Chromosome Mutations and Disorders

CHROMOSOME MUTATIONS

  • Overview:
    • Chromosome mutations are changes that occur in chromosome structure or number, which can lead to various genetic disorders.
Learning Objectives
  • Aneuploidy:
    • Definition: Some gametes contain the wrong number of copies of a particular chromosome.
    • Cause: Non-disjunction during meiosis.
    • Consequences: Generally deleterious effects, which may lead to abnormalities or death.
  • Chromosomal Rearrangements:
    • Types:
    • Deletion
    • Duplication
    • Inversion
    • Translocation
      • Reciprocal Translocation
      • Robertsonian Translocation
Chromosome Basics
  • The correct number of chromosomes must be passed to offspring via gametes.
  • Upon fertilization, the human genome consists of:
    • One pair of homologous autologous chromosomes from each parent.
    • One pair of sex chromosomes: XY (male) or XX (female).
  • Total Chromosome Count: Normal diploid human cells have 46 chromosomes (2 sets of 22 autologous chromosomes + 1 pair of sex chromosomes).
Karyotype
  • Definition: Karyotype describes:
    • Number of chromosomes
    • Chromosome length
    • Position of the centromeres
    • Banding pattern
    • Differences between sex chromosomes
Chromosomal Disorders
  • Causes of Disorders:
    • Problems during cell division, leading to:
    • Incorrect number of chromosomes (Aneuploidy, Polyploidy)
    • Chromosomal Rearrangements (e.g., deletion, duplication)
Aneuploidy vs. Polyploidy
  • Aneuploidy:
    • Occurs when gametes have an incorrect number of a particular chromosome due to non-disjunction during meiosis.
    • Generally harmful, causing abnormality or death.
  • Polyploidy:
    • A complete duplicated set of chromosomes.
    • Occurs due to complete non-disjunction, often seen in plants and some animals, suggesting evolutionary advantages.
Types of Aneuploidy
  • Common Terms:
    • Monosomy: One copy instead of two (2n-1)
    • Trisomy: Three copies instead of two (2n+1)
    • Nullisomy: No copies instead of two (2n-2)
Causes of Aneuploidy
  • Non-disjunction can occur during meiosis due to:
    • Homologous chromosomes failing to separate during Meiosis I.
    • Sister chromatids failing to separate during Meiosis II.
Types of Chromosomal Rearrangements
  • Deletion: A segment of a chromosome is lost.
  • Duplication: A segment of a chromosome is duplicated, creating extra genetic material.
  • Inversion: A segment of DNA is flipped and reinserted.
  • Translocation: A segment of one chromosome is exchanged with another.
    • Reciprocal translocation: Exchange of segments between non-homologous chromosomes without loss of genetic information.
    • Robertsonian translocation: One entire chromosome attaches to another, typically involves acrocentric chromosomes and results in fewer total chromosomes due to loss of a small segment.
Specific Chromosomal Disorders
  • Trisomy:
    • Trisomy 21 (Down Syndrome): Occurs in approximately 1 in 650 births, often results in learning disabilities.
    • Trisomy 18 (Edwards Syndrome): Severe, many pre-term deaths.
    • Trisomy 13 (Patau Syndrome): High rates of miscarriage or death shortly after birth.
  • Monosomy:
    • Example: Turner’s syndrome (X0) is generally fatal, but can be viable.
  • Klinefelter Syndrome (XXY):
    • Symptoms include reduced fertility, developmental delays, and learning difficulties.
Age-Related Risks
  • Risk Factors: Age of the mother corresponds with increased risk for chromosomal abnormalities in offspring.
Conclusion
  • Chromosomal mutations are critical for understanding genetic diseases and their potential impacts on human health. Their effects range from benign to severe, underlining the importance of genetic study and screening.