To further examine X-linked inheritance, let's consider another X-linked characteristic: red-green color blindness in humans. Within the human eye, color is perceived in light-sensing cone cells that line the retina. Each cone cell contains one of three pigments capable of absorbing light of a particular wavelength; one absorbs blue light, a second absorbs red light, and a third absorbs green light. The human eye actually detects only three colors — red, green, and blue — but the brain mixes the signals from different cone cells to create the wide spectrum of colors that we perceive. Each of the three pigments is encoded by a separate locus; the locus for the blue pigment is found on chromosome 7, and those for green and red pigments lie close together on the X chromosome.
The most common types of human color blindness are caused by defects of the red and green pigments; we will refer to these conditions as red-green color blindness. Mutations that produce defective color vision are generally recessive and, because the genes coding for the red and green pigments are located on the X chromosome, red-green color blindness is inherited as an X-linked recessive characteristic.
We will use the symbol Xe to represent an allele for red-green color blindness and the symbol X+ to represent an allele for normal color vision. Females possess two X chromosomes; so there are three possible genotypes among females: X+X+ and X+Xe, which produce normal vision, and XeXe, which produces color blindness. Males have only a single X chromosome and two possible genotypes: X+Y, which produces normal vision, and XeY which produces color blindness.
If a color-blind man mates with a woman homozygous for normal color vision ( FIGURE 4.16a), all of the gametes produced by the woman will contain an allele for normal color vision. Half of the man's gametes will receive the X chromosome with the color-blind allele, and the other half will receive the Y chromosome, which carries no alleles affecting color vision. When an Xe-bearing sperm unites with the X+-bearing egg, a heterozygous female with normal vision (X+Xe) is produced. When a Y-bearing sperm unites with the X+-bearing egg, a hemizygous male with normal vision (X+Y) is produced (see Figure 4.16a).
In the reciprocal cross between a color-blind woman and a man with normal color vision ( FIGURE 4.16b), the woman produces only Xe-bearing gametes. The man produces some gametes that contain the X+ chromosome and others that contain the Y chromosome. Males inherit the X chromosome from their mothers; because both of the mother's X chromosomes bear the Xc allele in this case, all the male offspring will be color blind. In contrast, females inherit an X chromosome from both parents; thus the female offspring of this reciprocal cross will all be heterozygous with normal vision. Females are color blind only when color-blind alleles have been inherited from both parents, whereas a color-blind male need inherit a color-blind allele from his mother only; for this reason, color blindness and most other rare X-linked recessive characteristics are more common in males.
In these crosses for color blindness, notice that an affected woman passes the X-linked recessive trait to her sons but not to her daughters, whereas an affected man passes the trait to his grandsons through his daughters but never to his sons. X-linked recessive characteristics seem to alternate between the sexes, appearing in females one generation and in males the next generation; thus, this pattern of inheritance exhibited by X-linked recessive characteristics is sometimes called crisscross inheritance.
Characteristics determined by genes on the sex chromosomes are called sex-linked characteristics. Diploid females have two alleles at each X-linked locus, whereas diploid males possess a single allele at each X-linked locus. Females inherit X-linked alleles from both parents, but males inherit a single X-linked allele from their mothers.
(a) Normal female and color-blind male
Normal-color-vision female X+X+
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