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.