Chapter 8 Genetics

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  1. Chromosome mutations or chromosome aberrations
    • a change in the total number of chromosomes
    • the deletion or duplication of genes or segments of a chromosome
    • rearrangements of the genetic material either within or among chromosomes
  2. Aneuploidy
    • variations in chromosome number when an organism gains or loses one or more chromosomes and has other than an exact multiple of the haploid set
    • 2n+/- x chromosomes
    • commonly causes an abnormal phenotype because it leads to an imbalance in the amount of gene products
    • ex. monosomy 2n-1, disomy 2n, trisomy 2n+1
  3. Euploidy
    • complete haploid sets of chromosomes are present
    • multiples of n
    • ex. diploidy 2n, triploidy 3n, tetraploidy 4n
  4. Polyploidy
    • occurs when more than two sets of chromosomes are present
    • ex. 3n,4n,5n
  5. Nondisjunction
    • chromosomal variation
    • chromosomes or chromatids fail to dis join and move to opposite poles during meiosis I or II
  6. Monosomy
    • the loss of one chromosome to produce a 2n-1 compleement
    • monosomy for x chromosome occurs in humans
    • monosomy for any of the autosomes is usually not tolerated in humans and other animals
  7. Trisomy
    • 2n+1 chromosomes
    • trisomy for autosomes are often lethal
    • sex chromosome has a less dramatic impact on the phenotype
    • three copies of one chromosome is present and so pairing configuartions are usually irregular
  8. Trivalent
    three copies of a chromosome are synapsed
  9. Univalent
    an unpaired chromosome
  10. In some cases, prior to the meiotic division in trisomy
    there is one bivalent and one univalent instead of a trivalent
  11. Down syndrome
    • trisomy of chromosome 21
    • 12 to 14 characteristics
    • affected individuals express 6 to 8 on average
    • usually due to nondisjunction of maternal chromosome 21 during meiosis
    • shows an increased incidence with increasing maternal age
  12. Familial down syndrome
    • down syndrome occasionally runs in families
    • involves a translocation of chromosome 21
  13. Amniocentesis or chorionic villus sampling (CVS)
    diagnostic testing for women who become pregnant late in their reproductive years
  14. Patau syndrome
    trisomy 13
  15. Edwards syndrome
    trisomy 19
  16. What is often found in spontaneously aborted fetuses?
    • trisomies but now monosomies
    • suggests that monosomic gametes may be very functionally impaired
  17. Autopolyploidy
    the addition of one or more sets of chromosomes identical to the haploid complement of the same species
  18. Allopolyploidy
    the combination of chromosome sets from different species as a consequence of interspecific mating
  19. Allotetraploid
    arise from hybridization of two closely related species
  20. Amphidiploid
    produced when the steril hybrid from allotetraploid undergoes a natural chromosomal doubling and produces a fertile amphidiploid
  21. Amphidiploid plants
    produced by somatic cell hybridization
  22. Endopolyploidy
    • the condition in which only certain cells in an otherwise diploid organism are polyploid
    • replication and segregation of chromosomes occur without nuclear division
  23. Total amount of genetic information in chromosome can change
    • deletions
    • duplications
  24. The genetic material remains the same, but is rearranged
    • inversions
    • translocations (reciprocal or nonreciprocal)
  25. Deletions
    • chromosome breaks in one or more places and a portion of it is lost, the missing piece is the deletion
    • can occur near one end (terminal deletion)
    • interior of the chromosome (intercalary deletion)
  26. Synapsis between a chromosome with a large intercalary deletion and a normal complete homolog what must occur?
    the unpaired region of the homolog must loop out of the linear structure into a deletion or compensation loop
  27. Cri-du-chat
    results from a segmental deletion of a small terminal portion of the short arm of chromosome 5
  28. Duplication
    • result of unequal crossing over during meiosis or through a replication error prior to meiosis
    • may play a role in evolution
  29. Gene redundancy
    organisms have multiple copies of the ribosomal RNA genes (rRNA)
  30. Gene amplification
    another mechanism to increase the rRNA
  31. Bar-eye phenotype in Drosophila
    results from duplication
  32. Inversion
    • rearrangement of the linear gene sequence rather than the loss of genetic information
    • a segment of a chromosome is turned around 180 degrees within a chromosome
    • requires two breaks in the chromosome and subsequent reinsertion of the inverted segment
    • may arise from chromosomal looping
  33. Paracentric inversion
    does not change the relative lengths of the two arms of a chromosome
  34. Pericentric inversion
    • includes the centromere
    • does change the relative lengths of the two arms of a chromosome
  35. Inversion loop
    synapsis of inverted chromosomes
  36. For a paracentric inversion crossover:
    one recombinant chromatid is dicentric and one is acentric
  37. Dicentric
    two centromeres
  38. Acentric
    lacking a centromere
  39. Translocation
    movement of a chromosomal segment to a new location in the genome
  40. Reciprocal translocation
    • involves the exchange of segments between two nonhomologous chromosomes
    • has an unusual synapsis configuration during meiosis
  41. Alternate segregation
    one that leads to a normal and balanced gamete
  42. Adjacent segregation
    one that leads to gametes containing duplications and deficiencies
  43. Robertsonian translocation or centric fusion
    • involves breaks at the extreme ends of the short arms of two nonhomologous acrocentric chromosomes
    • ex. familial down syndrome
  44. Fragile sites
    are more susceptible to chromosome breakage when cells are cultured in the absence of certain chemicals such as folic acid
  45. Fragile X syndrome (martin-bell syndrome)
    • the most common form of inherited mental retardation
    • affects 1 in 4000 males and 1 in 8000 femalesĀ 
    • it is a dominant trait
    • fragile site FMR-1 has a trinucleotide sequence of CGG
    • normal individual has 6-54 repeats and carriers have 55-230
    • fragile x has more than 230 repeats
    • link between an autosomal fragile site and lung cancer was reported
    • FRA3B (fragile site on p arm of chromosome 3) is often altered or missing in cells taken from individuals with lung cancer
    • normal protein product of this gene is absent in cells of many other cancers
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Chapter 8 Genetics
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