Abnormal chromosome structure

• Abnormal Chromosome Structure

  Translocation

  reciprocal translocation

  Centric fusion and fission

  Inverison

  Deletion or Deficiency

  Isochromosomes

  Interspecific hypbridization
• Translocation is a chromosome abnormality caused by rearrangement of parts between non homologous chromosomes.

  can be balanced (in an even exchange of material with no genetic information extra or missing)

  unbalanced (where the exchange of chromosome material is unequal resulting in extra or missing genes)
• Reciprocal translocation- This involves each of two non-homologous chromosomes breaking into two segments, followed by an exchange of segments between the two chromosomes.
• Many reciprocal translocations have been reported in domestic animals, including exchanges between three different chromosomes.
• Animals carrying a reciprocal translocation have a normal phenotype, but show a significant reduction in fertility.
• The reason for the normal phenotype in reciprocal translocation is that animals with a reciprocal translocation still have a normal complement of chromosomal material.
• The decrease in fertility in reciprocal translocation arises from unbalanced gamete because unbalanced gamete combines with another gamete to form zygote, that zygote is unbalanced and dies before term.
• the important of translocation is reproduction reduction in fecundity and it is passed on from parents to offspring.
• Centric fusion and fissions A type of translocation is one in which two acrocentric chromosomes fuse to produce one metacentric chromosome. This is known as a centric fusion or Robertsonian translocation .
• Since centric fusions involve the replacement of two chromosomes by one, it follows that the total number of separate chromosome in individual heterozygous for a centric fusion is one less than the normal number
• individual heterozygous for a centric fusion have a complete genome and hence have normal phenotype,
• Meiosis in individuals heterozygous for a centric fusion is certainly unusual, because three chromosomes have to synapase, forming a a trivalent.
• If the centric-fusion chromosome disjoins from the other two a balanced gametes result
• In cattle, most centric fusions have an adverse effect, giving rise to a varying proportion of monosomic and trisomic embryos that fail to survive to term.
• Occasionally a metacentric chromosome splits into two acrocentric, ie. there is a centric fission producing an animal having one additional chromosome but not being trisomic. This has been reported in the donkey.
• Inversions Inversions arise when a segment of chromosome becomes inverted following breakage of a chromosome in two positions.
• If the segment includes the centromere, the inversion is said to be pericentric, if the centromere is not included, the inversion is paracentric.
• Inversions involve the rearrangement of existing genes without any loss or addition, individuals carrying inversions usually have a normal phenotype.
• So inversion has exactly the same shape and size as a normal chromosome. However as their differences in banding pattern, inversion can be detected by banding technique
• The most common inversion seen in humans is on chromosome 9, at inv 19.
• Inv 9 have no harmful effect andis some suspicion it could lead to an increased risk for miscarriage or infertility for some affected individuals
• Deletion or deficiencyDeletion arise if, following the breakage of a chromosome in two positions, the segment between the two break points is lost (portion of chromosome without a centromere lag in anaphasic movement and are lost from recognizing nuclei).
• It is obvious that deletions results in the loss of genetic material.
• deletions are responsible for an array of genetic disorders, including some cases of male infertility and thirds of two cases of Duchenne muscular dystrophy.
• Deletion of part of the short arm of chromosome 5 results in Cri du chat syndrome.
• Inversions and deletions are the least frequently observed of chromosomal abnormalities in domestic animals.
• Isochromosomes

  This type of chromosomal abnormality arises when the centromere of a metacentric chromosome splits transversely (ie. perpendicular to the chromatids) rather than longitudinally (ie, parallel to the chromatids) during mitosis or meiosis
• Isochromosome splitting leaves two identical chromatid arms joined to the one centromere, so that each arm contains exactly the same of genes, in the same centromere to telomere order.
• Sometimes one of the products of the split is lost in isochromosome, especially if the centromere split unequally.
• Interspecific hybridizationIn most cases, the result of interspecific hybridization is primarily determined by the degree of similarity between the chromosomes of the two species, rather than the degree of physical resemblance
• Even if the animals closely resemble each other, and thus appear to belong to exactly the same species, it is possible that differ sufficiently at the chromosome level for there to be only a low chance of a viable, fertile offspring being produced,