CHPTER 9 LECTURE NOTES: CHROMOSOME MUTTION II: CHNGES IN NUMBERS I. berrnt euploidy. Generl info. Euploidy refers to the sitution in which n orgnism hs one complete set of chromosomes or n integer multiple of set.. The number of chromosomes in bsic set clled monoploid number (x).. The norml situtions re x = hploid or x=diploid.. Polyploids (>x) re by definition bnorml lthough they exist in plnts in nture. 5. Hploid number (n) refers only to number of chromosomes in gmetes. For most diploid orgnisms tht we re fmilir with the hploid nd monoploid number is the sme. However, this is not lwys the cse: whet is hexploid contining six sets of similr but not identicl chromosomes with 7 chromosomes per set (x=7 nd 6x = ). The gmetes however hve chromosomes (n= nd n=). x = 7 6x = Type Type Type Type Type 5 Type 6 Type 7 n = n = Meiosis Type Type Type Type Type 5 Type 6 Type 7
. Monoploidy (x = set of chromosomes insted of norml ). Monoploidy cn rise from spontneous development of n unfertilized egg (prthenogenesis).. Lethl in most nimls becuse unmsking of lethl lleles.. Monoploids usully sterile becuse of problem with meiosis: No piring prtner for ech chromosome. Gmetes contining ll the chromosomes in the set re formed t frequency of (/) x-. Exception in nimls re mle bees, wsps, nd nts which develop prthenogeneticlly nd in which gmetes re formed by mitosis 5. Monoploids re importnt in plnt breeding. For exmple: Diploids msk dvntgeous new muttions. Cn use monoploidy to circumvent this: Induced in plnts nthers by tking cells destined to become pollen nd treting with cold will become smll mss of dividing cells embryoid Plte on gr nd grows into plntlet look for desirble trits Use colchicine to redouble chromosomes returning to diploidy in sector of plnt tht cn then be trnsplnted. C. Generl info on polyploidy.. In plnts ) utoploidy (multiple of sets from one species) vs. lloploidy (multiple sets from different species) b) Even # ploidy vs. odd # ploidy: Odd # ploidy lwys hve unpired chromosome for ech chromosome type during meiosis; thus, the probbility of producing geneticlly blnced gmetes is low (see below), nd odd # ploidies re effectively sterile.. In nimls ) Polyploidy is rre. Resons for rreness of polyploidy in nimls is not certin. Hypothesis is the sex-determintion mechnism depends on delicte blnce in chromosomes. lso, since most nimls do not undergo self-fertiliztion, there is no chnce for polyploids to successfully produce progeny tht will be fertile (i.e. n X n = x (sterile). b) In humns, polyploid zygotes usully die in utero (ccount for pprox. 0% of spontneous bortions. c) There re few exmples of polyploids in nimls including fltworms, leeches, brine shrimp, some polyploid frogs nd tods, nd some fish.. Humns hve gint polyploid nuclei in such tissues s liver nd kidneys. Somtic cell polyploidy clled endopolyploidy.
D. Triploids (x = sets of chromosomes insted of norml sets). Usully utopolyploids. Cn rise from spontneous berrnt meiosis in nture to give n gmete tht fuses with norml n gmete to give x zygote OR from tetrploid (x) nd diploid (x) cross.. Usully sterile becuse the frequency of getting exctly set or exctly sets of ll the chromosomes to migrte to one pole is (/) x- Gmetes tht contin intermedite between n nd n exhibit genome imblnce becuse there re different rtios of genes depending upon if the gmete hs or copies of ech chromosome.. Seedless wtermelons nd bnns re triploid. They re sterile, but re propgted vegttively. E. utotetrploids in plnts (x where ll sets ll derived from one species). rise from spontneous berrnt meiosis in nture to give n gmete tht fuses with nother n gmete to give x zygote OR by ccidentl doubling of x genome to x genome in the cell stge of development. Importnt in crop plnts becuse greter # chromosomes = lrger plnt. (lflf, coffee, penuts, McIntosh pples). Becuse there re n even # of chromosome sets, piring of homologous chromosomes llows for norml meiosis.. Single locus segregtion rtios in re different thn simple Mendelin rtios ) With plnt where / is close to the centromere: Gmete rtios of : : Figure on the next pge shows how this is derived:
b) With BBbb plnt where B/b is fr from the centromere, there re mny crossovers between the loci nd the centromere. This effectively uncouples the segregtion of the B/b lleles from the segregtion of the centromere. s such you cn consider the chnces of obtining the following gmetes s : BB (/56) Bb (/56) nd bb (/56) F. llotetrploids ( sets from different species: sets from species # nd sets from species #). Chromosomes re homeologous (similr but not completely homologous). Importnt in plnt breeding: Consider species (n) which you ttempt to cross: n X n zygote with n + n ususlly sterile becuse homeologues cn not pir tret with colchicine n + n now hve llotetrploid (x) (K mphidiploid). In nture, the sme events s () hppen except tht chromosome doubling is rre spontneous event. llotetrploidy ws importnt in the evolution of plnts (cotton, whet).
II. neuploidy. Generl informtion. The sitution in which one or severl chromosomes re lost from or dded to the norml set of chromosomes. Cused by nondisjunction during meiosis to generte n+ or n- gmetes which when fertilized with n gmete to generte n+ or n- zygotes (from n Introduction to Genetic nlysis, 6 th ed. by Griffiths et l. (W. H. Freemn nd Compny). If utosome is ffected, usully lethl in nimls becuse of either chromosome imblnce or unmsking of lethl lleles; exception is the sex chromosomes (see below).
B. Nullsomics. Missing two homologous chromosomes for certin chromosome #. n-. Lethl in humn diploids becuse whole chromosome type hs been deleted. Cn be ok in some polyploid plnts C. Monosomics. Missing chromosome of certin chromosome #. For diploids: n-. In humns, lethl for utosomes (die in utero). Turner syndrome (only X chromosome) Figure 9-7 5. Cn be ok in some plnts in fct cn use monosomic bnks to mp recessive lleles to the chromosomes D. Trisomy. Hve extr chromosome of certin chromosome #. For diploids: n+. sex chromosome trisomies ) Klinefelter s syndrome (XXY) Figure 9-0 b) XYY syndrome (XYY) c) Triplo-X (XXX). utosoml trisomies ) Trisomy (Down s syndrome) Figure 9- b) Trisomy (Ptu syndrome) c) Trisomy 8 (Edwrds syndrome) 5. Cn be ok in some plnts. Single locus segregtion rtios in re different thn simple Mendelin rtios ) With n plnt where / is close to the centromere: Gmete rtios of : : : (from n Introduction to Genetic nlysis, 6 th ed. by Griffiths et l. (W. H. Freemn nd Compny)
E. Disomics berrtion in hploid orgnisms where there is n extr chromosome F. Somtic neuploidy In bove mentioned cses, the chromosome muttions ll rose in the germ cells during meiosis. However, neuploidy cn rise in the somtic tissues during mitosis resulting in genetic mosic of cell types.