[Intro music] During meiotic division in germ cells, errors can sometimes occur. One such error is called nondisjunction. Nondisjunction occurs when a pair of homologous chromosomes (or homologs) don’t separate properly during meiosis I, or when sister chromatids don’t separate properly during meiosis II. Nondisjunction can result in some cells having an extra copy of a given chromosome, while other cells will be missing a copy of a given chromosome. An increase or decrease in the normal number of chromosomes in a cell is referred to as aneuploidy. Let’s examine what happens if nondisjunction occurs in meiosis to find out more about how aneuploidy arises. We’ll use a hypothetical example of a diploid germ cell that has 2 pairs of homologs. The total number of chromosomes in diploid cells is designated as 2<i>n</i>, so we refer to this hypothetical germ cell as being 2<i>n</i> = 4. Let’s start by focusing on nondisjunction during meiosis I. During anaphase I, pairs of homologs typically separate from one another and move toward opposite spindle poles. When nondisjunction occurs, a pair of homologs doesn’t separate during anaphase I, and both homologs are pulled toward the same spindle pole. The 2 cells that result following nondisjunction in meiosis I have different numbers of chromosomes. In our example, 1 resulting cell has a total of 3 chromosomes, and the other has 1 chromosome. Now let’s consider the outcome if these 2 resulting cells undergo meiosis II as expected. During anaphase II, the sister chromatids separate as usual. However, the 4 cells that result from meiosis II still vary in the number of chromosomes. In our example, 2 resulting cells each have a total of 3 chromosomes, and the other 2 each only have 1 chromosome. If these 4 resulting cells go on to form gametes (or reproductive cells), the gametes will also vary in their number of chromosomes. A typical haploid gamete has 1 copy of each chromosome, and is designated as <i>n</i>. Gametes with an extra copy of a given chromosome are denoted as <i>n</i> + 1. During fertilization, if an <i>n</i> + 1 gamete fuses with a normal haploid gamete, the resulting zygote will be aneuploid, as it has an extra copy of a given chromosome. This addition of a third copy of a given chromosome in a diploid organism is called trisomy and is denoted as 2<i>n</i> + 1. Gametes that are missing a copy of a given chromosome are denoted as <i>n</i> – 1. During fertilization, if an <i>n</i> – 1 gamete fuses with a normal haploid gamete, the resulting zygote will also be aneuploid, as it is missing a copy of a given chromosome. This loss of a copy of a given chromosome in a diploid organism is called monosomy and is denoted as 2<i>n</i> – 1. Now let’s focus on nondisjunction during meiosis II. We’ll assume that meiosis I occurred as expected, where each resulting cell received 1 copy of each homolog. During anaphase II, the sister chromatids typically separate and move toward opposite spindle poles. When nondisjunction occurs, the sister chromatids of a chromosome in one of the cell’s don’t separate properly to opposite spindle poles during anaphase II. The 4 cells that result following nondisjunction in meiosis II have different numbers of chromosomes. In our example, 1 resulting cell has a total of 3 chromosomes, 1 has only 1 chromosome, and 2 each have 2 chromosomes. If these 4 resulting cells go on to form gametes, the gametes will also vary in the number of chromosomes. One gamete would be <i>n</i> + 1, 1 gamete would be <i>n</i> – 1, and 2 gametes would each be <i>n</i>. During fertilization, if each of these gametes fuses with a normal haploid gamete, the resulting zygotes will vary in their total number of chromosomes. One zygote would be trisomic (or 2<i>n</i> + 1), 1 zygote would be monosomic (or 2<i>n</i> – 1), and 2 zygotes would each be diploid (or 2<i>n</i>). The trisomic and monosomic zygotes are aneuploids. Often in humans, cells that result from a nondisjunction event during meiosis will naturally be eliminated. However, some of these cells may survive and can result in conditions like Down syndrome, which is due to trisomy of chromosome 21. [Outro music]