Problem 18 - Campbell Biology Concepts & Connections

Question

Suppose you read in the newspaper that a genetic engineering laboratory has developed a procedure for fusing two gametes from the same person (two eggs or two sperm) to form a zygote. The article mentions that an early step in the procedure prevents crossing over from occurring during the formation of the gametes in the donor's body. The researchers are in the process of determining the genetic makeup of one of their new zygotes. Which of the following predictions do you think they would make? Justify your choice, and explain why you rejected each of the other choices. a. The zygote would have 46 chromosomes, all of which came from the gamete donor (its one parent), so the zygote would be genetically identical to the gamete donor. b. The zygote could be genetically identical to the gamete donor, but it is much more likely that it would have an unpredictable mixture of chromosomes from the gamete donor's parents. c. The zygote would not be genetically identical to the gamete donor, but it would be genetically identical to one of the donor's parents. d. The zygote would not be genetically identical to the gamete donor, but it would be genetically identical to one of the donor's grandparents.

Step-by-Step Solution

Answer

The zygote would be genetically identical to the gamete donor (Option A).

Step 1: Understand the Zygote Formation

When a zygote forms from two gametes, it typically receives one set of chromosomes from each parent. In this scenario, the procedure fuses two gametes from the same person, bypassing the usual two-parent contribution.

Step 2: Consider the Prevented Crossing Over

Crossing over typically occurs during meiosis and is responsible for genetic variation by swapping segments of DNA between homologous chromosomes. The absence of crossing over means that no genetic exchange between homologous chromosomes happens.

Step 3: Analyze Chromosome Contribution

Given that the gametes originate from the same individual and there is no crossing over, the chromosomes in the gametes will be direct copies of the donor's chromosomes.

Step 4: Evaluate Option A

Option A states that the zygote would have 46 chromosomes, all from the donor, making it genetically identical to the donor. This seems consistent with the single individual providing both sets of chromosomes.

Step 5: Evaluate Option B

Option B mentions an unpredictable mixture due to potential crossing over. Since crossing over is prevented, this option is incorrect.

Step 6: Evaluate Option C

Option C suggests genetic identity to one of the donor's parents, which is unlikely as the zygote receives a mix of the donor’s chromosomes without crossing over.

Step 7: Evaluate Option D

Option D implies genetic identity to a grandparent, which is also implausible. The genetic contribution goes through two generations (the donor’s parents), making it impossible for the zygote to match one grandparent.

Step 8: Conclusion

Based on the steps above, Option A is the correct choice as the zygote would have 46 chromosomes all from the donor, making it genetically identical to the donor.

Key Concepts

zygote formation

A zygote forms when two gametes (sperm and egg) combine. Typically, each gamete contributes half of the genetic material, resulting in a full set of 46 chromosomes. This process usually involves two different parents. In genetic engineering, as described in the exercise, if the laboratory fuses two gametes from the same person, it bypasses the contribution from two parents.
Because both gametes come from the same individual, each gamete would have a set that mirrors the donor’s chromosomes. This is crucial to understand as it forms the fundamental basis of the scenario described.

chromosome contribution

Chromosomes are structures that contain DNA. Humans have 23 pairs, with one set from each parent. The described procedure results in both sets originating from the same donor, thus providing more chromosomes from the same genetic pool.
Because crossing over is prevented, there is no DNA exchange between homologous chromosomes. Chromosomes in each gamete stay as direct copies of the donor’s original chromosomes. This makes the zygote an exact duplicate in terms of chromosomal composition.

genetic identity

Genetic identity refers to how similar the DNA of the resulting zygote is to its source. Since both gametes come from the same donor and there’s no crossing over, the zygote’s genetic makeup closely matches that of the donor.
Each of the 46 chromosomes in the zygote is a direct replica of those in the donor, making the zygote genetically identical to the single parent donor. This is a key reason why option A was chosen as the correct answer in the exercise.

crossing over

Crossing over is a process in meiosis where homologous chromosomes exchange DNA segments. This genetic recombination increases diversity. In the scenario, crossing over is prevented, meaning such an exchange does not occur.
This absence of crossing over keeps the genetic material in each gamete uniform. Thus, the zygote ends up identical to the donor since the chromosomes remain unaltered through the process. This lack of mixing reaffirms why the resulting zygote would be genetically identical to the donor.