I stumbled upon this 2005 letter to Nature by Michael Lynch and I thought I'd share it with you since it emphasizes one of my main pet peeves about intelligent design creationists. I've highlighted the relevant sentences. Check out the video.
Intelligent design or intellectual laziness?
SIR – Much of the concern over ID (Nature 434, 1053 and 1062–1065; 2005) has focused on veiled attempts to inject religion into public education. Sheltered within the confines of academia, most biologists find it hard to believe that the slain need to be slain again. Those in the trenches—school boards, school biology teachers and their national representatives—often don’t know how to respond, in part because they themselves never really achieved a deep understanding of evolutionary biology at college.
However, there is a related and equally disturbing issue: the legitimization of intellectual laziness. Have a problem explaining something? Forget about it: the Designer made it that way. Any place for diversity of opinion as to who/what the Designer is/was? The ID literature makes it very clear that there is no room for scientific discourse on that. Think I’m exaggerating? To get a good idea of what IDers would have the face of science look like, check out the journal Perspectives on Science and Christian Faith (www.asa3.org/ASA/PSCF.html).
Two factors have facilitated the promotion of ID. First, IDers like to portray evolution as being built entirely on an edifice of darwinian natural selection. This caricature of evolutionary biology is not too surprising. Most molecular, cell and developmental biologists subscribe to the same creed, as do many popular science writers. However, it has long been known that purely selective arguments are inadequate to explain many aspects of biological diversity. Building a straw man based on natural selection alone makes it easy for opponents to poke holes in evolution. But features of the genome, such as genomic parasites or non-coding introns, which aren’t so evolutionarily favourable (nor obviously ‘intelligent’ innovations), can be more readily explained by models that include random genetic drift and mutation as substantial evolutionary forces.
Second, IDers like to portray evolution as a mere theory. But after a century of close scrutiny, evolutionary theory has passed so many litmus tests of validation that evolution is as much a fact as respiration and digestion.
Less widely appreciated is that evolution has long been the most quantitative field of biology, well grounded in the general principles of transmission genetics. Yet few students at university, and almost none at high school, are exposed to the mathematical underpinnings of evolutionary theory. The teaching of evolution purely as history, with little consideration given to the underlying mechanisms, reinforces the false view that evolution is one of the softer areas of science.
Here is a missed opportunity. Our failure to provide students with the mathematical skills necessary to compete in a technical world is a major concern in the United States. Mathematics becomes more digestible, and even attractive, when students see its immediate application. What better place to start than with the population-genetic theory of evolution, much of which is couched in algebraic terms accessible to school students?
Michael Lynch
Department of Biology, Indiana University,
Bloomington, Indiana 47405, USA
Back in 1980, the idea that large fractions of animal and plant genomes could be junk was quite controversial. Although the idea was consistent with the latest developments in population genetics, most scientists were unaware of these developments. They were looking for adaptive ways of explaining all the excess DNA in these genomes.
Some scientists were experts in modern evolutionary theory but still wanted to explain "junk DNA." Doolittle & Sapienza, and Orgel & Crick, published back-to-back papers in the April 17, 1980 issue of Nature. They explained junk DNA by claiming that most of it was due to the presence of "selfish" transposons that were being selected and preserved because they benefited their own replication and transmission to future generations. They have no effect on the fitness of the organism they inhabit. This is natural selection at a different level.
This prompted some responses in later editions of the journal and then responses to the responses.
The human genome has about 14,000 pseudogenes that are derived from protein-coding genes and an unknown number derived from genes that specify functional noncoding RNAs. There is abundant evidence that the vast majority of these pseudogenes are nonfunctional by all measurable criteria.
It would be perverse to deny the existence of pseudogenes. Almost all of them are junk DNA with no known function. Anyone who claims otherwise can be dismissed as a kook and it's not worth debating those people.
The presence of a single well-characterized pseudogene at the same locus in the genomes of different species is powerful evidence of common descent. For example, Ken Miller has long argued that the existence of common pseudogenes in chimpanzees and humans is solid evidence that the two species share a common ancestor. He uses the β-globin pseudogene and the gene for making vitamin C as examples in Only a Theory: Evolution and the Battle for America's Soul.
A few months ago I highlighted a paper by Casane et al. (2015) where they said ...
In September 2012, a batch of more than 30 articles presenting the results of the ENCODE (Encyclopaedia of DNA Elements) project was released. Many of these articles appeared in Nature and Science, the two most prestigious interdisciplinary scientific journals. Since that time, hundreds of other articles dedicated to the further analyses of the Encode data have been published. The time of hundreds of scientists and hundreds of millions of dollars were not invested in vain since this project had led to an apparent paradigm shift: contrary to the classical view, 80% of the human genome is not junk DNA, but is functional. This hypothesis has been criticized by evolutionary biologists, sometimes eagerly, and detailed refutations have been published in specialized journals with impact factors far below those that published the main contribution of the Encode project to our understanding of genome architecture. In 2014, the Encode consortium released a new batch of articles that neither suggested that 80% of the genome is functional nor commented on the disappearance of their 2012 scientific breakthrough. Unfortunately, by that time many biologists had accepted the idea that 80% of the genome is functional, or at least, that this idea is a valid alternative to the long held evolutionary genetic view that it is not. In order to understand the dynamics of the genome, it is necessary to re-examine the basics of evolutionary genetics because, not only are they well established, they also will allow us to avoid the pitfall of a panglossian interpretation of Encode. Actually, the architecture of the genome and its dynamics are the product of trade-offs between various evolutionary forces, and many structural features are not related to functional properties. In other words, evolution does not produce the best of all worlds, not even the best of all possible worlds, but only one possible world.
How did we get to this stage where the most publicized result of papers published by leading scientists in the best journals turns out to be wrong, but hardly anyone knows it?
Back in September 2012, the ENCODE Consortium was preparing to publish dozens of papers on their analysis of the human genome. Most of the results were quite boring but that doesn't mean they were useless. The leaders of the Consortium must have been worried that science journalists would not give them the publicity they craved so they came up with a strategy and a publicity campaign to promote their work.
Their leader was Ewan Birney, a scientist with valuable skills as a herder of cats but little experience in evolutionary biology and the history of the junk DNA debate.
The ENCODE Consortium decided to add up all the transcription factor binding sites—spurious or not—and all the chromatin makers—whether or not they meant anything—and all the transcripts—even if they were junk. With a little judicious juggling of numbers they came up with the following summary of their results (Birney et al., 2012) ..
The human genome encodes the blueprint of life, but the function of the vast majority of its nearly three billion bases is unknown. The Encyclopedia of DNA Elements (ENCODE) project has systematically mapped regions of transcription, transcription factor association, chromatin structure and histone modification. These data enabled us to assign biochemical functions for 80% of the genome, in particular outside of the well-studied protein-coding regions. Many discovered candidate regulatory elements are physically associated with one another and with expressed genes, providing new insights into the mechanisms of gene regulation. The newly identified elements also show a statistical correspondence to sequence variants linked to human disease, and can thereby guide interpretation of this variation. Overall, the project provides new insights into the organization and regulation of our genes and genome, and is an expansive resource of functional annotations for biomedical research.
The bottom line is that these leaders knew exactly what they were doing and why. By saying they have assigned biochemical functions for 80% of the genome they knew that this would be the headline. They knew that journalists and publicists would interpret this to mean the end of junk DNA. Most of ENCODE leaders actually believed it.
That's exactly what happened ... aided and abetted by the ENCODE Consortium, the journals Nature and Science, and gullible science journalists all over the world. (Ryan Gregory has published a list of articles that appeared in the popular press: The ENCODE media hype machine..)
Almost immediately the knowledgeable scientists and science writers tried to expose this publicity campaign hype. The first criticisms appeared on various science blogs and this was followed by a series of papers in the published scientific literature. Ed Yong, an experienced science journalist, interviewed Ewan Birney and blogged about ENCODE on the first day. Yong reported the standard publicity hype that most of our genome is functional and this interpretation is confirmed by Ewan Birney and other senior scientists. Two days later, Ed Yong started adding updates to his blog posting after reading the blogs of many scientists including some who were well-recognized experts on genomes and evolution [ENCODE: the rough guide to the human genome].
Within a few days of publishing their results the ENCODE Consortium was coming under intense criticism from all sides. A few journalists, like John Timmer, recongized right away what the problem was ...
Yet the third sentence of the lead ENCODE paper contains an eye-catching figure that ended up being reported widely: "These data enabled us to assign biochemical functions for 80 percent of the genome." Unfortunately, the significance of that statement hinged on a much less widely reported item: the definition of "biochemical function" used by the authors.
This was more than a matter of semantics. Many press reports that resulted painted an entirely fictitious history of biology's past, along with a misleading picture of its present. As a result, the public that relied on those press reports now has a completely mistaken view of our current state of knowledge (this happens to be the exact opposite of what journalism is intended to accomplish). But you can't entirely blame the press in this case. They were egged on by the journals and university press offices that promoted the work—and, in some cases, the scientists themselves.
Nature may have begun to realize that it made a mistake in promoting the idea that most of our genome was functional. Two days after the papers appeared, Brendan Maher, a Feature Editor for Nature, tried to get the journal off the hook but only succeeded in making matters worse [see Brendan Maher Writes About the ENCODE/Junk DNA Publicity Fiasco].
Meanwhile, two private for-profit companies, illumina and Nature, team up to promote the ENCODE results. They even hire Tim Minchin to narrate it. This is what hype looks like ...
Soon articles began to appear in the scientific literature challenging the ENCODE Consortium's interpretation of function and explaining the difference between an effect—such as the binding of a transcription factor to a random piece of DNA—and a true biological function.
Niu, D. K., and Jiang, L. (2012) Can ENCODE tell us how much junk DNA we carry in our genome?. Biochemical and biophysical research communications 430:1340-1343. [doi: 10.1016/j.bbrc.2012.12.074]
Doolittle, W.F. (2013) Is junk DNA bunk? A critique of ENCODE. Proc. Natl. Acad. Sci. (USA) published online March 11, 2013. [PubMed] [doi: 10.1073/pnas.1221376110]
Graur, D., Zheng, Y., Price, N., Azevedo, R. B., Zufall, R. A., and Elhaik, E. (2013) On the immortality of television sets: "function" in the human genome according to the evolution-free gospel of ENCODE. Genome Biology and Evolution published online: February 20, 2013 [doi: 10.1093/gbe/evt028
Eddy, S.R. (2013) The ENCODE project: missteps overshadowing a success. Current Biology, 23:R259-R261. [10.1016/j.cub.2013.03.023]
Hurst, L.D. (2013) Open questions: A logic (or lack thereof) of genome organization. BMC biology, 11:58. [doi:10.1186/1741-7007-11-58]
Morange, M. (2014) Genome as a Multipurpose Structure Built by Evolution. Perspectives in biology and medicine, 57:162-171. [doi: 10.1353/pbm.2014.000]
By March 2013—six months after publication of the ENCODE papers—some editors at Nature decided that they had better say something else [see Anonymous Nature Editors Respond to ENCODE Criticism]. Here's the closest thing to an apology that they have ever written ....
The debate over ENCODE’s definition of function retreads some old battles, dating back perhaps to geneticist Susumu Ohno’s coinage of the term junk DNA in the 1970s. The phrase has had a polarizing effect on the life-sciences community ever since, despite several revisions of its meaning. Indeed, many news reports and press releases describing ENCODE’s work claimed that by showing that most of the genome was ‘functional’, the project had killed the concept of junk DNA. This claim annoyed both those who thought it a premature obituary and those who considered it old news.
There is a valuable and genuine debate here. To define what, if anything, the billions of non-protein-coding base pairs in the human genome do, and how they affect cellular and system-level processes, remains an important, open and debatable question. Ironically, it is a question that the language of the current debate may detract from. As Ewan Birney, co-director of the ENCODE project, noted on his blog: “Hindsight is a cruel and wonderful thing, and probably we could have achieved the same thing without generating this unneeded, confusing discussion on what we meant and how we said it”.
Oops! The importance of junk DNA is still an "important, open and debatable question" in spite of what the video sponsored by Nature might imply.
(To this day, neither Nature nor Science have actually apologized for misleading the public about the ENCODE results. [see Science still doesn't get it ])
The ENCODE Consortium leaders responded in April 2014—eighteen months after their original papers were published.
Kellis, M., Wold, B., Snyder, M.P., Bernstein, B.E., Kundaje, A., Marinov, G.K., Ward, L.D., Birney, E., Crawford, G. E., and Dekker, J. (2014) Defining functional DNA elements in the human genome. Proc. Natl. Acad. Sci. (USA) 111:6131-6138. [doi: 10.1073/pnas.1318948111]
In that paper they acknowledge that there are multiple meanings of the word function and their choice of "biochemical" function may not have been the best choice ....
However, biochemical signatures are often a consequence of function, rather than causal. They are also not always deterministic evidence of function, but can occur stochastically.
This is exactly what many scientists have been telling them. Apparently they did not know this in September 2012.
They also include in their paper a section on "Case for Abundant Junk DNA." It summarizes the evidence for junk DNA, evidence that the ENCODE Consortium did not acknowledge in 2012 and certainly didn't refute.
In answer to the question, "What Fraction of the Human Genome Is Functional?" they now conclude that ENCODE hasn't answered that question and more work is needed. They now claim that the real value of ENCODE is to provide "high-resolution, highly-reproducible maps of DNA segments with biochemical signatures associate with diverse molecular functions."
We believe that this public resource is far more important than any interim estimate of the fraction of the human genome that is functional.
There you have it, straight from the horse's mouth. The ENCODE Consortium now believes that you should NOT interpret their results to mean that 80% of the genome is functional and therefore not junk DNA. There is good evidence for abundant junk DNA and the issue is still debatable.
I hope everyone pays attention and stops referring to the promotional hype saying that ENCODE has refuted junk DNA. That's not what the ENCODE Consortium leaders now say about their results.
Casane, D., Fumey, J., et Laurenti, P. (2015) L’apophénie d’ENCODE ou Pangloss examine le génome humain. Med. Sci. (Paris) 31: 680-686. [doi: 10.1051/medsci/20153106023]
There's one sub-category of pseudogenes that deserves mentioning. It's called "polymorphic pseudogenes." These are pseudogenes that have not become fixed in the genome so they exist as an allele along with the functional gene at the same locus. Some defective genes might be detrimental, representing loss-of-function alleles that compromise the survival of the organism. Lots of genes for genetic diseases fall into this category. That's not what we mean by polymorphism. The term usually applies to alleles that have reached substantial frequency in the population so that there's good reason to believe that all alleles are about equal with respect to natural selection.
Polymorphic pseudogenes can be examples of pseudogenes that are caught in the act of replacing the functional gene. This indicates that the functional gene is not under strong selection. For example, a newly formed processed pseudogene can be polymorphic at the insertion site and newly duplicated loci may have some alleles that are still functional and others that are inactive. The fixation of a pseudogene takes a long time.
The third type of pseudogene is the "unitary" pseudogene. Unitary pseudogenes are genes that have no parent gene. There is no functional gene in the genome that's related to the pseudogene.
Unitary psedogenes arise when a normally functional gene becomes inactivated by mutation and the loss of function is not detrimental to the organism. Thus, the mutated, inactive, gene can become fixed in the population by random genetic drift.
The classic example is the gene for L-glucono-γ-lactone oxidase (GULO), a key enzyme in the synthesis of vitamin C (L-ascorbate, ascorbic acid). This gene is functional in most vertebrate species because vitamin C is required as a cofactor in several metabolic reactions; notably, the processing of collagen [Vitamin C]. This gene has become inactive in primates so primates cannot synthesize Vitamin C and must obtain it from the food they eat.
A pseudogene can be found at the locus for the L-glucono-γ-lactone oxidase gene[GULOP = GULO Pseudogene]. It is a highly degenerative pseudogene with multiple mutations and deletions [Human GULOP Pseudogene]
This is a unitary pseudogene. Unitary pseudogenes are rare compared to processed pseudogenes and duplicated pseudogenes but they are distinct because they are not derived from an existing, functional, parent gene.
Note: Intelligent design creationists will go to great lengths to discredit junk DNA. They will even attempt to prove that the GULO pseudogene is actually functional. Jonathan Wells devoted an entire chapter in The Myth of Junk DNA to challenging the idea that the GULO pseudogene is actually a pseudogene. A few years ago, Jonathan McLatchie proposed a mechanism for creating a functional enzyme from the bits and pieces of the human GULOP pseudogene but that proved embarrasing and he retracted [How IDiots Would Activate the GULO Pseudogene] Although some scientists are skeptical about the functionality of some pseudogenes, they all accept the evidence showing that most psuedogenes are nonfunctional.
Of the three different kinds of pseudogenes, the easiest kind of pseudogene formation to understand is simple gene duplication followed by inactivation of one copy. [see: Processed pseudogenes for another type]
I've assumed, in the example shown below, that the gene duplication event happens by recombination between sister chromosomes when they are aligned during meiosis. That's not the only possibility but it's easy to understand.
These sorts of gene duplication events appear to be quite common judging from the frequency of copy number variations in complex genomes (Redon et al., 2006; MacDonald et al., 2013).
Let's look at the formation of a "processed" pseudogene. They are called "processed" because they are derived from the mature RNA produced by the functional gene. These mature RNAs have been post-transcriptionally processed so the pseudogene resembles the RNA more closely than it resembles the parent gene.
This is most obvious in the case of processed pseudogenes derived from eukaryotic protein-coding genes so that's the example I'll describe first.
In the example below, I start with a simple, hypothetical, protein-coding gene consisting of two exons and a single intron. The gene is transcribed from a promoter (P) to produce the primary transcript containing the intron. This primary transcript is processed by splicing to remove the intron sequence and join up the exons into a single contiguous open reading frame that can be translated by the protein synthesis machinery (ribosomes plus factors etc.).1 [See RNA Splicing: Introns and Exons.]
I define a gene as "DNA sequence that is transcribed to produce a functional product" [What Is a Gene? ]. Genes can encode proteins or the final product can be a functional RNA other than mRNA.
A pseudogene is a broken gene that cannot produce a functional RNA. They are called "pseudogenes" because they resemble active genes but carry mutations that have rendered them nonfunctional. The human genome contains about 14,000 pseudogenes related to protein-coding genes according to the latest Ensembl Genome Reference Consortium Human Genome build [GRCh38.p3]. There's some controversy over the exact number but it's certainly in that ballpark.1
The GENCODE Pseudogene Resource is the annotated database used by Ensembl and ENCODE (Pei et al. 2012).
There are an unknown number of pseudogenes derived from genes for noncoding functional RNAs. These pseudogenes are more difficult to recognize but some of them are present in huge numbers of copies. The Alu elements in the human genome are derived from 7SL RNA and there are similar elements in the mouse genome that are derived from tRNA genes.
There are three main classes of pseudogenes and one important subclass. The categories apply to pseudogenes derived from protein-coding genes and to those derived from genes that specify functional noncoding RNAs. I'm going to describe each of the categories in separate posts. I'll mostly describe them using a protein-coding gene as the parent.
The answers are the platypus and the opossum. The overall impression she conveys to the general public is that these species have not evolved for millions and millions of years.
I don't agree. I think it's important to teach the general public that such statements flatly contradict modern evolutionary theory. If, in fact, we discovered modern species that showed no signs of having evolved for millions of years, this would refute modern evolutionary theory.
The accepted minimal definition of evolution is ... [What Is Evolution?]
Evolution is a process that results in heritable changes in a population spread over many generations.
... or something similar like "change in the frequency of alleles in a population."
The main accepted mechanisms of evolution are natural selection and random genetic drift.
The only way positive natural selection1 can stop is if an organism is so perfectly adapted to its current environment (external and internal) that every possible mutation is either deleterious or neutral. That includes all metabolic processes and every structure in the cell.
Nobody could rationally advocate such a claim.
The only way to stop random genetic drift is if there's no such thing as a new neutral or nearly neutral mutation and all such variation in the population has been eliminated.
No evolutionary biologist could possibly make such a claim with a straight face.
It's easy to test such ridiculous claims by looking at the genomes of the opossum and the platypus. The evidence shows that they have evolved at the same rate as all other species.
The article actually mentions this problem ...
“'Unchanged' is a tricky word,” Nizar Ibrahim, a paleontologist at the University of Chicago and 2014 National Geographic Explorer, says via email.
With only fossils to go by, scientists can examine an ancient animal's skeletal structure, but it's not the whole story. Physiology and DNA change somewhat over time, he says, both through the basic process of evolution as well as random genetic changes.
That said, two mammals that have undergone the fewest evolutionary shifts are the platypus and the opossum, says Samantha Hopkins, associate professor of geology at the University of Oregon.
Liz Langley did not pick up on this comment so she missed a wonderful teaching moment.
It's possible that Liz Langley isn't aware of modern evolutionary theory and that she actually believes that evolution comes to a halt as long as species live in a relatively constant environment. It's possible that she disagrees with the minimal definition of evolution and prefers a definition that only counts significant changes in external phenotype. Or, it's possible that she thinks that National Geographic readers can't handle modern evolutionary theory. If it's the latter, I disagree.
1. You can't stop negative natural selection unless there are no new deleterious mutations. That's also impossible.
Casey Luskin: ... and, as we all know, or many ID the Future listeners probably know, for years Darwinian theorists and evolutionary scientists have said that our genomes ought to be full of junk if evolution is true
.....
Casey Luskin: So, Dr. Hunter, you think, just for the record, that in the long term there is going to be function found for probably the vast majority of the genome and so, maybe, you might call it a prediction you would make coming out of an intelligent design paradigm. Is that correct?
Cornelius Hunter: Yes, that's correct Casey, I'll definitely go on the record on that. Not to say I have a hundred percent confidence and also I wanna be clear that from a metaphysical perspective, from my personal belief, I don't have a problem wherever it lands. It doesn't matter to me whether it's 10% or 90% or any where in between or 100% ... just from the scientific perspective and just from the history of science and the history of what we've found in biology, it really does look like it's gonna be closer to 100 than zero.
Casey Luskin: Okay, great, I always like it when people put clear and concrete predictions out there and and I think that's very helpful.
I predict that about 90% of our genome will turn out to be junk DNA—DNA with no function. I base my prediction on the scientific perspective and the history of what we've found in biology. That's interesting because Cornelius Hunter and I are apparently reaching opposite conclusions based on the same data.
I also love it when people make predictions. Will the intelligent design paradigm be falsified if I turn out to be right?
Does it sound to you that Cornelius Hunter personally doesn't care if the prediction coming out of the intelligent design paradigm is correct or not?
1. How come in the podcast they never refer to Dan Graur as Dr. Graur? Isn't that strange?
... those ignorant of history are not condemned to repeat it; they are merely destined to be confused.
Stephen Jay Gould Ontogeny and Phylogeny (1977)Back when the Nobel Prize in Chemistry was announced I was surprised to learn that it was for DNA repair but Phil Hanawalt wasn't a winner. I blogged about it on the first day [Nobel Prize for DNA repair ].
I understand how difficult it is to choose Nobel Laureates in a big field where a great many people make a contribution. That doesn't mean that the others should be ignored but that's exactly what happened with the Nobel Prize announcement [The Nobel Prize in Chemsitry for 2015].
In the early 1970s, scientists believed that DNA was an extremely stable molecule, but Tomas Lindahl demonstrated that DNA decays at a rate that ought to have made the development of life on Earth impossible. This insight led him to discover a molecular machinery, base excision repair, which constantly counteracts the collapse of our DNA.
Maybe it's okay to ignore people like Phil Hanawalt and others who worked out mechanisms of DNA repair in the early 1960s but this description pretends that DNA repair wasn't even discovered until ten years later.
By that time I was in touch with David Kroll who was working on an article about the slight to early researchers. He had already spoken to Phil Hanawalt and discovered that he (Hanawalt) wasn't too upset. Phil is a really, really nice guy. It would be shocking if he expressed disappointment or bitterness about being ignored. I'll do that for him!
OK, Larry. I assume you mean to say that I do not understand the basics of Darwinism. I challenge you, therefore, to demonstrate your claim.
This was the kind of challenge that's like shooting fish in a barrel but I thought I'd do it anyway in case it could serve as a teaching moment. Boy, was I wrong! Turns out that ID proponents are unteachable.
I decided to concentrate on Arrington's published statements about junk DNA where he said ...
I was alerted to this video by a post on Facebook. I had never seen it before. The occasion is the celebration of the 20th anniversary of McLean v. Arkansas— one of the legal victories of Americans who are fighting to keep creationism out of the classroom.
It's a 30 minute presentation by Stephen J. Gould on the fossil record. The event took place in February 2001, just a year before he died. You should watch it for many reasons—too many to mention them all here but here are some of the most important ones.
There are several different kinds of genes. Some of them encode proteins, some of them specify abundant RNAs like tRNAs and ribosomal RNAs, some of them are responsible for making a variety of small catalytic RNAs, and some unknown fraction may specify regulatory RNAs (e.g. lncRNAs).
This jumble of different kinds of genes makes it difficult to estimate the total number of genes in the human genome. The current estimates are about 20,000 protein-coding genes and about 5,000 genes for functional RNAs.
Aside from the obvious highly conserved genes for ubiquitous RNAs (rRNA, tRNAs etc.), protein-coding genes are the easiest to recognize from looking at a genome sequence. If the protein is expressed in many different species then the exon sequences will be conserved and it's easy for a computer program to identify the gene. The tough part comes when the algorithm predicts a new protein-coding gene based on an open reading frame spanning several presumed exons. Is it a real gene?
