Chapter 12

  • Chromosomal Rearrangements
  • Two main themes underlying the observations on chromosomal changes
    • Karyotypes generally remain constant within a species
    • most genetic imbalances result in a selective disadvantage
    • Related species usually have different karyotypes
    • closely-related species differ by only a few rearrangements on their karyotypes
    • distantly-related species differ by many rearrangements on their karyotypes
    • Correlation between karyotypic rearrangements and speciation
  • Four types of chromosomal rearrangement: deletion, duplication, inversion, and translocation
  • Fluorescent in situ hybridization can detect large chromosomal rearrangements using spectral karyotyping with probes specific for two different chromosomes showing chromosomal translocation
  • Phenotypic and genetic effects of deletions
    • homozygosity for deletions is often lethal or harmful depending on size of deletions and affected genes
    • deletion in heterozygotes can have a mutant phenotype due to gene dosage effects (haploinsufficiency)
    • deletion in heterozygotes increases risk of altered phenotype due to mutation in the other copy of the gene, may uncover recessive mutant alleles
  • Transposable elements (TEs) are movable genetic elements
    • TEs are any segment of DNA that has the ability to move from place to place within a genome
    • Marcus Rhoades (1930s) and Barbara McClintock (1950s) inferred existence of TEs from genetic studies of corn
    • TEs have now been found in all organisms with some functions beneficial to the host, can be present in hundreds of thousands of copies per genome
  • Instability of TE assiciated alleles
    • a nonautonomous TE (Ds) inserted into gene C and disrupted its function. In kernals that have autonomous TE (AC), Ds can hop out, restoring gene function
  • Structure of DNA transposons
    • Most DNA transposons contain inverted repeats (IRs) of 10-200 bp long at each end, gene encoding trasnposase recocgnizes IRs and cuts at border between the IR and genomic DNA
  • TEs can disrupt genes and alter genomes
    • TE insertion can result in altered phenotype. TE can insert within coding region of a gene or insert near a gene and affect its expression. TE-associated alleles can be unstable
    • TEs can trigger spontaneous chromosomal rearragnements due to unequal crossing over between TEs
    • Gene relocation due to transposition, formation of composite TE
  • Aneuploidy is loss or gain of one or more chromosomes
    • Aneuploids: individuals whose chromosome number is not an exact multiple of the diploid number for that species
    • Euploidy: 2n
    • Nullisomy: 2n-2
    • Monosomy: 2n-1
    • Trisomy: 2n+1
  • Aneuploidy usually results in multiple abnormalities or lethality
    • autosomal monosomy is usually lethal
    • most trisomies are lethal; a few result in characteristic syndromes
    • sex chromosome aneuploidy is tolerated due to X chromosome inactivation
    • Trisomic 21: down syndrome
    • Trisomic 13: Patau syndrome
    • Trisomic 18: Edwards syndrome
    • XO, monosomic: turner syndrome
    • XYY trisomic, normal
    • XXY: trisomic Klinefelter syndrome
    • XXYY, XXXY: tetrasomic Klinefelter syndrome
    • XXXXY: pentasomic Klinefelter syndrome
    • XXXXXY: hexasomic Klinefelter syndrome
  • Aneuploidy is caused by nondisjunction
    • nondisjunction: failure of chromosomes to segregate normally, can occur during meiosis I or meiosis II
  • Some euploid species are not diploid
    • basic chromosome number (number of chromosomes in a single, complete set = x
    • monoploidy and polyploidy are rarely observed in animals, except ants, bees, hermaphroditic worms, and some fish
  • Formation of a triploid (3x) organism
    • diploid gametes may arise from a 4x parent or from a diploid with defects in meiosis (defect in spindle or at cytokinesis)
  • Meiosis in a triploid organism
    • regardless of how chromosomes pair, there is no way to ensure that gametes contain a complete, balanced set of chromosomes. all polyploids with odd numbers of chromosome sets are sterile because they cannot produce balanced gametes.
  • Generation of tetraploid (4x) cells
    • tetraploid cells occur during mitosis in a diploid when chromosomes fail to separate into two daughter cells
    • if tetraploidy occurs in gamete precursors, diploid gametes are produced
    • union of two diploid gates produces a tetraploid organism
    • autopolyploid: all chromosome sets are derived from the same species
  • In tetraploid, pairing of chromosomes as bivalents generates balanced gametes
    • four copies of each group of homologs pair two by two to form two bivalents
    • successful tetraploids produce balanced 2X gametes and are fertile
  • Polyploids in agriculture
    • 1/3 of all known flowering plant species are polyploid
    • polyploidy often results in increased size and vigor in plants
    • many polyploid plants have been selected for agricultural cultivation
    • tetraploids: alfalfa, coffee, peanuts, Macintosh apples, Bartlett pear
    • octaploids: strawberries
    • Euploid: a complete set of chromosomes (usually diploid)
    • Polyploid: a euploid species that carries 3+ complete sets of chromosomes
    • Autopolyploid: a kind of polyploid that derives all its chromosome sets from the same species
    • Allopolyploid: hybrids in which chromosome sets come from distict, but related, species. Usually infertile ebcause different chromosome sets cannot easily pair and segregate properly
    • Amphidiploid: two diploid genomes, each from a different parental species
    • Raphanobrassica: sterile F1 from crossing cabbages and radishes, has 18 chromosomes (9 from each parent)
  • Creation of the allopolyploid Triticale
    • F1 hybrid of wheat and rye, sterile because there are no pairing partners for the rye chromosomes
    • fertile Triticale can be created from infertile F1 hybrid Triticale
    • some triticale hybrids combine high yield of wheat with ability of rye to grow in unfavorable environments
    • some triticale hybrids combine high level of protein from wheat and high level of lysine from rye
  • Colchicine treatment prevents spindle formation and results in doubling of chromosome numbers
  • Duplicate genes may acquire new functions
    • gene families: sets of related genes with slightly different functions
    • genome in common ancestor of all cereal grasses was duplicated. The two copies are related, but have diverged in sequence and function
  • Rapid chromosomal evolution in house mice on the island of Madeira
    • one population of mice introduced to island in 1400s; two populations evolved different sets of Robertsonian translocations, hybrid offspring are sterile