Question 1

A man has extra digits (six fingers on each hand and six toes on each foot). His wife and their daughter have the normal number of digits (five fingers on each hand and five toes on each foot.) Having extra digits is a dominant trait. The couple's second child has extra digits. What is the probability that their next (third) child will have extra digits?

Accepted Answer

The trait for extra digits is dominant, so the man must be heterozygous (Dd) since his daughter is normal (dd). The wife is normal (dd). The probability of their next child having extra digits is 1/2 (Dd or dd).

Question 2

Radish flowers may be red, purple, or white. A cross between a red-flowered plant and a white-flowered plant yields all-purple offspring. The part of the radish we eat may be oval or long, with long being the dominant trait. If true-breeding red long radishes are crossed with true-breeding white oval radishes, the F₁ will be expected to exhibit which of the following phenotypes?

Accepted Answer

According to the information provided, a cross between a red-flowered plant and a white-flowered plant yields all-purple offspring. This suggests that the allele for purple flowers is dominant over both the allele for red flowers and the allele for white flowers. 
Additionally, the information provided indicates that the allele for long radishes is dominant over the allele for oval radishes. This suggests that the F1 generation of true-breeding red long radishes crossed with true-breeding white oval radishes will be expected to be purple and long.

Question 3

Which of the following phenotypes is an example of polygenic inheritance?

Accepted Answer

Skin pigmentation in humans is an example of polygenic inheritance, as it is controlled by multiple genes that interact with each other to produce a continuum of shades of skin color. Pink flowers in snapdragons and white and purple flower color in peas are examples of incomplete dominance, while the ABO blood group in humans is an example of codominance.

Question 4

What is the probability of producing the genotype AABBCC in a cross of individuals who both possess this genotype: AaBbCc?

Accepted Answer

Each gene pair (Aa, Bb, Cc) has a 1/4 chance of producing the homozygous dominant genotype (AA, BB, CC) when crossed. Therefore, the probability of producing AABBCC is 1/4 * 1/4 * 1/4 = 1/64.

Question 5

Which of the following inheritance patterns describes the ability of a single allele to have multiple phenotypic effects?

Accepted Answer

Pleiotropy occurs when a single gene affects multiple phenotypic traits. It is often seen in genetic disorders where one gene can cause a variety of symptoms, such as sickle-cell anemia or Marfan syndrome. Incomplete dominance refers to a situation where a heterozygote displays an intermediate phenotype between the two homozygotes, while multiple alleles refer to the existence of more than two alleles of a gene in a population. Epistasis occurs when one gene affects the expression of another gene.

Question 6

Phenylketonuria is an inherited disease caused by a recessive autosomal allele. If a woman and her husband are both carriers, what is the probability that their first child will be a phenotypically normal girl?

Accepted Answer

The probability of having a phenotypically normal child is 3/4 (since 1/4 will be affected). The probability of having a girl is 1/2. Therefore, the probability of having a phenotypically normal girl is 3/4 * 1/2 = 3/8.

Question 7

In cattle, roan coat color (mixed red and white hairs) occurs in the heterozygous (CᴿCᵂ) offspring of red (CᴿCᴿ) and white (CᵂCᵂ) homozygotes. Which of the following crosses would produce offspring in the ratio of 1 red:2 roan:1 white?

Accepted Answer

The roan × roan cross (CᴿCᵂ × CᴿCᵂ) results in a 1:2:1 ratio of red (CᴿCᴿ), roan (CᴿCᵂ), and white (CᵂCᵂ) offspring.

Question 8

Hydrangea plants of the same genotype are planted in a large flower garden. Some of the plants produce blue flowers and others pink flowers. This can be best explained by which of the following?

Accepted Answer

The color of hydrangea flowers can vary due to environmental factors such as soil pH. In acidic soils, hydrangeas produce blue flowers while in alkaline soils they produce pink flowers. Therefore, the presence of different flower colors in the hydrangea plants of the same genotype can be best explained by the influence of soil pH on the phenotype.

Question 9

Assuming independent assortment for all gene pairs, what is the probability that a cross between the following parents, AABbCc × AaBbCc, will produce an AaBbCc offspring?

Accepted Answer

For each gene pair (A/a, B/b, C/c), the probability of the offspring inheriting the desired alleles from both parents (i.e. Aa, Bb, Cc) is 1/2. Since there are three gene pairs, the probability of all three occurring together is (1/2) x (1/2) x (1/2) = 1/8.

Question 10

In pea plants, the tall phenotype is dominant to the dwarf phenotype. If a heterozygous pea plant is crossed with a homozygous tall pea plant, what is the probability that the offspring will be dwarf in size?

Accepted Answer

A heterozygous tall plant (Tt) crossed with a homozygous tall plant (TT) will produce offspring that are either TT or Tt, both of which are tall. There is no possibility of a dwarf (tt) offspring, so the probability is 0.

Question 11

Two true-breeding stocks of pea plants are crossed. One parent has red, axial flowers, and the other has white, terminal flowers; all F₁ individuals have red, axial flowers. The genes for flower color and location assort independently. Among the F₂ offspring, what is the probability of producing plants with white axial flowers?

Accepted Answer

The F₁ generation shows that red and axial are dominant traits. In the F₂ generation, the probability of getting white flowers (recessive) is 1/4, and the probability of getting axial flowers (dominant) is 3/4. Since the genes assort independently, the probability of both occurring together is 1/4 * 3/4 = 3/16.

Question 12

Gray seed color in peas is dominant to white. Assume that Mendel conducted a series of experiments where plants with gray seeds were crossed among themselves, and the following progeny were produced: 302 gray and 98 white. What is the most probable genotype of each parent?

Accepted Answer

Since gray seed color is dominant and there are 302 gray progeny, we know that at least one parent must be heterozygous for the gray seed color gene (Gg). If both parents were homozygous dominant (GG), then all progeny would be gray. If one parent was homozygous recessive (gg), then all progeny would be white. Thus, the only option that fits is that both parents are heterozygous (Gg), which would result in a 3:1 ratio of gray to white progeny.

Question 13

Suppose two individuals with the genotype AaBbCc are mated. Assuming that the genes are not linked, what fraction of the offspring are expected to be homozygous recessive for the three traits?

Accepted Answer

Each gene pair (Aa, Bb, Cc) segregates independently, and the probability of being homozygous recessive (aa, bb, cc) for each gene is 1/4. Therefore, the probability for all three traits is (1/4) * (1/4) * (1/4) = 1/64.

Question 14

When Mendel crossed yellow-seeded and green-seeded pea plants, all the offspring were yellow-seeded. When he took these F₁ yellow-seeded plants and crossed them to green-seeded plants, what genotypic ratio was expected?

Accepted Answer

The F₁ yellow-seeded plants are heterozygous (Yy). Crossing them with green-seeded plants (yy) results in a 1:1 genotypic ratio (Yy:yy).

Question 15

Which of the following calculations require the use of the addition rule of probability?

Accepted Answer

The addition rule of probability is used when calculating the probability of two mutually exclusive events occurring. In this case, having sickle-cell anemia and cystic fibrosis are mutually exclusive events, and thus the addition rule is used. A, B, and C require different forms of probability calculations, such as Punnett squares and the product rule of probability.

Question 16

Given the following genotypes for two parents, AABBCc × AabbCc, assume that all traits exhibit simple dominance and independent assortment. What proportion of the progeny of this cross will be expected to phenotypically resemble the first parent with the genotype AABBCc?

Accepted Answer

The answer of Given the following genotypes for two parents,...

Question 17

Black fur in mice (B) is dominant to brown fur (b). Short tails (T) are dominant to long tails (t). What fraction of the progeny of crosses BbTt × BBtt will be expected to have black fur and long tails?

Accepted Answer

The cross BbTt × BBtt results in offspring with genotypes: BBTt, BBtt, BbTt, and Bbtt. All offspring will have black fur (B is dominant). For long tails, only those with genotype tt will have long tails. The probability of tt is 1/2, so the fraction with black fur and long tails is 1/2.

Question 18

Which of the following statements best describes the addition rule of probability?

Accepted Answer

The addition rule of probability states that the probability of either one of two independent events occurring is equal to the sum of their individual probabilities.

Question 19

Radish flowers may be red, purple, or white. A cross between a red-flowered plant and a white-flowered plant yields all-purple offspring. The flower color trait in radishes is an example of which of the following inheritance patterns?

Accepted Answer

The fact that a cross between a red-flowered plant and a white-flowered plant yields all-purple offspring indicates that neither the red nor the white allele is completely dominant over the other. This is a characteristic of incomplete dominance.

Question 20

Which of the following scenarios describes an example of epistasis?

Accepted Answer

Epistasis occurs when the effect of one gene is dependent on the presence of one or more 'modifier genes'. In option B, the genotype (ee) prevents any fur color from developing, which is an example of epistasis where one gene masks the effect of another gene.

Quiz 14: Mendel and the Gene Idea