Counting Chromosomes: Nondisjunction Quiz

Nondisjunction is the failure of chromosomes to separate correctly during cell division. It can occur during meiosis I when homologous chromosomes fail to separate, or during meiosis II when sister chromatids fail to separate. It can also occur in mitosis. The result is the production of cells or gametes with an abnormal chromosome number. When a gamete resulting from nondisjunction participates in fertilization, the resulting offspring has aneuploidy.

Aneuploidy is defined as a chromosome number that is not an exact multiple of the haploid set. It results from having one or more extra or missing individual chromosomes. Conditions such as trisomy and monosomy are forms of aneuploidy. This is distinct from polyploidy, which involves having complete extra sets of the entire genome. Aneuploidy is most commonly caused by nondisjunction during meiosis and is a major cause of miscarriage and congenital disorders.

Down syndrome is caused by trisomy 21, the presence of three copies of chromosome 21 instead of the normal two. It is the most common chromosomal cause of intellectual disability. Most cases result from nondisjunction during meiosis I in the mother, and the risk increases with maternal age. Characteristics include distinctive facial features, developmental delays, and increased risk of cardiac defects and certain cancers including leukemia.

Turner syndrome results from monosomy of the sex chromosomes, typically written as 45, X, meaning the individual has only one X chromosome and no second sex chromosome. It occurs exclusively in females. Individuals with Turner syndrome typically have short stature, gonadal dysgenesis, infertility, and may have heart or kidney defects. Turner syndrome is one of the few viable human monosomies, as complete monosomy of autosomes is generally lethal early in development.

Turner syndrome, Klinefelter syndrome, and Triple X syndrome are all examples of sex chromosome aneuploidies involving abnormal numbers of sex chromosomes. Down syndrome is caused by trisomy of chromosome 21, which is an autosomal rather than a sex chromosome aneuploidy. Sex chromosome aneuploidies tend to be better tolerated than autosomal aneuploidies because of X-inactivation and the relatively gene-poor nature of the Y chromosome.

When nondisjunction occurs during meiosis I, the homologous chromosomes fail to separate, meaning both chromosomes move to the same pole. All four resulting gametes will be abnormal, either carrying two copies of a chromosome or none. When nondisjunction occurs during meiosis II, sister chromatids fail to separate in one secondary oocyte or spermatocyte, so two of the four gametes produced are normal while two are abnormal, carrying either two copies or no copies of the affected chromosome.

Klinefelter syndrome occurs in individuals who have two X chromosomes and one Y chromosome, giving a total of 47 chromosomes with the complement 47, XXY. It affects males and is characterized by small testes, reduced testosterone production, infertility, and often tall stature. Most cases result from nondisjunction during meiosis in either parent. Klinefelter syndrome is the most common sex chromosome aneuploidy in males and is often not diagnosed until puberty or adulthood.

Monosomy refers to the presence of only one copy of a particular chromosome in a diploid cell instead of the normal two. It results from a gamete produced by nondisjunction that carries no copy of a specific chromosome fertilizing a normal gamete. Most autosomal monosomies are lethal in early development. The most viable human monosomy is Turner syndrome, which involves monosomy of the X chromosome, though even this condition results in significant developmental consequences.

Human oocytes are arrested in prophase I of meiosis from before birth and remain in this arrested state for decades until ovulation. Over time, the spindle apparatus and cohesion proteins that hold homologous chromosomes together degrade, increasing the likelihood of nondisjunction. This explains why the risk of trisomy 21 and other aneuploidies increases significantly with advancing maternal age, a well-established epidemiological finding in human reproductive medicine.

A normal gamete contains 23 chromosomes, including one copy of chromosome 21. A gamete resulting from nondisjunction that carries two copies of chromosome 21 contains 24 chromosomes. When these two gametes fuse at fertilization, the resulting zygote has 47 chromosomes total, with three copies of chromosome 21, which is the karyotype characteristic of Down syndrome. This illustrates how a single nondisjunction event during gamete formation leads directly to trisomy in the offspring.

Nondisjunction during meiosis can produce gametes with an extra chromosome or no chromosome, leading to trisomy or monosomy in offspring after fertilization with a normal gamete. Developmental abnormalities are common outcomes of autosomal aneuploidy. However, not all offspring of an individual who experiences nondisjunction will be aneuploid, since nondisjunction typically affects only some cells or one meiotic division, and most gametes produced may be chromosomally normal.

Trisomy refers to having three copies of one specific chromosome in an otherwise diploid genome, for example three copies of chromosome 21 in a total of 47 chromosomes. Triploidy, by contrast, refers to having three complete sets of all chromosomes, resulting in a total of 69 chromosomes. Triploidy is caused by fertilization errors such as dispermy and is generally lethal in humans. These are fundamentally different conditions despite both involving the number three.

Mosaic Down syndrome results from nondisjunction occurring during mitosis in an early embryonic cell division after fertilization, rather than during meiosis. This produces an individual with two cell populations, one with the normal 46 chromosomes and one with 47 chromosomes including an extra chromosome 21. Because not all cells are trisomic, individuals with mosaic Down syndrome often show a milder range of features compared to those with full trisomy 21.

Patau syndrome is caused by trisomy 13, the presence of three copies of chromosome 13. It is associated with severe intellectual disability, heart defects, brain malformations, and extra digits. Patau syndrome is a rare and severe condition with a median survival of only days to weeks after birth, although some individuals survive longer. Like other trisomies, Patau syndrome most commonly arises from nondisjunction during maternal meiosis, with risk increasing with maternal age.

Nondisjunction during meiosis I causes all resulting gametes to be abnormal. Nondisjunction is the primary mechanism causing aneuploidy in humans. When an aneuploid gamete fuses with a normal gamete during fertilization, the resulting zygote will have an abnormal chromosome number. Aneuploidy is not limited to errors in meiosis II; it can arise from errors in meiosis I, meiosis II, or even post-fertilization mitotic divisions, making option C incorrect.