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 5 chromosomes, and after fertilization by a normal sperm, the resulting zygote will have 13 chromosomes, creating an abnormal chromosome condition.

Step-by-step solution

Review the process of oogenesis and meiosis

Oogenesis is the process of egg formation in animals. It starts within the ovary, where diploid cells divide by meiosis to produce haploid cells. The haploid number of this species is 6, which means that there are 6 chromosomes in each gamete. Meiosis is a type of cell division in which a diploid cell undergoes two successive cell divisions, meiosis I and meiosis II, to produce four haploid cells. The key events during meiosis are DNA replication, pairing and exchange of genetic material between homologous pairs (crossing over), separation of homologous chromosomes in meiosis I, and separation of sister chromatids in meiosis II.

Analyze the consequences of nondisjunction during meiosis II

In this case, one dyad undergoes nondisjunction during meiosis II, which means that the sister chromatids do not separate properly and end up intact in the ovum. This abnormal event will result in the ovum having an extra chromosome, and the second polar body having one less chromosome.

Determine the number of chromosomes in the mature ovum and second polar body

Mature ova typically have the haploid number of chromosomes; in this species, that would be 6. However, since there was nondisjunction during meiosis II, the ovum will have an extra chromosome from the dyad. Therefore, the mature ovum will have 7 chromosomes. As for the second polar body, it is also supposed to have a haploid number of chromosomes. Due to the nondisjunction, the second polar body will have one less chromosome than expected. Thus, the second polar body will have 5 chromosomes.

Determine the chromosome condition after fertilization by a normal sperm

Normal sperm cells have a haploid number of chromosomes, so they will have 6 chromosomes in this species. Following fertilization by a normal sperm, the number of chromosomes in the zygote will be the sum of the chromosomes from the sperm and the ovum. Since the sperm has 6 chromosomes and the ovum has 7 chromosomes, the resulting zygote will have 13 chromosomes. In conclusion, (a) The mature ovum has 7 chromosomes. (b) The second polar body has 5 chromosomes. (c) After fertilization by a normal sperm, the resulting zygote will have 13 chromosomes, which is an abnormal chromosome condition.

Key concepts

Meiosis in Animals

Meiosis is a fundamental biological process through which animals produce gametes, or sex cells (sperm in males and eggs in ova or ovum in females). Understanding this process is crucial as it ensures that offspring have the correct chromosome number from each parent. Meiosis occurs in two distinct phases: meiosis I and meiosis II.

During meiosis I, homologous chromosomes—one set from each parent—pair up and exchange genetic material. This is known as recombination or crossing over, which increases genetic diversity. The homologous pairs are then separated so that each new cell receives one chromosome from each pair. Meiosis II resembles a regular cell division, or mitosis, where the sister chromatids (the replicated forms of each chromosome) separate into different cells.

It is important to note that these steps must occur precisely. Any errors can lead to conditions such as nondisjunction, where chromosomes fail to separate correctly, leading to gametes with an incorrect number of chromosomes. This can lead to genetic disorders in the offspring, demonstrating the significance of accurate chromosome distribution during meiosis in animals.

Chromosome Number

In sexually reproducing animals, each species has a characteristic chromosome number. Chromosome number is significant because it ensures the stability of a species' genome over generations. Except for the gametes, cells in an organism are typically diploid, meaning they have two sets of chromosomes—one from each parent. In contrast, gametes are haploid, with only one set of chromosomes.

When a gamete is produced during meiosis, the chromosome number is halved so that upon fertilization, when the two gametes merge, the resulting zygote restores the diploid number. In the exercise provided, the haploid number is 6, indicating that normal gametes should contain six chromosomes. However, nondisjunction can disrupt this balance, leading to gametes with more or fewer chromosomes than normal, challenging the embryo's viability or resulting in genetic abnormalities.

Fertilization and Zygote Formation

Fertilization is the process of a sperm cell merging with an ovum, leading to zygote formation. This is the first step in the creation of a new individual with a unique combination of genetic material. The zygote's chromosome number is the sum of the haploid numbers from both parents, typically resulting in a diploid cell.

In the context of the textbook exercise, a sperm (with 6 chromosomes) fertilizes an ovum, which, due to nondisjunction, has 7 chromosomes. This results in a zygote with 13 chromosomes, an aberrant condition known as aneuploidy. This zygote would carry an abnormal number of chromosomes, possibly leading to developmental issues. Knowledge of these processes is not only pivotal for academic purposes but also for understanding underlying causes of genetic conditions and reproductive health.