Chapter 1. Chapter 2 Online Worked Problem: Meiosis

1.1 Problem Statement

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?

For help with this problem:

  1. Review Meiosis (Pages 28-29) and Meiosis in the Life Cycles of Animals and Plants (Pages 35-37) in Section 2.3: Sexual Reproduction Produces Genetic Variation through the Process of Meiosis.
  2. Review Figures 2.13 (Page 28), 2.15 (Page 30-31) and 2.20 (Page 36), and Table 2.2 (Page 29).

Question

Begin by thinking about the steps to take to do this problem. Use the dropdown menu to put the steps in the order in which they should be performed.

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=
2
Correct.
Check with the “Instant TA” and try again.
It is always good practice to first define any unfamiliar terms. Next, you’ll need to identify the question that is being asked and figure out what information is available to help you answer the question. Before performing calculations, it is best to choose a problem-solving approach. In problems that ask for combined probabilities, you’ll need to calculate individual probabilities first and then use the multiplication rule with those individual probabilities to calculate the combined probabilities.

Instant TA

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.

1.2 Step 1: Define terms you may not know or understand.

Question

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
2
Correct.
Incorrect.
Check with the “Instant TA” and try again.

Instant TA

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.

1.3 Step 2: Identify the question(s) being asked.

Question

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2
Correct.
Incorrect.
Check with the “Instant TA” and try again.

Instant TA

Review the problem to determine which answer is closest to what is being asked.

1.4 Step 3: Identify the information provided that will enable you to answer the question.

Question

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
2
Correct.
Check with the “Instant TA” and try again.
Incorrect.

Instant TA

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.

1.5 Step 4: Choose a problem-solving approach.

Question

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2
Correct.
Check with the “Instant TA” and try again.
Incorrect.

Instant TA

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.

1.6 Step 5: Calculate the number of chromosomes that will be present in the egg produced by the female horse; calculate the number of chromosomes that will be present in the sperm produced by the male donkey.

Question

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2
Correct.
Incorrect.
Check with the “Instant TA” and try again.

Instant TA

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.

1.7 Step 6: 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 formed by their fusion.

Question

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2
Correct.
Incorrect.
Check with the “Instant TA” and try again.

Instant TA

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.

1.8 Step 7: Use your knowledge of the mechanisms of meiosis to explain why most mules are sterile.

Question

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2
Check with the “Instant TA” and try again.

Correct.

Further Exploration

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.

Further Exploration

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.

Instant TA

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?