Genetic Chromosomal Changes

Chromosomal Structure Changes

• Diploid Organisms:
  • Have intact chromosomes with two copies of each chromosome.
  • Mistakes in chromosome replication or segregation can lead to:
  1. Changes in structure (rearrangements)
  2. Changes in chromosome number.

Rearrangements from Double Strand Breaks (DSBs)

• Double Strand Breaks (DSBs):
  • Often caused by accidents.
• Rearrangements can occur and lead to different structural changes:
  • Deletion: Loss of a chromosome segment
  • Duplication: A segment is duplicated, leading to extra copies
  • Inversion: A segment is reversed within the chromosome
  • Translocations: Segments of one chromosome break off and attach to another chromosome.
  • Simple translocation
  • Reciprocal translocation

Types of Deletions

• Deletions: Can result from one or two DSBs.
  • Terminal Deletion: Loss of the end segment of the chromosome.
  • Interstitial Deletion: Loss of a segment from the middle of the chromosome after two breaks and reattachment of outer pieces.
• Example: Cri du chat syndrome - which can result in fatality in the first year or reach a normal age.

Duplications

• Duplications: Can arise from DSBs or unequal crossing over.
  • Result in an extra segment of the chromosome, resulting in multiple copies of genes.
• Effects of Duplications:
  • Can be detrimental (e.g., dominant mutations in the Bar gene).
  • Can be neutral and lead to new gene functions (e.g., red opsin and green opsin genes).
  • Example: The globin family includes myoglobins and hemoglobins that have evolved through duplications over millions of years.

Finding Changes in Chromosome Structure

• Karyotyping: A technique used to find large deletions or duplications in chromosomes by observing the number and structure of chromosomes under a microscope.

Inversions

• Inversions: Result from two DSBs.
  • Effects: They do not typically lead to loss or gain of DNA, but can result in fertility problems due to issues during meiosis and improper chromosome segregation.

Translocations

• Translocations: Can result from breaks in two different chromosomes.
  • Do not usually lead to loss or gain of DNA but can cause fertility issues due to improper segregation in meiosis.
  • Three types of segregation patterns:
  • Alternate segregation
  • Adjacent-1 segregation
  • Adjacent-2 segregation (very rare).

Changes in Chromosome Number

• Nondisjunction: Mistakes during chromosome segregation in meiosis or mitosis can lead to changes in chromosome numbers.
  • Meiosis I Nondisjunction: Both chromosomes in a homologous pair go into one cell, leading to an extra chromosome in one daughter cell.
  • Meiosis II Nondisjunction: Similar situation arises but occurs in the sister chromatids.

Aneuploidy in Humans

• Aneuploid Conditions: Mostly lethal; certain trisomies and monosomies can arise from nondisjunction.

  • Common Conditions:

  • Trisomy 13 (Patau syndrome)

  • Trisomy 18 (Edwards syndrome)

  • Trisomy 21 (Down syndrome)

  • Sex Chromosome Aneuploidies:

    • Klinefelter syndrome (XXY)
    • Turner syndrome (X0).

Ploidy Changes

• Ploidy: Refers to the number of sets of chromosomes in a cell.
  • Polyploidy: Organisms with more than two sets (e.g., triploid 3n, tetraploid 4n), more tolerable in plants than animals.
  • Odd-numbered Ploidy: Leads to sterility, such as triploidy (3n).