Passage V

Three students observed that Trait R is present in some people but not in others. The presence or absence of Trait R in a person is determined entirely by alleles of Gene R. Gene R has 2 alleles: R, which is dominant, and r, which is recessive.

A teacher gave the students a pedigree showing the inheritance of Trait R in a particular family (see Figure 1). Each individual represented in the pedigree was assigned a number (shown below the symbol for the individual) for reference. Based on the pedigree, each student proposed a hypothesis explaining the pattern of inheritance for Trait R in this family.

Student 1

Gene R is located on the X chromosome, so females (who inherit two X chromosomes) inherit 2 copies of Gene R, whereas males (who inherit one X chromosome) inherit only 1 copy of Gene R. All females with Trait R inherited 2 copies of the r allele, and all males with Trait R inherited 1 copy of the r allele. Table 1 shows the possible Gene R genotypes for females and for males and indicates whether the genotypes result in Trait R.

Student 2

Gene R is not located on the X chromosome or the Y chromosome. All individuals with Trait R inherited 2 copies of the r allele.

Student 3

Gene R is not located on the X chromosome or the Y chromosome. All individuals with Trait R inherited either 1or 2 copies of the R allele.