Crosses between female horses and male donkeys produce mules. Gene flow between the gene pools of horses and donkeys is prevented by

I. hybrid sterility.
II. a pre-zygotic isolating mechanism.
III. a post-zygotic isolating mechanism.

Question

Crosses between female horses and male donkeys produce mules. Gene flow between the gene pools of horses and donkeys is prevented by

I. hybrid sterility.
II. a pre-zygotic isolating mechanism.
III. a post-zygotic isolating mechanism.

BlackTom AI · Accepted Answer

To analyze the question, I will break down what each statement implies about barriers to gene flow between horses and donkeys.
Option 1: I and II. If this were true, both hybrid sterility and a pre-zygotic isolating mechanism would prevent gene flow. While pre-zygotic barriers can stop mating or fertilization, in the real-world pairing of a female horse and a male donkey, fertilization can occur to produce a mule, so a pre-zygotic barrier is not the primary driver here. This makes this option questionable.
Option 2: II only. This suggests that only a pre-zygotic barrier prevents gene flow. Since mules are produced and the mating can occur, a pre-zygotic barrier does not fully explain the isolation between the gene pools. Therefore, this choice does not fit the biological reality.
Option 3: III only. A post-zygotic barrier would explain reduced or absent gene flow due to issues arising after fertilization. However, while post-zygotic barriers are important, the question specifically involves crosses that do occur but produce sterile offspring, which is a hallmark of hybrid sterility rather than an inability to form hybrids.
Option 4: I and III. Here, hybrid sterility (I) directly prevents gene flow by making the hybrids sterile, and post-zygotic effects (III) reinforce this barrier by ensuring that even if hybrids are produced, they cannot contribute to future generations. This combination accurately captures how gene flow between horse and donkey gene pools is prevented.
In summary, pre-zygotic barriers (II) are not the sole or primary reason for the genetic separation in this case because mating and hybrid formation do occur in horses and donkeys, yielding sterile offspring. The most accurate depiction involves both hybrid sterility (I) and post-zygotic barriers (III), which together block gene flow between the two species. Thus, the correct choice is the option that lists I and III together.

BIOL 1009 (001-006/050-055) Final Exam

Crosses between female horses and male donkeys produce mules. Gene flow between the gene pools of horses and donkeys is prevented by I. hybrid sterility. II. a pre-zygotic isolating mechanism. III. a post-zygotic isolating mechanism.

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