chapter 13 pt4

Nondisjunction in Meiosis

  • Nondisjunction: The failure of homologous chromosomes or sister chromatids to separate properly during cell division, specifically during meiosis.

Types of Nondisjunction

  • Meiosis I: Nondisjunction of homologous chromosomes occurs.

  • Meiosis II: Nondisjunction of sister chromatids occurs.

Chromosome Number Consequences

  • Resulting gametes from nondisjunction:

    • Gametes with n + 1 chromosomes

    • Gametes with n - 1 chromosomes

    • Gametes with n chromosomes

Visual Representation:

  • (a) Nondisjunction of homologous chromosomes in Meiosis I:

    • Produces gametes with n + 1 and n - 1.

  • (b) Nondisjunction of sister chromatids in Meiosis II:

    • Produces gametes with n + 1 and n - 1.

Genetic Disorders Caused by Nondisjunction

  • Autosomal Disorders

    • Trisomy 13 (Patau Syndrome):

    • Caused by having three copies of chromosome 13.

    • Symptoms include serious brain and organ defects.

    • Survival Rates:

      • 50% survival rate at 7-10 days.

      • 13% survival rate at age 10.

    • Trisomy 18 (Edwards Syndrome):

    • Caused by having three copies of chromosome 18.

    • Symptoms include serious organ and joint defects.

    • Survival Rates:

      • 50% survival rate at 7-10 days.

      • 10% survival rate at age 10.

    • Trisomy 21 (Down Syndrome):

    • Caused by having three copies of chromosome 21.

    • Can also arise from a translocation error between chromosomes 14 and 21.

    • Symptoms may include developmental delays, heart defects, and varying degrees of intellectual disability.

    • Life expectancy is approximately 60 years.

    • Most survivable of all trisomy conditions.

Patau Syndrome Karyotype

  • Example Karyotype:

    • 47XY, +13 (indicates an extra chromosome 13).

Genetic Disorders Caused by Nondisjunction (Sex Chromosome Disorders)

  • Involve X and/or Y chromosomes.

Trisomy Conditions:

  • XXX (Triple X Syndrome):

  • XXY (Klinefelter Syndrome):

  • XYY (Jacob's Syndrome):

Monosomy Condition:

  • XO (Turner Syndrome):

    • Only known survivable monosomy.

Characteristics of Turner Syndrome:

  • Individuals are female with only one X chromosome.

  • Conditions vary; the most common characteristic is sterility.

Alterations in Chromosome Structure

  • Errors in Meiosis

    • Lead to breakage of chromosomes.

Major Types of Errors with Outcomes:

  1. Deletion Error:

    • Part of the chromosome is broken off.

    • Creates a sequence with missing genetic material.

  2. Duplication Error:

    • The deleted part can attach to the sister chromatid or homologous chromosome.

    • This results in extra copies of a section of the chromosome.

  3. Inversion Error:

    • The deleted part reattaches to the same chromosome but in an inverted order.

  4. Translocation Error:

    • The deleted part reattaches to a nonhomologous chromosome.

    • Can lead to a balanced translocation, where genetic material is rearranged but overall genetic information is preserved.

Visual Representation of Chromosome Errors

  • Deletion

    • Breaks in chromosome leading to loss of genetic material.

  • Duplication

    • Illustrates how a section may be attached to sister chromatids or nonsister chromatids.

  • Paracentric Inversion

    • Shows reinsertion of a deleted piece of DNA but in reversed orientation.

  • Translocation:

    • Shows how genetic material can be exchanged between nonhomologous chromosomes.