Lionel Mandrake
Lionel Mandrake's JournalYosemite sleep deprivation
Sleep has always been flaky under Windows, but it has always been solid under Mac OS X until I "upgraded" to Yosemite. For the first time ever, I am having trouble putting a Mac to sleep. Sometimes it goes to sleep and stays asleep. Other times it wakes up almost immediately. Then I have to shut it down instead.
Color vision and X-inactivation.
Normal color vision in humans is said to be trichromatic, since there are normally three types of cones in our retinas. The most common types of color deficiencies result from abnormal or missing genes on our X chromosomes. Most of the readers in the Science Group have probably heard that many men, but few women, are "red-green colorblind". This trait is said to be sex-linked and recessive, like baldness and many other abnormalities that affect more men than women.
Roughly speaking, if a man has such a trait and a woman is not a carrier for that trait, their children will not have the trait, but half of their daughters will be carriers. If a woman is a carrier and a man does not have the trait, half of their sons will have the trait, and half of their daughters will be carriers. If a woman is a carrier and a man has the trait, then half of their children of either sex will have the trait, and the daughters who don't have the trait will be carriers. These statements follow from the facts that every female cell has two X chromosomes and every male cell has only one X chromosome.
But the real situation is more complicated. In very precise color-matching experiments, women who are normally considered to be carriers have been shown to have a slight deficit compared to women who are not carriers. In other words, the genes for color deficiencies are not completely recessive. This has to do with the fact that one of the X chromosomes in each cell of a woman's body is inactivated, i.e., most of its genes are not expressed. The reason for X inactivation is to prevent a mismatch between males and females in the amounts of messenger RNA (and hence the amounts of protein) that are produced. In animals (but not plants) if the level of gene expression is off by a factor of two, the results are usually deleterious and often fatal. Even if the level is only off by 50%, as in trisomy 21 (Down Syndrome), there are serious consequences.
A female "carrier" of a sex-linked trait will have clumps of cells in which the X chromosome inherited from her father is inactivated, and other clumps in which the X chromosome inherited from her mother is inactivated. The patches of fur in a calico cat are similarly explained. The retina in each eye will have abnormal patches and normal patches. It's not surprising that such an eye does not perform quite as well as a completely normal eye.
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