PCAT |VERIFIED QUESTIONS AND CORRECT DETAILED
ANSWERS|RATED AND GRADED A+ NEW UPDATE| 2026/2027
At two independently assorting loci, a man has the following genotype: GgHH. He marries a woman with
the genotype ggHh. What is the probability that they will have a child who has the same genotype as the
father?
0
1/8
1/4
1/2 - ANSWER✔ The correct answer is (C)
This is a "probability" genetics question that can be answered by practical application of Mendel's Laws.
Mendel's Law of Segregation states that alleles segregate during meiosis, resulting in gametes that carry
only one allele for any given inherited trait (i.e., haploid gametes). Mendel's Law of Independent
Assortment states that unlinked genes assort independently during meiosis. By applying Mendel's Laws,
we can conclude that each parent in the problem can produce two possible gametes. The father can
produce the gametes GH and gH, and the mother can produce the gametes gH and gh. The probability
of the father's genotype (GgHH) appearing in the progeny can be determined by calculating the number
of different gamete combinations that will produce this genotype. Thus, a GgHH zygote can only be
produced by the fusion of a GH gamete and a gH gamete. The probability that one parent will donate a
particular gamete is independent of the probability that the other parent will donate a particular
gamete. Thus, the probability of the father donating a GH gamete is 1/2, and the probability of the
mother donating a gH gamete is 1/2. The probability of producing a genotype that requires the
occurrence of both these independent events is equal to the product of the individual probabilities that
these events will occur. Thus, 1/2 x 1/2 = 1/4, so the probability that this couple will have a child with
the genotype GgHH is 1/4, or choice C.
In a certain genetically stable population, the frequency of a recessive allele (for a trait with two alleles)
is 0.6. What is the frequency of individuals expressing the dominant trait?
0.16
0.24
0.36
, 0.64 - ANSWER✔ The correct answer is (D)
The question stem asks you to determine the frequency of individuals expressing the dominant trait in a
genetically stable population. However, before you do that, you need to determine the allelic
frequencies in the population. This question involves a practical application of the Hardy-Weinberg
equation. The Hardy-Weinberg equilibrium states that within a genetically stable population, the gene
frequencies of dominant and recessive alleles will not change over time. Two mathematical expressions
are associated with the Hardy-Weinberg equilibrium. The first relationship, p + q = 1, describes the
relative allelic frequencies in a population. p is defined as the frequency of the dominant allele and q is
defined as the frequency of the recessive allele, and the sum of both those frequencies adds up to 1, or
100%. The second relationship, p2 + 2 pq+ q2 = 1, describes the relative genotypic frequencies in the
population. p2 represents homozygous, or dominant pp genotypes; q2 represents homozygous, or
frequency of the dominant allele, p, by the mathematical relationship p + q = 1. Therefore, the
frequency of p is .4 because .6 + .4 = 1. Next, you need to determine the frequency of individuals
expressing the dominant trait by recessive qq genotypes; and 2pq represents the frequency of
heterozygotes, or hybrids.applying the second relationship, p2 + 2 pq+ q2 = 1. The individuals expressing
the dominant trait are those that have the pp and pq genotypes, so to find the total frequency of
individuals expressing the dominant trait, you add p2 and 2pq. Thus, p2 = .4 x .4, or .16 and 2pq = 2 x .6 x
.4, or .48. If you add the two together, you get .16 + .48, or .64. Thus, .64 is the correct frequency of
individuals expressing the dominant trait, and choice D is correct.
200 mL of an ideal gas is placed in a piston and is held at a pressure of 500 torr. If the temperature is
held constant and the pressure is increased to 650 torr, what is the new volume of the gas?
75 mL
154 mL
220 mL
300 mL - ANSWER✔ The correct answer is (B)
When the pressure of a gas increases at constant temperature, the volume decreases. Therefore, before
calculation, choices C and D can be eliminated. Since PV/nT is constant, and the temperature and
number of moles of gas are kept constant, we can derive the relationship, P1V1 = P2V2. Rearranging this
equation to find the final volume of gas, we get: V2 = P1V1/P2. By substituting numbers into this
equation, we get: V2 = (500 × 200)/650, or 154 mL, choice B. Another way to arrive at the correct
answer is to notice that the original pressure (500 mL) divided by the new pressure (650 mL) is
approximately 3/4, so the new volume is around 3/4 of 200 mL or 150 mL, and this figure is closest to
choice B.
ANSWERS|RATED AND GRADED A+ NEW UPDATE| 2026/2027
At two independently assorting loci, a man has the following genotype: GgHH. He marries a woman with
the genotype ggHh. What is the probability that they will have a child who has the same genotype as the
father?
0
1/8
1/4
1/2 - ANSWER✔ The correct answer is (C)
This is a "probability" genetics question that can be answered by practical application of Mendel's Laws.
Mendel's Law of Segregation states that alleles segregate during meiosis, resulting in gametes that carry
only one allele for any given inherited trait (i.e., haploid gametes). Mendel's Law of Independent
Assortment states that unlinked genes assort independently during meiosis. By applying Mendel's Laws,
we can conclude that each parent in the problem can produce two possible gametes. The father can
produce the gametes GH and gH, and the mother can produce the gametes gH and gh. The probability
of the father's genotype (GgHH) appearing in the progeny can be determined by calculating the number
of different gamete combinations that will produce this genotype. Thus, a GgHH zygote can only be
produced by the fusion of a GH gamete and a gH gamete. The probability that one parent will donate a
particular gamete is independent of the probability that the other parent will donate a particular
gamete. Thus, the probability of the father donating a GH gamete is 1/2, and the probability of the
mother donating a gH gamete is 1/2. The probability of producing a genotype that requires the
occurrence of both these independent events is equal to the product of the individual probabilities that
these events will occur. Thus, 1/2 x 1/2 = 1/4, so the probability that this couple will have a child with
the genotype GgHH is 1/4, or choice C.
In a certain genetically stable population, the frequency of a recessive allele (for a trait with two alleles)
is 0.6. What is the frequency of individuals expressing the dominant trait?
0.16
0.24
0.36
, 0.64 - ANSWER✔ The correct answer is (D)
The question stem asks you to determine the frequency of individuals expressing the dominant trait in a
genetically stable population. However, before you do that, you need to determine the allelic
frequencies in the population. This question involves a practical application of the Hardy-Weinberg
equation. The Hardy-Weinberg equilibrium states that within a genetically stable population, the gene
frequencies of dominant and recessive alleles will not change over time. Two mathematical expressions
are associated with the Hardy-Weinberg equilibrium. The first relationship, p + q = 1, describes the
relative allelic frequencies in a population. p is defined as the frequency of the dominant allele and q is
defined as the frequency of the recessive allele, and the sum of both those frequencies adds up to 1, or
100%. The second relationship, p2 + 2 pq+ q2 = 1, describes the relative genotypic frequencies in the
population. p2 represents homozygous, or dominant pp genotypes; q2 represents homozygous, or
frequency of the dominant allele, p, by the mathematical relationship p + q = 1. Therefore, the
frequency of p is .4 because .6 + .4 = 1. Next, you need to determine the frequency of individuals
expressing the dominant trait by recessive qq genotypes; and 2pq represents the frequency of
heterozygotes, or hybrids.applying the second relationship, p2 + 2 pq+ q2 = 1. The individuals expressing
the dominant trait are those that have the pp and pq genotypes, so to find the total frequency of
individuals expressing the dominant trait, you add p2 and 2pq. Thus, p2 = .4 x .4, or .16 and 2pq = 2 x .6 x
.4, or .48. If you add the two together, you get .16 + .48, or .64. Thus, .64 is the correct frequency of
individuals expressing the dominant trait, and choice D is correct.
200 mL of an ideal gas is placed in a piston and is held at a pressure of 500 torr. If the temperature is
held constant and the pressure is increased to 650 torr, what is the new volume of the gas?
75 mL
154 mL
220 mL
300 mL - ANSWER✔ The correct answer is (B)
When the pressure of a gas increases at constant temperature, the volume decreases. Therefore, before
calculation, choices C and D can be eliminated. Since PV/nT is constant, and the temperature and
number of moles of gas are kept constant, we can derive the relationship, P1V1 = P2V2. Rearranging this
equation to find the final volume of gas, we get: V2 = P1V1/P2. By substituting numbers into this
equation, we get: V2 = (500 × 200)/650, or 154 mL, choice B. Another way to arrive at the correct
answer is to notice that the original pressure (500 mL) divided by the new pressure (650 mL) is
approximately 3/4, so the new volume is around 3/4 of 200 mL or 150 mL, and this figure is closest to
choice B.
