Nondisjunction and chromosomal disorders Knowledge-Based Study

Study notes

1. Normal Chromosome Segregation in Meiosis

   During normal sexual reproduction, meiosis reduces the diploid (2n) chromosome number by half to produce haploid (n) gametes. This reduction relies on precise chromosome segregation across two consecutive nuclear divisions:

   • Meiosis I: Homologous chromosome pairs align along the metaphase plate and are separated to opposite poles during anaphase I.
   • Meiosis II: Sister chromatids of each chromosome align along the metaphase plate and are pulled apart during anaphase II.

   When segregation proceeds without error, each resulting gamete receives exactly one copy of each chromosome. Fertilization of two normal haploid gametes restores the diploid chromosome number in the zygote.

   What is Nondisjunction?

   Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. While it can occur during mitosis, its occurrence in meiosis leads to gametes with abnormal chromosome numbers.

   Timing and Consequences in Meiosis

   The outcome of nondisjunction depends on the division stage at which the error takes place:

   • Nondisjunction in Meiosis I: A pair of homologous chromosomes fails to separate. As a result, two daughter cells receive an extra chromosome and two lack that chromosome entirely. After meiosis II, all four resulting gametes are abnormal: two gametes have $n + 1$ chromosomes and two gametes have $n - 1$ chromosomes (100% aneuploid gametes).
   • Nondisjunction in Meiosis II: Sister chromatids of one chromosome fail to separate during anaphase II. This error affects only the products of that specific secondary spermatocyte or oocyte. Consequently, 50% of the gametes are normal ($n$), while 25% contain an extra chromosome ($n + 1$) and 25% lack that chromosome ($n - 1$).

   Aneuploidy: Trisomy and Monosomy

   When an abnormal gamete formed by nondisjunction fuses with a normal haploid gamete ($n$), the resulting zygote exhibits aneuploidy, which is the presence of an abnormal number of chromosomes in a cell.

   • Trisomy ($2n + 1$): Fertilization of an $n + 1$ gamete with an $n$ gamete results in three copies of a specific chromosome instead of the normal two.
   • Monosomy ($2n - 1$): Fertilization of an $n - 1$ gamete with an $n$ gamete leaves the zygote with only a single copy of a specific chromosome. In humans, complete autosomal monosomies are typically non-viable and lead to early embryonic lethality.

   Aneuploidy disrupts normal gene dosage, causing imbalances in gene expression that impair development and cellular physiology.

   Disorders of Autosomes

   Autosomal aneuploidies occur when nondisjunction involves non-sex chromosomes. Because autosomes carry many essential genes, most autosomal aneuploidies are fatal during embryonic or fetal development. A few trisomies are compatible with live birth:

   • Down Syndrome (Trisomy 21): The most common viable autosomal trisomy, characterized by three copies of chromosome 21 (total karyotype: 47 chromosomes). Features include characteristic facial features, intellectual disability, congenital heart defects, and increased susceptibility to specific medical conditions. Advanced maternal age significantly increases the frequency of nondisjunction in oocytes, elevating the risk of Trisomy 21.
   • Edwards Syndrome (Trisomy 18): Caused by an extra copy of chromosome 18. Affected infants present with severe developmental delays, clenched fists with overlapping fingers, and severe organ malformations, leading to high neonatal mortality.
   • Patau Syndrome (Trisomy 13): Caused by an extra copy of chromosome 13. Manifestations include microcephaly, cleft lip or palate, polydactyly, and severe structural defects of the central nervous system.

   Disorders of Sex Chromosomes

   Nondisjunction involving the sex chromosomes (X and Y) produces conditions that are generally better tolerated than autosomal aneuploidies, partly due to normal X-inactivation (formation of Barr bodies) and the relatively small number of genes on the Y chromosome.

   • Turner Syndrome (45,X or 45,X0): The only viable human monosomy. Individuals are phenotypic females with a single X chromosome. Characteristics include short stature, a webbed neck, broad chest, cardiovascular abnormalities, and primary ovarian insufficiency resulting in sterility.
   • Klinefelter Syndrome (47,XXY): Occurs in phenotypic males carrying an extra X chromosome. Features include tall stature, reduced testosterone production, hypogonadism, gynecomastia, and infertility. Additional X chromosomes are inactivated as Barr bodies.
   • Triple X Syndrome (47,XXX): Phenotypic females with three X chromosomes. Most individuals have mild or no obvious physical abnormalities, normal fertility, and tall stature, as two of the three X chromosomes undergo Lyonization (inactivation).
   • Jacob's Syndrome (47,XYY): Phenotypic males with an extra Y chromosome resulting from paternal nondisjunction during meiosis II. Individuals are typically taller than average, with normal fertility and development.