Lec 11 &12

• Cytogenetics


  The study of chromosomes and their role in heredity
• Chromosomal defect


  Arises from change in the autosomes or sex chromosomes
• Causes of chromosomal defects

  1. Failure of chromosome reduction during meiosis

  2. Fertilization by polyspermy

  3. Abnormalities of the mitotic spindle at critical stages during cell division

  4. One or more breaks on a chromosome with or without reunion of the broken segments
• Types of chromosomal changes

  • Numerical change
  • Structural change
• Euploidy


  Variation in the number of complete sets of chromosomes, an exact multiple of the haploid number
• Aneuploidy


  Variation in the number of particular chromosomes within a set, an inexact number of the normal chromosome set
• Most species of animals are 2n
• In many cases, changes in euploidy are not tolerated in animals
• Polyploidy in animals is lethal
• Some euploidy variations are naturally occurring
• Female bees are 2n, male bees (drones) are monoploid or haploid
• Goldfish, salmons and salamanders are examples of vertebrate polyploids
• Diploid animals sometimes produce tissues that are polyploid, a phenomenon termed endopolyploidy
• Highly differentiated human tissues like heart muscles, bone marrow and liver cells can be triploid, tetraploid or even octaploid
• In contrast to animals, plants commonly exhibit polyploidy
• 30-35% of ferns and flowering plants are polyploid
• Many of the fruits and grain we eat come from polyploid plants
• Polyploid plant strains display outstanding agricultural characteristics, they are often larger in size and more robust
• Aneuploidy


  Leads to an imbalance in the amount of gene products, three copies will lead to 150% production
• Aneuploidy commonly causes an abnormal phenotype
• Mechanisms leading to variation in chromosome number

  • Meiotic nondisjunction
  • Mitotic abnormalities
  • Interspecies crosses
• Meiotic nondisjunction


  Failure of chromosomes to separate properly during anaphase, can produce haploid cells with too many or too few chromosomes
• Mitotic abnormalities


  1. Mitotic disjunction - sister chromatids separate improperly, leading to trisomic and monosomic daughter cells

  2. Chromosome loss - one sister chromatid does not migrate to a pole, leading to normal and monosomic daughter cells
• Genetic abnormalities that occur after fertilization lead to mosaicism, where part of the organism contains cells that are genetically different from other parts
• Structural alterations


  • Deficiency/Deletion - when a segment of a chromosome is lost
  • Duplication - when a segment is copied/repeated
  • Inversion - when a segment is in reverse sequence
  • Insertion - when a segment is deleted and inserted into another chromosome
  • Translocation - when a segment or whole arm is transposed to another chromosome
• Deletion/Deficiency


  When a segment of a chromosome is lost
• Duplication


  When a segment of a chromosome is copied/repeated, resulting in extra genetic material
• Inversion


  When a segment is in reverse sequence along a single chromosome
• Insertion


  When a segment of a chromosome is deleted and inserted into another chromosome
• Translocation


  When a segment or a whole arm is transposed to another chromosome, two types: reciprocal and Robertsonian
• Reciprocal translocations lead to rearrangement of genetic material but not total amount, they are usually without phenotypic consequences
• Unbalanced translocations are associated with phenotypic abnormalities or even lethality
• Robertsonian translocations


  Breaks occur at the extreme ends of the short arms of two nonhomologous acrocentric chromosomes, the small acentric fragments are lost, and the larger fragments fuse at their centromeric regions to form a single chromosome
• Robertsonian translocations are the most common type of chromosomal rearrangement in cattle
• Chromosomal insertion


  A rare event involving three chromosomal breaks where a segment is removed from one chromosome and inserted into a broken region of a nonhomologous chromosome
• Majority of chromosomal defects are not seen frequently in animals as they are incompatible with life
• However, there are defects that can cause reproductive problems like infertility, subfertility and intersexuality, one such problem is translocation of chromosomes
• Translocation


  A mutation that occurs when a fragment of DNA joins either a non-homologous chromosome (interchromosome) or another segment of the chromosome itself (intrachromosome)
• Types of translocation


  • Reciprocal translocation
  • Robertsonian translocation
• Reciprocal translocation


  Corresponding/mutual translocation