You Are Reading the Chapter of Your Textbook That Covers

You are reading the chapter of your textbook that covers mental retardation, and you come to a discussion of phenylketonuria, Tay-Sachs disease, Hurler syndrome, and Lesch-Nyhan syndrome. They are all included in the same section of this chapter because they

A) are all mild forms of mental retardation.
B) only affect boys.
C) are caused by recessive gene pairings.
D) are all linked to chromosome 21.