ACT science practice test 24

Directions: Each passage is followed by several questions. After reading a passage, choose the best answer to each question and fill in the corresponding oval on your answer document. You may refer to the passages as often as necessary.

You are NOT permitted to use a calculator on this test.


A corn seed, or kernel, is made up of pericarp, aleurone, and endosperm layers. Figure 1.1 shows the basic anatomy of a corn seed. The endosperm layer may be yellow or white. The aleurone layer may be purple, red, or colorless. Unless the aleurone is colorless, the color of the aleurone layer masks the color of the endosperm layer.

Figure 1.1

Endosperm color is determined by a single gene with two versions, or alleles. A corn seed's specific combination of alleles, or genotype, determines the physical color of the endosperm (phenotype). Aleurone color is determined by the interactions of three independently assorting genes. A genotype that contains at least one aleurone color allele R and one aleurone color inhibitor allele C will produce a purple aleurone. A genotype that contains at least one R, one C, and two aleurone color modifier alleles pp will produce a red aleurone. All other allele combinations will produce a colorless aleurone.

Table 1.1 shows the phenotypes that result from all possible allele combinations for each of the four corn seed color genes.

TABLE 1.1 Corn Seed Cross

In a single ear of corn, each individual kernel is a separate seed representing an independent outcome from the cross of the parental corn. This means that individual kernels on the same ear of corn can have different genotypes and phenotypes.

Students in a biology class examined several ears of corn that resulted from three different parental crosses. Students were told the parental phenotypes for each cross and were instructed to count the number of kernels of each color present on each ear of corn. Table 1.2 shows the students' kernel color data for each of the three crosses.

TABLE 1.2 Corn Seed Genetic Cross

1. Which structure's color is only visible when the aleurone layer is colorless?

A. Pericarp
B. Tip cap
C. Cotyledon
D. Endosperm

2. It can most logically be inferred that the pericarp layer of a corn seed:

A. is colorless.
B. is beneath the aleurone and endosperm layers.
C. has the same phenotype as the endosperm layer.
D. is absent in most corn seeds.

3. According to Table 1.1, which trait has more than two alleles?

A. Aleurone color modifier
B. Aleurone color
C. Aleurone color inhibitor
D. Endosperm color

4. According to Table 1.1, how many unique kernel color phenotypes are possible?

A. Two
B. Four
C. Five
D. Nine

5. Based on the information in Table 1.1, a corn seed with the genotype rrC'cPPyy would appear:

A. white.
B. purple.
C. yellow.
D. colorless.

6. Based on the information in Table 1.1, which of the following genotypes would produce a red kernel?

A. rrCCppyy
B. rrCCPpyy
C. RRCCppyy
D. RRCCPpyy

7. The term allele relationships describes how multiple alleles for the same gene interact. Based on the information in Table 1.1, which statement accurately describes the relationship between the alleles of the aleurone color modifier gene?

A. When both P and p are present, an intermediate phenotype is produced.
B. When P is present, the phenotype of p is masked.
C. When p is present, the phenotype of P is masked.
D. The relationship between P and p cannot be determined from the information in the table.

8. Table 1.2 shows two different kernel colors on the same ear of corn. This is possible because

A. different kernels have different parent plants.
B. some kernels do not have an aleurone layer.
C. each kernel only gets two of the four seed color genes.
D. each kernel represents a separate offspring.

9. If the genotype of the yellow parent in Cross 1 is rrCCppYY, which of the following could be the genotype of the white parent?

A. rrCCppyy
B. rrccppYY
C. RrCCppYY
D. rrCCPPYY

10. In Cross 2, two red parents are shown to produce yellow kernels. What is the most likely explanation for this outcome?

A. A mutation occurring when the two parent plants were crossed resulted in a new color phenotype.
B. The crossing of parent alleles resulted in some kernels with a colorless aleurone phenotype.
C. The two parent plants for Cross 2 were incorrectly identified, resulting in mismatched phenotypes.
D. One of the parent plants passed on a yellow allele instead of a red aleurone color modifier allele.

11. The ratio of red to yellow kernels in Cross 2 is approximately:

A. 2:01
B. 3:02
C. 3:01
D. 4:01

12. To have the greatest probability of producing a yellow kernel, it would be most appropriate to repeat Cross(es):

A. 1
B. 3
C. 1 and 3.
D. 1, 2, and 3.

13. Based on the relationships information in Table 1.1, what would be the outcome if the yellow parent plant in Cross 3 were replaced with a white parent?

A. The ratio of purple to yellow kernels would increase.
B. The ratio of purple to yellow kernels would remain constant.
C. The resulting ears would contain purple and white kernels.
D. The resulting ears would contain purple, yellow, and white kernels.

14. Corn seed color is considered a polygenic trait. Based on the information in the passage, the term polygenic refers to a trait that

A. results from a single gene with multiple alleles.
B. can exhibit a variety of phenotypes over time.
C. has a phenotype that is influenced by multiple genes.
D. affects many different functions of an organism.

15. Most of the corn sold in grocery stores is yellow. This means that an ear of corn seen at the grocery store possesses

A. a different combination of genes than is shown in Table 1.1.
B. fewer color genes than the corn in the crosses shown here.
C. the same genotype as the yellow kernels produced in Cross 2.
D. a genotype that produces a colorless aleurone.