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Colorblindness is a sex-linked trait that is inherited along the X chromosome. Males have one X chromosome and one Y chromosome. If a male is born with red-green colorblindness, the X chromosome with the colorblind gene is inherited from his mother. Females are XX and get an X chromosome from each parent. Let X represent the normal vision gene and let X- represent the colorblindness gene. If a male inherits XY, he would have normal vision. If a male inherits X-Y, he would be colorblind. The Y chromosome carries no corresponding genes and is not involved in this trait. For females, they can be of three different genotypes--XX is a normal female, X-X is a female carrier and X-X- is a colorblind female. As long as the female has one normal gene, her vision will be normal. The only way to produce a colorblind female is both parents had to each contribute one colorblind gene on the X chromosome!
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