nondisjunction

Nondisjunction

Overview of Nondisjunction

  • Nondisjunction is a failure of chromosomes to separate properly during cell division, specifically during meiosis.

    • In Meiosis, spindles are responsible for distributing chromosomes evenly to daughter cells.

    • Uneven distribution can lead to genetic disorders due to aneuploidy.

Meiosis and Its Stages

  • Meiosis I:

    • Homologous chromosomes fail to separate.

      • Result: The gametes produced have an abnormal chromosome number, referred to as aneuploid.

        • Gametes can be categorized as:

          • Trisomic: A gamete that receives an extra chromosome (n+1).

          • Monosomic: A gamete that loses a chromosome (n-1).

  • Meiosis II:

    • Sister chromatids fail to separate.

      • Result: Similar aneuploid gametes (n+1, n-1) are produced, but in this case, the issue occurs with chromatids rather than homologous pairs.

Genetic Disorders Caused by Nondisjunction

  • Consequences of nondisjunction lead to various genetic disorders, primarily associated with alterations in chromosome number.

Down Syndrome (Trisomy 21)

  • Genetic Makeup: 47,XX,+21

  • Incidence: Approximately 1 in 200 live births.

  • Characteristics:

    • Distinct facial features.

    • Congenital heart defects.

    • Intellectual disabilities.

    • Correlation with advanced maternal age.

Edward’s Syndrome (Trisomy 18)

  • Genetic Makeup: 47,XX,+18

  • Incidence: Approximately 1 in 8,000 live births.

  • Characteristics:

    • Low birth weight.

    • Intellectual disabilities.

    • Congenital heart defects.

    • Cleft palate.

    • Various skeletal and organ defects.

Patau Syndrome (Trisomy 13)

  • Genetic Makeup: 47,XX,+13

  • Incidence: Around 1 in 20,000 live births.

  • Prognosis: 50% of affected infants die within the first month, and 75% die within six months.

  • Characteristics:

    • Brain defects (seizures, apnea, deafness).

    • Cleft palate.

    • Congenital heart defects.

    • Numerous skeletal and organ defects.

Nondisjunction of Sex Chromosomes

  • Generally causes fewer symptoms and is better tolerated than autosomal nondisjunctions.

Klinefelter's Syndrome (XXY)

  • Incidence: Approximately 1 in 2,000 live births.

  • Characteristics:

    • Male sex organs but underdeveloped testes leading to sterility.

    • Some female characteristics (e.g., breast development).

    • Intellectual abilities remain unchanged.

Triple X Syndrome (XXX)

  • Characteristics:

    • Extra X chromosome in females.

    • Taller than average.

    • Generally normal fertility.

    • Occasional learning difficulties, kidney problems, or seizures.

Turner’s Syndrome (X, also referred to as XO)

  • Incidence: About 1 in 5,000 live births.

  • Significance: The only known viable monosomy in humans.

  • Characteristics:

    • Phenotypically female.

    • Non-mature sex organs at puberty.

    • Failure to develop secondary sex characteristics.

    • Sterility.

Deletions and Genetic Disorders

  • Many diseases and disorders arise from a part of a chromosome being either missing or moved.

Cri-Du-Chat Syndrome (5P Minus Syndrome)

  • Incidence: Approximately 1 in 50,000 live births.

  • Characteristics:

    • Deletion of a portion of the short (P) arm of chromosome #5.

    • High-pitched, cat-like cry in affected infants.

    • Intellectual disabilities.

    • Skeletal and organ defects.