Nondisjunction is an error in cell division where chromosomes fail to separate correctly during meiosis. This matters because it can produce gametes with too many or too few chromosomes, which can lead to chromosomal disorders after fertilization. Many well known conditions, including Down syndrome, Turner syndrome, and Klinefelter syndrome, are caused by aneuploidy.
Studying nondisjunction helps explain how chromosome number affects development and inheritance.
During meiosis I, homologous chromosomes should separate into different cells, and during meiosis II, sister chromatids should separate. If either step fails, the resulting egg or sperm may have n + 1 or n - 1 chromosomes instead of the normal haploid number. After fertilization with a normal gamete, these errors can create a zygote with 2n + 1, called trisomy, or 2n - 1, called monosomy.
Karyotypes allow scientists and doctors to count chromosomes and identify the specific chromosome abnormality.
Key Facts
- Normal human body cells are diploid: 2n = 46 chromosomes.
- Normal human gametes are haploid: n = 23 chromosomes.
- Nondisjunction in meiosis can produce gametes with n + 1 = 24 or n - 1 = 22 chromosomes.
- Fertilization with a normal gamete can produce trisomy: 24 + 23 = 47 chromosomes.
- Fertilization with a normal gamete can produce monosomy: 22 + 23 = 45 chromosomes.
- Common karyotypes include Down syndrome: 47, +21; Turner syndrome: 45, X; Klinefelter syndrome: 47, XXY.
Vocabulary
- Nondisjunction
- Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division.
- Aneuploidy
- Aneuploidy is an abnormal chromosome number caused by the gain or loss of one or more chromosomes.
- Trisomy
- Trisomy is a condition in which a cell has three copies of one chromosome instead of two.
- Monosomy
- Monosomy is a condition in which a cell has only one copy of a chromosome instead of two.
- Karyotype
- A karyotype is an organized image or diagram of a person's chromosomes arranged by size, shape, and number.
Common Mistakes to Avoid
- Confusing nondisjunction with mutation in a gene, because nondisjunction changes chromosome number while a gene mutation changes DNA sequence within a chromosome.
- Thinking trisomy means three total chromosomes, because trisomy means three copies of one specific chromosome in a cell.
- Assuming nondisjunction only occurs in meiosis I, because it can occur in meiosis I when homologous chromosomes fail to separate or in meiosis II when sister chromatids fail to separate.
- Writing Turner syndrome as 46, X, because Turner syndrome has only one sex chromosome and is correctly written as 45, X.
Practice Questions
- 1 A sperm cell has 24 chromosomes because of nondisjunction. If it fertilizes a normal egg with 23 chromosomes, how many chromosomes will the zygote have, and is this trisomy or monosomy?
- 2 A human egg is missing one chromosome and has 22 chromosomes. If it is fertilized by a normal sperm, how many chromosomes will the zygote have, and what general type of aneuploidy is present?
- 3 Explain how a nondisjunction event in meiosis I can produce a different pattern of abnormal gametes than a nondisjunction event in meiosis II.