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?

Key Concepts

Oogenesis and Meiosis

Oogenesis is the process of egg cell development in females, involving meiosis, which reduces the chromosome number by half. In this case, the organism has a haploid number of 6, meaning that normally, each gamete (egg or sperm) would contain 6 chromosomes after meiosis. During meiosis, two divisions occur: meiosis I and meiosis II, leading to the formation of one mature ovum and polar bodies.

Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. In this scenario, nondisjunction occurs during meiosis II, resulting in one gamete (the ovum) receiving an extra chromosome. This can lead to aneuploidy, where the resulting cells have an abnormal number of chromosomes, affecting the genetic makeup of the offspring.

Fertilization and Chromosome Condition

Fertilization is the process where a sperm cell unites with an ovum, resulting in a zygote. If the ovum has an abnormal chromosome number due to nondisjunction, the zygote will inherit this condition. In this case, if the ovum has 7 chromosomes (6 normal plus 1 extra) and is fertilized by a normal sperm (which has 6 chromosomes), the resulting zygote will have 13 chromosomes, leading to a condition known as aneuploidy.