Question

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

Answer: The mature ovum has 7 chromosomes, the second polar body has 6 chromosomes, and following fertilization by a normal sperm, the zygote will have 13 chromosomes.

Step-by-step solution

Understanding oogenesis and nondisjunction

Oogenesis is the process of formation of an ovum or mature egg in females. In this process, a diploid cell (called oogonium) undergoes meiosis to produce four haploid cells, one of which becomes the mature ovum while the other three become polar bodies. The haploid number in the given exercise is \(6,\) which means a diploid cell will have \(12\) chromosomes. Nondisjunction is an event that occurs during meiosis when two homologous chromosomes or sister chromatids fail to separate properly. As a result, one cell receives both chromosomes instead of one, and the other cell does not get any. In this exercise, nondisjunction happens during meiosis II, so a dyad (two sister chromatids) fails to separate and ends up intact in the ovum.

Determine the number of chromosomes in the mature ovum

A normal ovum will have the haploid number of chromosomes, which is \(6\) in this species. Since there was nondisjunction during meiosis II, one dyad (two sister chromatids) ended up intact in the ovum instead of being separated. So, in this case, the mature ovum will have one extra chromosome, making the total number of chromosomes \(6+1=7\). (a) The mature ovum has \(7\) chromosomes.

Determine the number of chromosomes in the second polar body

Normally, the second polar body should have inherited the separated sister chromatid from the said dyad. However, due to nondisjunction during meiosis II, the entire dyad remained in the ovum, and the second polar body did not inherit the additional chromosome. Therefore, the second polar body has the normal haploid number of chromosomes, which is \(6\) in this species. (b) The second polar body has \(6\) chromosomes.

Determine the chromosome condition following fertilization

When a mature ovum is fertilized by a normal sperm, the sperm provides a haploid set of chromosomes, which is \(6\) in this species. Since the ovum in this case has \(7\) chromosomes, the resulting zygote will have the diploid number of chromosomes plus an extra chromosome: \(6 + 7 = 13.\) (c) Following fertilization by a normal sperm, the chromosome condition created is a zygote with \(13\) chromosomes, which indicates trisomy (the presence of an extra chromosome) in the zygote for the chromosome involved in the initial nondisjunction event.

Key concepts

Meiosis

Meiosis is an essential biological process that occurs in the reproductive cells of animals, including humans, to produce gametes, which are the egg and sperm cells. Unlike normal cell division, or mitosis, meiosis involves two rounds of division. It starts with a single diploid cell, which contains two sets of chromosomes, and ends with four haploid cells, each holding half the original number of chromosomes.

This reduction is crucial because it ensures that when an egg and sperm unite during fertilization, the resulting zygote has the proper number of chromosomes. During meiosis, cells go through two distinct phases: meiosis I, which separates homologous chromosomes, and meiosis II, which divides the sister chromatids. The process allows for genetic diversity due to the exchange of genetic material during crossing over.

Issues can arise during meiosis, such as nondisjunction, where chromosomes fail to separate properly. The exercise provided explores the outcomes of nondisjunction during the second meiotic division, meiosis II, affecting the number of chromosomes in gametes and the potential zygote.

Haploid and Diploid

The terms haploid and diploid describe the number of chromosome sets found in a cell. Diploid cells, represented by the symbol 2n, have two complete sets of chromosomes, one from each parent. In humans and many other organisms, most cells are diploid, including the cells that make up our organs and tissues.

On the other hand, haploid cells, denoted by n, contain only one set of chromosomes. Gametes, the egg and sperm cells, are haploid, which ensures that upon fertilization, the resulting zygote restores the diploid state with the appropriate number of chromosomes. This meticulously orchestrated exchange of genetic material between generations is key to sexual reproduction and biological diversity.

Trisomy

Trisomy is a form of chromosomal abnormality characterized by the presence of an extra chromosome in the cells. While a normal diploid cell has two copies of each chromosome, a cell with trisomy has three copies of a particular chromosome. This can happen as a result of nondisjunction during meiosis, when chromosomes fail to separate correctly. Trisomy often leads to developmental and health issues, as seen in disorders like Down syndrome, Patau syndrome, and Edwards syndrome, each caused by an extra copy of chromosomes 21, 13, and 18, respectively.

In the exercise we are discussing, nondisjunction results in an ovum with an extra chromosome, leading to trisomy after fertilization. This underlines the critical importance of precise chromosomal segregation during meiosis for the development of a healthy zygote.

Zygote Chromosome Number

The zygote chromosome number is fundamental to an organism's development and is typically the sum of the haploid numbers of chromosomes from an egg and a sperm. In organisms with diploid life cycles, zygotes must have a precise diploid number to develop normally. Any deviation from this number, such as the example provided in the exercise, can lead to genetic disorders or developmental anomalies.

In the scenario of the exercise, fertilization of the ovum, which has undergone nondisjunction, with a normal sperm results in an uneven number of chromosomes in the zygote. This unusual zygote chromosome number can be the root of various genetic disorders, making the proper progression of meiosis and resulting chromosomal count in gametes of incredible significance for the health of the offspring.