are roughly 85 percent the same.

U.S. Department of Energy Genome Programs: http://genomics.energy.gov

This picture (click to blow it up) shows that we could come close to creating a mouse genome by taking a full set of human chromosomes, cutting them into about 150 pieces, and reassembling them, like a collage.

Although dogs have a whopping 39 pairs of chromosomes, I could use my 23 pairs to make a not-too-shabby Fergus collage, too.

We take for granted, maybe, how much we have in common.

Fergus and I have, for example, a mouth in front, a butt in the back and a left side that is a lot like our right. This “bilateral symmetry” is found all over the animal kingdom with very few exceptions, among them the starfish, the sea urchin and the wonky-eyed jewel squid.

Fergus and I both have teeth, eyes, and noses. As embryos we both sprouted four limbs. As housemates, we often share food. Fergus can’t eat onions or chocolate, and he has a few eating habits that I find disgusting, but more or less, we can eat off the same plate.

We share a lot, metabolically, with the dog.

We are like houses on the same street that have different architectural details but are basically fitted with the same plumbing, wiring, and fixtures.

Another way to look at this is to imagine that we are both houses made of bricks (genes), and that bricks across species are just a little different– bigger or smaller, darker or lighter, but still at essence just regular bricks.

A lot of differences across species involve evolution through duplication and modification– in other words, taking a good concept and changing it slightly. So if one species has a good metabolic “idea” like being able to break down glucose and other sugars from flowers, fruits and vegetables into energy, well, future generations might take the idea and possibly tweak it a little. It’s a lot easier to take something that’s known, like a brick, and modify it just a little when you copy it to see if you can’t get the next evolutionary step in that metabolic pathway.

That kind of duplication and modification at the molecular level is just one way in which species are similar but different. Another thing that accounts for similarity and variation across species is the collage we talked about at the chromosome level, and that’s where we look at the human and dog chromosomes and see that there are huge chunks that are simply rearranged.  Following the evolutionary family tree down to its roots (explore the tree at http://tolweb.org/tree/!), we see that neither humans nor our dog ancestors were made de novo.

In fact, Fergus and I shared a single ancestor that lived somewhere on earth about 83 million years ago.

Slow evolutionary shifts happen at the replication stage of meiosis. Those shifts tend not to affect the development of an animal all that much: A gene changes here or there and that gets duplicated. That happens every generation. Each one of us represents the next evolutionary step.

But bigger, more radical variations can happen with sex in what’s called the “hybrid zone,” where usually because of limited food resources or some huge calamity one species has babies with another species that is closely related. It’s a little bit like genetic roulette, because there are only certain combinations of the chromosomes coming together from the different gametes that will work. But all you really need to do is to keep having these hybrid-zone matings and the rearrangements across the chromosomes start to actually become functional, and the dance of attraction that usually brings together like chromosomes from like species now brings different chromosomes together so that in the next meiosis, chromosome 1 from species 1 could swap chunks with chromosome 2 from species 2.

In the picture of the human and mouse chromosomes we see that our X chromosome, which is important for most metabolic functions, is almost entirely the same as the mouse’s X chromosome. We share a big segment of chromosome 1 with the mouse, but a lot of our chromosome 1 appears on the mouse chromosome 4. If we compared my chromosomes to Fergus’s, we’d see similar patterns. Each species is like a mosaic of the species before it. In other words, we’ve got sections of the genomes of other species smattered all over us.

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