Why Are Some Traits Only on the X Chromosome?
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Color Blindness
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Men are more likely to be colorblind than women. They only inherit one X chromosome, while women inherit two. The second X chromosome a woman inherits can complement the one carrying the color blindness trait and therefore make the trait recessive. The X chromosome a male carries that may contain the trait for color blindness will be dominant because there is no other X chromosome to cancel it out.
Baldness
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For the same reason men are more likely to be color blind, they are also more likely to be bald, as this trait is found on the X chromosome and the second sex chromosome the male receives, the Y chromosome, is unable to cancel out X's trait for baldness. Some women also exhibit pattern baldness and hair thinning, but in far fewer numbers than men.
Fraternal Twins
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When identical twins occur, one fertilized egg splits into two separate embryos and there seem to be no inherited factors for this. When fraternal twins, or other fraternal multiples occur, it is because more than one egg was released during ovulation and fertilized. This trait is found on the X chromosome, which is why fraternal multiple births are said to run on the mother's side of the family rather than the father's, since the female can only pass on X chromosomes.
Hemophilia
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Hemophilia is a serious disease responsible for various blood-clotting disorders. England's Queen Victoria was a carrier of this X-linked trait. She passed it onto one son and two daughters. The son had the disease, but the daughters were only carriers, because they had a second X chromosome that made the trait recessive. However, they were able to pass the trait on to male offspring who were then infected with the disease and not carriers like their mothers.
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