Analysis: when it comes to the size of the genome, humans are way down the pecking order behind the locust and garden pea

By David DuffyUniversity of Florida

Humans have long been fascinated with measuring and comparing certain physical attributes, with the always implicit notion of the bigger the better. Maybe that's just men? But this uniquely human trait can also be found when it comes to our endeavours to understand how organisms, including humans themselves, work.

Ever since it was discovered that DNA was the molecule of life, coding the instructions for how to make a human and transmitting those instructions through the generations, we have sought to find out the size of the human genome. A genome is the name for the entirety of the DNA encoding an organism, and all the genes therein. Another way of putting this is: how many genes does it take to make a human?

Every cell in your body contains a complete genome. The exceptions that prove the rule are gametes (egg and sperm cells), which only contain half of the regular DNA content, and red blood cells, which shed their genome entirely. Each person has a unique genome (with the exception of identical twins), but the more closely related two individuals are the more in common their genomes will have.

From Harvard University, an introduction to the human genome

The genome of each individual is much the same size. Heritable differences between individuals are determined by the specific code of their genome, not by gross changes in genome size. How disappointing to us as a species that genome size could not be used for a bit of one-upmanship between individuals. But never fear, instead we were able to turn to different species to reassure ourselves that our genomes where the most handsomely endowed.

To be fair, we knew the size of some other species' genomes before we knew the size of our own. Given that it took us a while to be able to measure and decode the large 3.2 billion base pairs that make up a human genome (base pairs consist of two nucleobases and form the building blocks of the DNA double helix, we had to begin with less daunting species.

Our efforts to measure ourselves against our animal brethren proved highly reassuring. Compared with our imposing 3,200 million base pairs, brewers yeast (used for beer making) had a mere 12 million, nematode worms had 97 million, the humble fruit fly has a genome of 180 million, the zebrafish had 1,500 million and a mouse had a genome of 2,700 million base pairs. Clearly, genome sizes increased the more you progressed from simple to complex animals. Phew! We could now rest assured that we safely resided at the top of the animal kingdom.

We need your consent to load this YouTube contentWe use YouTube to manage extra content that can set cookies on your device and collect data about your activity. Please review their details and accept them to load the content.Manage Preferences

From SciShow the mystery of the biggest genomes

If only we had stopped looking, we would never have burst our own bubble. You see, something fishy had happened with our comparison. Those species we had decided to compare ourselves with were essentially the species which were routinely used in scientific labs. One of the reasons these species were commonly used was because they had short generation times, making them ideal for genetic study.

However, we later discovered that peculiar things can happen when animals evolve to have short life spans. As they race to shorten their generation times, they tend to make biological processes more efficient and streamlined, including the copying of the genome which needs to happen during growth and reproduction. We now know that short-lived animals often have smaller genomes than their longer-lived ancestors, with genes having been lost/shed from their genomes during their evolution.

If we really want to measure our genome against other species, we needed to look to wild animals and especially to long-lived animals. Doing so broke the simplistic link of genome size with animal complexity. For example 'simple’ species like jellyfish and corals could have genomes the same size as vertebrates such as zebrafish. We also found other animals have genomes nearly as large as ours such as sea turtles.

We need your consent to load this YouTube contentWe use YouTube to manage extra content that can set cookies on your device and collect data about your activity. Please review their details and accept them to load the content.Manage Preferences

From National Human Genome Research Institute, scientists involved in the Human Genome Project reflect on the lessons learned

But even more appalling to our sensibilities, we discovered a species with an even larger genome than us! Worse, it was our movie nightmare and occasional foe (though admittedly they receive a lot of unfair press), the great white shark. The great white shark genome stacks up at an impressive 4.3 billion base pairs, compared to the puny human genome of 3.2 billion base pairs. Even more indignantly, even by embracing our shark cousin, we can no longer claim that vertebrates (humans are vertebrates too) have the biggest genome, It was recently discovered that the locust's genome comes in at 6.5 billion base pairs - yes, over double the genome of humans.

If we look beyond the animal kingdom, our indignity and embarrassment does not end there as common plants also have us beat hands down. Some of those unassuming foodstuffs in your kitchen cupboard would put you to shame as the genome size of the garden pea, onion and wheat weight in in at 3.9, 14.2 and 15.9 billion base pairs respectively. I guess the moral of the story is to be careful who you start a pissing contest with.

READ: The viruses in your DNA

READ: How are genes named?

READ: How to recreate Planet Earth with 90g of DNA

However, humans were determined to regain our place at the top of the pecking order and we needed another excuse to elevate ourselves above mere beasts. After much soul searching about our now apparently lacklustre genome size, we have decided that it's not the number of genes that is important, but how you use them. We now say that humans’ unique complexity arises not from the number of genes we have, but the added complexity of how they are put to work in humans.

While there does appear to be some scientific truth to this argument, our 20,000 genes produce about 500,000 different gene variants (known as transcripts). Perhaps this time we will make comparisons with our animal brethren from a more modest and chastened stance. 

Dr David Duffy is an assistant professor of Wildlife Disease Genomics at the Whitney Laboratory for Marine Bioscience and Sea Turtle Hospital at the University of Florida

The views expressed here are those of the author and do not represent or reflect the views of RTÉ