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It was in 2002 that the X chromosome was seen to possess a strange genetic makeup, with just a small amount of genes required for cell function. This has baffled us for years, but researchers from the University of Bath and Uppsala University, along with the FANTOM consortium, seem to have found a solution.

"Since we showed that X-linked genes tend to be relatively tissue specific over a decade ago, the reason as to why the X chromosome is so odd has bugged me," said Laurence Hurst, who participated in the research, in a press release. "In the end, we have found the answer to be quite simple. Whereas most chromosomes operate in pairs, meaning there are two copies of each gene in every cell, in contrast, we only have one active copy of the X chromosome."

Looking at the gene activity in the world and the X chromosome compared to other chromosomes, it was seen that the X chromosome's "peak level of gene expression" was less than half the levels in other chromosomes with two active copies.

"This means it is not sustainable for highly active genes to be on the X chromosome. Housekeeping genes tend also to be highly active - they just couldn't survive on the X," he added.

Particular genes moving from the X chromosome to other chromosomes through evolution showed scientists that the genes making their way onto the X chromosome show lower peak rates of expression compared to genes that have migrated away from the X chromosome.

"It's a bit like traffic on a busy road - a highway with two lanes can have a lot more and faster traffic on it than a single lane highway," said Hurst. "A consequence of having a single chromosome is that, like a one lane road, there will be gene expression traffic tailbacks on the X chromosome, especially at peak periods. Hence, our X chromosome will not be a tolerable home for the most highly expressed genes."

Even the "most highly expressed genes" on the X chromosome seemed less likely to increase their expression through time or evolution, discovered the scientists.

"This fits with our traffic analogy as, if there is a tailback, it is hard to increase the speed of the cars on the road," said senior author Lukasz Huminiecki.

The results were published in Dec. 18,2015 issue of PLOS Biology.