Posted by jlwile on January 18, 2011
Susumu Ohno is famous for postulating the existence of “junk DNA.” In his paper introducing the term, here is what he wrote about DNA sequences that he thought were nonfunctional:1
Our view is that they are the remains of nature’s experiments which failed. The earth is strewn with fossil remains of extinct species; is it a wonder that our genome too is filled with the remains of extinct genes?
Of course, as time went on, we slowly learned how wrong Ohno was in this assessment. While many DNA sequences are not used to produce proteins, specific functions have been found for much of this supposed “junk.” Indeed, as more and more functions have been found for more and more “junk” sequences, it is becoming increasingly clear that very little junk exists in the genome.
While Ohno did some marvelous work in his illustrious career, much of it was hampered by the blinders of evolution. When you are compelled to believe that nothing but natural processes are responsible for life, you simply cannot see the deep complexity of creation. As a result, you force simplistic ideas on science, whether the data support them or not. The idea that much of an organism’s genome could be filled with “junk DNA” is a perfect example of how evolutionary thinking produces absurd conclusions.
Recently, Yuanyan Xiong and colleagues have laid to rest another evolution-inspired idea that originated with Susumu Ohno.
Many organisms have two different kinds of chromosomes. Gender is determined by sex chromosomes, while all the other chromosomes are called autosomes. People, for example, have 22 pairs of autosomes. Each pair is perfectly matched, so that the genes correspond. This means you have two genes for every trait, and they can both be found in the same autosome pair.
This isn’t necessarily the case for the sex chromosomes, however. If you are a woman, your sex chromosomes consist of a pair of X chromosomes. Thus, your sex chromosomes are like your autosomes in the sense that they come in a pair, and the pair is perfectly matched. If you are a man, however, your sex chromosomes consist of an X chromosome and a Y chromosome. This means your sex chromosomes are not a matched set. Instead, they are two different chromosomes.
Now if you think about it, this could cause a problem when it comes to the amount of protein each gender produces from its sex chromosomes. After all, women have two X chromosomes, but men have only one. If women used both X chromosomes, they would produce twice the amount of X-related proteins as compared to men. That doesn’t happen, though, because one of the two X chromosomes in women is inactivated. As a result, even though they have two X chromosomes, they use only one of them. That way, when it comes to the proteins that are encoded on the X chromosome, men and women produce equal amounts. This is called X-inactivation, and it is was actually first detected by Ohno himself.2
This leads to a big question in terms of evolution. Remember, evolutionists believe that gender was something that arrived later in the evolutionary process. Initially, all organisms were asexual. Thus, they had only autosomes. Later on, some evolutionary process had to eventually produce sex chromosomes from autosomes so that sexual reproduction could evolve. Of course, evolutionists have no idea how this actually happened, but they are confident that it must have.
Well, both of the chromosomes in each autosome pair are fully active, producing proteins over and over again. However, in the sex chromosomes, only one X is active in both genders. If sex chromosomes evolved from autosomes, then, something must have happened to make the the X chromosome genes expressed twice as much as the genes on any single autosome. Otherwise, the fact that both genders have only one active X chromosome would mean that both genders would produce only half of the needed X-related proteins. Because of this, Ohno suggested that during evolution, the expression of genes on the X chromosome was boosted by a factor of 2.
Some early genetic studies seemed to indicate that this was, indeed, the case. These studies suggested that the genes on the only active X chromosome in both males and females were expressed as much as the genes on the autosomes. This, of course, would indicate that the single active X chromosome is producing twice as much protein as any single autosome.3-4 However, in a new paper, Xiong and colleagues show that those previous studies were not correct, at least not for mice and people. They show that the expression of genes coming from the X chromosome in mice and people is (on average) half as much as the expression of genes coming from any autosome pair.5
The authors, therefore, conclude that Ohno’s idea is incorrect. Of course, they call for new theories to try to understand how sex chromosomes could have evolved from autosomes without the increase in activity that Ohno realized was crucial. However, it would be more fruitful if some researchers were willing to consider the idea that sex chromosomes did not evolve from autosomes. Instead, they were specifically designed for gender delineation and as such, there is no reason to believe that the expression of their genes is related to the expression of autosomal genes in any way.
1. Susumu Ohno, “So Much ‘Junk’ DNA in Our Genome,” Evolution of Genetic Systems. Brookhaven symposia in biology, 23:366-370, 1972.
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2. Ohno S, Kaplan WD, and Kinosita R, “Formation of the sex chromatin by a single X-chromosome in liver cells of rattus norvegicus,” Cell Research, 18:415-419, 1959.
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3. Nguyen, D.K. & Disteche, C.M., “Dosage compensation of the active X chromosome in mammals,” Nat. Genet., 38:47-53, 2006.
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4. Gupta, V,et al, “Global analysis of X-chromosome dosage compensation,” J. Biol, 5:3, 2006.
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5. Yuanyan Xiong, et al, “RNA sequencing shows no dosage compensation of the active X-chromosome,” Nat. Genet., 42:1043-1049, 2010.
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