Genetics - Chapter 6 (after test)

• Duplications arise as the result of unequal crossing over during meiosis or through a replication error prior to meiosis
• •Organisms have multiple copies of the ribosomal RNA genes (rRNA)

  •This is an example of gene redundancy
• •Gene amplification is another mechanism to increase the rRNA - multiplying by itself
• Gene redundancy – 2 or more genes are performing the same function
• Polyploidy, in which more than 2 haploid sets of chromosomes are present, is prevalent in plants.
• Each identical set of chromosomes is identical to parent species.
• Autotriploids can arise in several ways.
• Autotetraploids are more prevalent in a natural population as they produce balanced gametes
• An allotetraploid arises from hybridization of 2 closely related species. If the sterile hybrid undergoes a natural chromosomal doubling, a fertile amphidiploid is produced.
• Amphidiploid plants can also be produced by somatic cell hybridization
• Endopolyploidy - the condition in which only certain cells in an otherwise diploid organism are polyploid. In these cells, replication and segregation of chromosomes occur without nuclear divisions.
• Variation occurs in the internal composition and arrangement of chromosomes
• Rearrangements of chromosome segments include:

  • Deletions
  • Duplications
  • Inversion
  • Nonreciprocal translocations
  • Reciprocal translocations
• There are 2 primary ways in which the structure of chromosomes can be altered.

  1. The total amount of genetics info in the chromosome can change (deletions, duplications)
  2. The genetic material remains the same, but is rearranged (inversions, translocations)
• Duplication types: normal, tandem, reverse, and displaced
• Deletion - missing region of a chromosome
• When a chromosome breaks in one or more places and a portion of it is lost, the missing piece is referred to as a deletion (or deficiency)
• The deletion can occurs

  • Near one end (terminal deletion)
  • From the interior of the chromosomes (intercalary deletion)
• For synapsis to occur between a chromosome with a large intercalary deletion and a normal complete homolog, the undpaired region of the normal homolog must loop out of the linear structure into a deletion or compensation loop
• Cri-du-chat results from a segmental deletion of a small terminal portion of the short arm of chromosome 5.
• Duplication - repeated segment of the genetic material
• Duplications arise as the result of unequal crossing over during meiosis or through a replication error prior to meiosis. Pairing didn’t occur properly.
• Organisms have multiple copies of the ribosomal RNA genes (rRNA). This is an example of gene redundancy, in which 2+ genes are performing the same function
• Gene amplification is another mechanism to increase the rRNA, in which the gene multiplies by itself
• Gene duplication may play a role in evolution. This hypothesis is supported by the discover of genes that have a substantial amount of their DNA sequence in common, but whose gene products are distinct
• Other support includes the presence of gene families, regional groups of genes whose products perform the same general function
• Inversions rearrange the linear gene sequence. An inversion involves a rearrangement of the linear gene sequence rather than the loss of genetic info.
• In an inversion, a segment of a chromosome is turned around 180 within the chromosome
• An inversion requires 2 breaks in the chromosomes and subsequent reinsertion of the inverted segment. An inversion may arise from chromosomal looping
• A paracentric inversion doesn’t change the relative lengths of the 2 arms of a chromosome.

  A pericentric inversion, which includes the centromere, does change the relative lengths of the 2 arms of a chromosome
• For a paracentric inversion crossover:

  • One recombinant chromatid is dicentric (2 centromeres)
  • One is acentric (lacking a centromere)
• Translocations alter the location of chromosomal segments in the genome

  Translocation - movement of a chromosomal segment to a new location in the genome
• A reciprocal translocation

  • Involves the exchange of segments between 2 non-homologous chromosomes
  • Has an unusual synapsis configuration during meiosis
• Alternate segregation - one that leads to a normal and a balanced gamete

  Adjacent segregation - one that leads to gametes containing duplications and deficiencies
• A Robertsonian translocation or centric fusion involves breaks at the extreme ends of the short arms of 2 nonhomologous acrocentric chromosomes. Familial Down syndrome is an example
• Fragile sites in humans are susceptible to chromosome breakage

  Fragile sites are more susceptible to chromosome breakage when cells are culture in the absence of certain chemicals such as folic acid
• Fragile X-syndrome (Martin-Bell syndrome) - the most common form of inherited mental retardation, affecting 1/4000 males and 1/8000 females, and is a dominant trait
• Fragile site FMR-1 has a trinucleotide sequence CGG

  Normal individual: 6-54 repeats, “carriers”: 55-230, fragile x-syndrome: 230+ repeats
• A link between autosomal fragile site and lung cancer was reported

  FRA3B (fragile site on p arm on chromosome 3) is often altered or missing in cells taken from individual with lung cancer

  Further studies have revealed that normal protein product of this gene is absent in cells of many other cancers