Chapter 2 Online Worked Problem: Meiosis

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:

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.
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.
Use your knowledge of the mechanisms of meiosis to explain why most mules are sterile.
Define terms you may not know or understand.
Identify relevant information in the problem that will enable you to answer the question.
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.
Identify the question(s) being asked.
Choose a problem-solving approach.

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.

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

Question

Choose the correct statement.

A horse is a cross between a donkey and a mule.

A donkey is a cross between a horse and a mule.

A mule is a cross between a horse and a donkey.

Correct.

Incorrect.

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

Question

What problem are you being asked to solve?

The number of chromosomes in the egg produced by the horse and why most mules are sterile

The number of chromosomes in the sperm produced by the donkey and why most mules are sterile

The number of chromosomes in the mule produced by the fusion of the horse egg and the donkey sperm and why most mules are sterile

The number of chromosomes in the offspring of mules who mate with other mules and why most mules are sterile

Correct.

Incorrect.

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

Question

What information will help you to solve the problem?

A horse has a diploid chromosome number of 64; a donkey has a diploid chromosome number of 62.

A horse has a haploid chromosome number of 64; a donkey has a diploid chromosome number of 62.

A horse has a diploid chromosome number of 64; a donkey has a haploid chromosome number of 62.

A horse has a haploid chromosome number of 64; a donkey has a haploid chromosome number of 62.

Correct.

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Incorrect.

Step 4: Choose a problem-solving approach.

Question

What approach will help you to solve this problem?

Calculate the number of chromosomes in the egg produced by the horse.

Calculate the number of chromosomes in the sperm produced by the donkey.

Use the numbers chromosomes in the egg and the sperm to calculate the number of chromosomes in the fertilized egg/mule.

All of the above

Correct.

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Incorrect.

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

How many chromosomes will be present in the egg produced by the horse? How many chromosomes will be present in the sperm produced by the donkey?

The horse will produce an egg with 64 chromosomes; the donkey will produce sperm with 62 chromosomes.

The horse will produce an egg with 32 chromosomes; the donkey will produce sperm with 31 chromosomes.

The horse will produce an egg with 64 chromosomes; the donkey will produce sperm with 31 chromosomes.

The horse will produce and egg with 32 chromosomes; they donkey will produce sperm with 62 chromosomes.

Correct.

Incorrect.

Check with the “Instant TA” and try again.

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

How many chromosomes will be present in the mule produced by crossing the female horse and the male donkey?

An offspring mule will have 64 (egg) + 62 (sperm) chromosomes = 126 chromosomes

An offspring mule will have 32 (egg) + 31 (sperm) chromosomes = 63 chromosomes

An offspring mule will have 64 (egg) + 31 (sperm) chromosomes = 95 chromosomes

An offspring mule will have 32 (egg) + 62 (sperm) chromosomes = 94 chromosomes

Correct.

Incorrect.

Check with the “Instant TA” and try again.

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

Question

Choose the explanation that you think best explains why most mules are sterile.

Mules are sterile because an even number of chromosomes is required for synapsis and recombination during prophase I, and for lining up homologous chromosomes in pairs during metaphase I. Mules have 63 chromosomes, an odd number that cannot properly align during prophase I and metaphase I to complete meiosis. Therefore, mules are generally unable to produce viable gametes.

Mules are sterile because an even number of chromosomes is required for synapsis and recombination during prophase II, and for lining up homologous chromosomes in pairs during metaphase II. Mules have 63 chromosomes, an odd number that cannot properly align during prophase II and metaphase II to complete meiosis. Therefore, mules are generally unable to produce viable gametes.

Mules are sterile because an even number of chromosomes is required for synapsis and recombination during prophase I, and for lining up homologous chromosomes in pairs during metaphase II. Mules have 63 chromosomes, an odd number that cannot properly align during prophase I and metaphase II to complete meiosis. Therefore, mules are generally unable to produce viable gametes.

Mules are sterile because an even number of chromosomes is required for synapsis and recombination during prophase II, and for lining up homologous chromosomes in pairs during metaphase I. Mules have 63 chromosomes, an odd number that cannot properly align during prophase II and metaphase I to complete meiosis. Therefore, mules are generally unable to produce viable gametes.

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?