A horse has 64 chromosomes and a donkey has 62 chromosomes. A cross between a female horse and a male donkey produces a mule, which is usually sterile. How many chromosomes does a mule have? Can you think of any reasons to explain the fact that most mules are sterile?
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| Calculate the number of chromosomes in the egg produced by the female horse; calculate the number of chromosomes in the sperm produced by the male donkey. | FnX3b7sE90v94vmLlulCQlp00JI= |
| Use your knowledge of the mechanisms of meiosis to explain why most mules are sterile. | O73vLd0T+HUDfyQSKU7TrAkO8X0= |
| Define terms you may not know or understand. | +ip/azOsQZAPIahPy8zcDjYRRHE= |
| Identify relevant information in the problem that will enable you to answer the question. | 5V6x9VtSGh7gXHp3Lu5E0GafRxw= |
| Using your calculations of the numbers of chromosomes in the egg and the sperm, calculate the number of chromosomes that will be present in the mule offspring. | hUcsH5b98uROz7gkVNx+4ZXXNQ0= |
| Identify the question(s) being asked. | qVfojwJTEkaJDZYx+Ja/4L1G1Bg= |
| Choose a problem-solving approach. | KoVvFfPzAcMh2SJqlYIOFDdT0Kw= |
The best first step in solving any problem is to make sure you understand all the terms that are used. Next, you’ll need to identify the question that is asked and what information is available to help you formulate your answer. After choosing a problem-solving approach, begin solving the problem. When a question asks about offspring, it is usually best to figure out the parents first.
This problem does not have many unfamiliar terms. While you may be familiar with horses, donkeys, and mules, you might not have understood their genetic relationships. Review the information provided in the question.
Review the problem to determine which answer is closest to what is being asked.
Read the problem carefully to figure out what information is provided. Because both the horse and the donkey are animals, you can safely assume that the problem is giving you their diploid (2n) chromosome numbers. As animals, the diploid adult individuals will produce haploid (1n) gametes.
In order to solve this problem, you’ll need to calculate the number of chromosomes in the gametes produced by each parent. Then you’ll need to calculate the number of chromosomes in the offspring produced by fusion of the gametes.
As with all sexually reproducing individuals, horses and donkeys will each produce gametes that contain half as many copies of each chromosome. Because horses and donkeys are both animals, we can safely assume that both are diploid, with two copies of each chromosome, and will produce gametes that are haploid, with one copy of each chromosome.
The mule offspring is formed by the fertilization of the horse egg by the donkey sperm. You’ll need to correctly calculate the numbers of chromosomes in the egg and sperm to answer this question.
Correct.
If a mule has an odd number of chromosomes and if an even number of chromosomes is required for meiosis to produce gametes, how can a mule ever be fertile?
The Problem Statement mysteriously specifies that only “most” mules are sterile. There have been a few scientifically validated reports of fertile female mules, but the mechanism(s) by which this occurs is not well understood.
Incorrect.
If a mule has an odd number of chromosomes and if an even number of chromosomes is required for meiosis to produce gametes, how can a mule ever be fertile?
The Problem Statement mysteriously specifies that only “most” mules are sterile. There have been a few scientifically validated reports of fertile female mules, but the mechanism(s) by which this occurs is not well understood.
Review the mechanisms of meiosis to understand why a mule that has a diploid chromosome number of 63 cannot produce gametes by meiosis. During which specific phase(s) is an even number of chromosomes required?