Passage IV

Four students observed that in a population of land plants, Population A, a plant could have a green stem or a purple stem. Each student proposed an explanation for this observation.

Student 1

All plants in Population A produce the green pigment chlorophyll. If a plant receives 8 hr or more of sunlight each day, it also produces a purple pigment, causing its stem to be purple. If a plant receives less than 8 hr of sun­light each day, it does not produce this purple pigment, so its stem is green. All plants in Population A are genetically identical, so they all have the ability to produce both pigments.

Student 2

All plants in Population A produce the green pigment chlorophyll. If a plant receives too little phosphorus (a nutrient), it also produces a purple pigment, causing its stem to be purple. If a plant receives enough phosphorus, it does not produce this purple pigment, so its stem is green. All plants in Population A are genetically identical, so they all have the ability to produce both pigments. The amount of sunlight received by a plant does not affect stem color.

Student 3

All plants in Population A produce the green pigment chlorophyll. The production of purple pigment is deter­mined by Gene Q, which has 2 alleles (Q and q) and 3 pos­sible genotypes (QQ, Qq, and qq). A plant with either the Gene Q genotype QQ or the Gene Q genotype Qq produces the purple pigment, causing its stem to be purple. A plant with the Gene Q genotype qq does not produce this purple pigment, so its stem is green. The amount of sunlight or nutrients received by a plant does not affect stem color.

Student 4

All plants in Population A produce the green pigment chlorophyll. The production of purple pigment is deter­mined by Gene Q, which has 2 alleles (Q and q) and 3 pos­sible genotypes (QQ, Qq, and qq). A plant with the Gene Q genotype qq produces the purple pigment, causing its stem to be purple. A plant with either the Gene Q genotype QQ or the Gene Q genotype Qq does not produce this purple pigment, so its stem is green. The amount of sunlight or nutrients received by a plant does not affect stem color.