Lecture 16: Errors in Meiosis

Lecture Objectives

• Understand various chromosomal aberrations:
  • Nondisjunction
  • Aneuploidy
  • Deletion
  • Duplication
  • Inversion
  • Translocation
• Describe how chromosomal rearrangements happen during meiosis and their contribution to birth defects in humans.
• Learn about different forms of polyploidy and their effects on meiosis and fertility.
• Explain the consequences of odd numbers of chromosomes during meiotic segregation.

Meiotic Nondisjunction

• Definition: Failure of chromosomes to separate properly during meiosis.
  • Meiosis I: Homologous chromosomes do not separate.
  • Meiosis II: Sister chromatids do not separate.
• Can occur in mitosis as well.
• Result: Abnormal number of chromosomes in gametes.

Aneuploidy

• Definition: Abnormal number of chromosomes in a zygote due to fertilization involving abnormal gametes.
  • Monosomy (2n-1) - Missing a chromosome.
  • Trisomy (2n+1) - Extra chromosome.
• Occurrence: 10-25% of all human conceptions, few are viable.
• Common aneuploid conditions:
  • Down Syndrome (Trisomy 21): Occurs in 1 in 750 live births. Increased incidence with maternal age.
  • Klinefelter Syndrome (XXY): Extra X chromosome, 1 in 1,000 males.
  • Turner Syndrome (XO): Only one X chromosome, 1 in 5,000 females.

Common Aneuploids Summary

Types of Chromosomal Aberrations

• Deletion: Loss of a chromosome segment, severe effects possible.
• Duplication: Extra segment present; usually harmful effects.
• Inversion: Segment of chromosome is reversed; changes gene order but not number. Can affect fertility through misalignment during meiosis.
• Translocation: Segment attaches to a nonhomologous chromosome. Affects gene expression without changing gene number.

Familial Down Syndrome

• Translocation: Parent carries a translocation between chromosome 14 and 21. Only 45 chromosomes present but phenotypically normal. Produces gametes with abnormal chromosome numbers.

Polyploidy

• Definition: Possession of more than two complete chromosome sets.
• Causes: Nondisjunction or failure of a diploid zygote to divide after chromosome replication.
• Common in plants (e.g., strawberries are octoploids). Less common in animals.
• Odd numbers of chromosome sets lead to infertility due to unbalanced gametes (e.g., triploid bananas).

Challenges for Polyploid Organisms

• Reproductive Issues: Odd chromosome numbers (e.g., triploids) cannot evenly segregate during meiosis, leading to infertility.
• Some polyploids are fertile due to proper chromosome pairing and balanced gene dosage.

Summary of Chromosomal Aberrations

• Errors during cell division can lead to various chromosomal disorders contributing to developmental issues in humans.
• Nondisjunction: Leads to aneuploidy, affecting birth viability.
• Chromosomal aberrations can disrupt protein production and affect reproductive capabilities.
• Understanding these concepts is crucial in genetics and their implications for human health.