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.
27. Which student, if any, would be likely to predict that the Gene R genotype of Individual 5 is rr?
Your Answer is
Correct Answer is B
Explanation
Comparing the descriptions of the three students, student 1 thinks that the gene R is on the X chromosome, student 2 thinks that it is on the autosome, and is recessive, and student 3 thinks that it is also on the autosome, but it is dominant, so Only Student 2 meets the requirements.