Genetics News

How do males avoid lethal defects without a second X chromosome?

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From “Study Confirms Males and Females Have at Least One Thing in Common: Upregulating X” ( ScienceDaily, Oct. 24, 2011), we learn:

Women have two X chromosomes, while men have one X and one Y. The lack of a ‘back up’ copy of the X chromosome in males contributes to many disorders that have long been observed to occur more often in males, such as hemophilia, Duchenne muscular dystrophy, and certain types of color blindness. Having only one copy of X and two copies of every other chromosome also creates a more fundamental problem — with any other chromosome, the gene number imbalance resulting from having only one copy would be lethal. How can males survive with only one X?

Turns out they upregulate, as needed.

In mammals — humans and mice — both males and females up-regulate X chromosome gene expression and females then equalize expression by turning off the one X chromosome. In roundworms (C. elegans) the both female X chromosomes stay active, but the genes on both Xs are down-regulated by half to compensate in the females. In fruit files (Drosophilia melanogaster), males increase the expression of X chromosome genes, with no upregulation of X in females.

Well, the guys had to be doing something.

7 Replies to “How do males avoid lethal defects without a second X chromosome?

  1. 1
    wd400 says:

    Err.. this isn’t about “lethal defects” on the X, it’s about dosage effects.

    Is anyone counting the number of times a headline from “News” doesn’t fit the linked story?

  2. 2
    Joseph says:

    LoL! The title says How do males avoid lethal defects without a second X chromosome?

    LETHAL DEFECTS WITHOUT A SECOND X CHROMOSOME and News quoted:

    with any other chromosome, the gene number imbalance resulting from having only one copy would be lethal. How can males survive with only one X?

    The word “lethal” is right there!

  3. 3
    wd400 says:

    But defects aren’t.

  4. 4
    John D says:

    “disorders” is in the second sentence. I’m sure news feels properly spanked. Good catch!!

  5. 5
    wd400 says:

    John. I don’t think you quite understand. The X doesn’t get upregulated to cover for defective genes that don’t have a “back up” – the most straight forward way of reading News’ take on this. It is up-regulated to avoid dosage problems from having twice as many copies in female cells as male ones. They certainly don’t regulate “as needed”.

    Lethal defects on the X remain lethal to males (and non lethal ones remain more common in males c.f. colour blindness)

  6. 6
    John D says:

    wd – you are the only one claiming the headline said there were lethal defects on the x. Obviously such a thing would be impossible to survive by definition and not be on the table to be misunderstood. If you didn’t understand that, please pardon me for thinking you might just be looking for non lethal defects for some incoherent reason.

  7. 7
    wd400 says:

    You don’t think the phrase “lethal defects” was a pointer there?

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