MEDizzy - A 22-year-old Caucasian female with no family history of

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Neurosensory and Neuromuscular Systems

A 22-year-old Caucasian female with no family history of color blindness (MIM*304000) marries a color-blind male. What is the risk for this couple of having a son or daughter who is color-blind?

ExplanationThe common forms of color blindness are X-linked recessive, as indicated by the initial 3 of the McKusick number (MIM*304000). The couple’s daughters will be obligate carriers—that is, carriers implied by the pedigree. Using a lowercase c to represent the recessive color blindness allele, the female is most likely XC XC, while her husband is X cY. The Punnett square below indicates that all daughters will be carriers (Xc XC), while sons will be normal (XC Y). Note again that loci on the X chromosome cannot be transmitted from father to son, since the son receives the father’s Y chromosome.