Passage VII

PTC is a chemical found in some foods. However. nor all humans can taste PTC. The ability to taste PTC is deter­mined by I gene (Gene P). Gene P has 2 alleles (possible DNA sequences), which are designated P and p.

Four students disagree on 2 questions about Gene P.

1. Which alleles are carried by PTC tasters and by PTC nontasters?

2. ls Gene P X-linked (on the X chromosome), or is it autosomal (not found on a sex chromosome)?

The students know that if Gene P is X-linked. females have 2 copies of it and males have 1. (Females receive 1 X chromosome from their mother and 1 X chromosome from their father. Males receive 1 X chromosome from their mother. but no X chromosomes from their father.) They also know that if Gene P is autosomal. each person has 2 copies of it.

Student 1

Gene P is autosomal. Tasters have I or 2 P alleles. The P allele codes for (leads to the production of) a protein. This protein is an enzyme required for producing PTC receptors. Without these receptors, one cannot taste PTC. The p allele codes for a different protein that cannot act as a PTC receptor.

Student 2

Gene P is autosomal. The P allele codes for a protein that inhibits the biochemical pathway leading to the production of PTC receptors. The p allele codes for a protein that does not inhibit the production of PTC receptors. individuals with either 1 or 2 P alleles do not produce PTC receptors and do not taste PTC. In individuals with no P alleles, the pathway is not interrupted, PTC receptors are produced, and PTC can be tasted.

Student 3

Gene P is X-linked. The P allele codes directly for the PTC receptor. So, tasters must have at least 1 P allele and nontasters lack P alleles. For tasters, sensitivity to PTC varies. Tasters with 2 P alleles produce more PTC recep­tors and can taste PTC at lower concentrations than can tasters with 1 P allele.

Student 4

Gene P is X-linked. However, it does not affect the production of PTC receptors. The P allele codes for 3 protein that binds to PTC receptors. When this protein is pre­sent, PTC cannot bind to PTC receptors and PTC cannot be tasted. The p allele codes for a different protein that does not bind to PTC receptors.

38. Based on the argument presented by Student 3, which of the following individuals would most likely be able to taste the lowest concentration of PTC ?

Answer and Explanation

Your Answer is

Correct Answer is H

Explanation

First, Student 3 thinks that the taster is X chromosome dominant, that is, X(P)X(P), or X(P)Y;

Look at the third to sixth lines of Student 3 For tasters, sensitivity to PTC varies. Tasters with 2 P alleles...that is to say, two dominants are more sensitive than one dominant.

Because male taster X(P)Y can only have one Pa allele, but females can have two X(P)X(P), so females of this genotype are most sensitive to PTC

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