Chapter 2: Problem 17

During oogenesis in an animal species with a haploid number of \(6,\) one dyad undergoes nondisjunction during meiosis II. Following the second meiotic division, this dyad ends up intact in the ovum. How many chromosomes are present in (a) the mature ovum and (b) the second polar body? (c) Following fertilization by a normal sperm, what chromosome condition is created?

Short Answer

Expert verified

Answer: The mature ovum has 7 chromosomes, the second polar body has 5 chromosomes, and the chromosome condition after fertilization is aneuploid with 13 chromosomes.

Step by step solution

Understand the normal oogenesis process

In the normal oogenesis process, a primary oocyte (2n = 12) will undergo two meiotic divisions, resulting in one mature ovum (n = 6) and two polar bodies. Meiosis I will produce a secondary oocyte (n = 6) and the first polar body (n = 6). Meiosis II will then produce the mature ovum (n = 6) and the second polar body (n = 6).

Determine the number of chromosomes in the ovum with nondisjunction

In this case, we have a nondisjunction event in one dyad during meiosis II, meaning one dyad stays intact and ends up in the ovum. Since a dyad contains two chromatids (sister chromatids), the ovum will have an extra pair of chromatids. Therefore, instead of having a haploid number of chromosomes (6), the ovum will have an additional chromatid, making the total number of chromosomes = 6 + 1 = 7. (a) So, the mature ovum has 7 chromosomes.

Determine the number of chromosomes in the second polar body

Since the second polar body is formed after meiosis II, and the dyad with nondisjunction did not separate, the second polar body will have one less chromatid than usual. Thus, the second polar body will have = 6 - 1 = 5 chromosomes. (b) Therefore, the second polar body has 5 chromosomes.

Determine the chromosome condition after fertilization by a normal sperm

Upon fertilization of the mature ovum (with 7 chromosomes) by a normal sperm (with 6 chromosomes), the resulting zygote will have an unequal number of chromosomes. It will have one extra chromatid, making the total number of chromosomes in the zygote = 7 + 6 = 13. (c) The chromosome condition created after fertilization is aneuploid, with 13 chromosomes.

Unlock Step-by-Step Solutions & Ace Your Exams!

Full Textbook Solutions
Get detailed explanations and key concepts
Unlimited Al creation
Al flashcards, explanations, exams and more...
Ads-free access
To over 500 millions flashcards
Money-back guarantee
We refund you if you fail your exam.

Start your free trial

Over 30 million students worldwide already upgrade their learning with Vaia!

Recommended explanations on Biology Textbooks

View all explanations

What do you think about this solution?

We value your feedback to improve our textbook solutions.

Short Answer

Step by step solution

Understand the normal oogenesis process

Determine the number of chromosomes in the ovum with nondisjunction

Determine the number of chromosomes in the second polar body

Determine the chromosome condition after fertilization by a normal sperm

One App. One Place for Learning.

Most popular questions from this chapter

Recommended explanations on Biology Textbooks

Biological Structures

Genetic Information

Energy Transfers

Communicable Diseases

Bioenergetics

Responding to Change

Study anywhere. Anytime. Across all devices.