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
| Syndrome | Chromosomes | Sex | Incidence |
|---|
| Down Syndrome | 21 | M/F | 1 in 750 |
| Patau Syndrome | 13 | M/F | 1 in 5,000 |
| Edwards Syndrome | 18 | M/F | 1 in 10,000 |
| Klinefelter | XXY | M | 1 in 1,000 |
| Turner | XO | F | 1 in 5,000 |
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.
