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BIOL 239 - Pre-Midterm Notes

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University of Waterloo
BIOL 239
Christine Dupont

BIOL  239  –  Genetics     Topic  1:  Mendelian  Genetics     Genetic  traits  are  passed  on  from  one  generation  to  the  next.     Artificial  Selection:  Purposeful  matings  to  select  for  particular  d esired  traits.   • Ex.  Domesticated  plants  and  animals     Belyaev’s  Domestic  Foxes  (1950s)   • Geneticist  in  Stalinist  Russia  who  claimed  to  be  developing  the  most  luxuriant  fur   • Manipulated  behavior  (nature  vs.  nurture)  by  selecting  for  specific  traits   o In  four  generations,  had  lines  of  foxes  that  were  aggressive  or  friendly   o Evidence  shows  that  they  were  selecting  for  cortisol  levels   o High  cortisol  levels  =  aggressive   o Low  cortisol  levels  =  friendly;  linked  to  different  coat  patterns     Gregor  Mendel  (1822-­‐1884)  is  known  as  the  father  of  genetics.   • Studied  garden  peas,  which  are  easy  to  cross -­‐fertilize  and  are  able  to  self-­‐fertilize   • Pure  breeding  lines:  offspring  carry  parental  traits  that  remain  constant  from  generation  to   generation   • Reciprocal  cross:  exchanged  breeding  be tween  plants  that  consistently  produced  either   white  or  purple  flowers   • Self-­‐fertilization  ensured  that  the  genetic  line  was  pure  by  breeding  the  plant  with  itself     Genotype:  genetic  makeup;  description  of  genetic  information  (genes  or  alleles)  that  are  car ried  by   an  individual     Phenotype:  observable  characteristics;  the  physical/visual  outcome  of  a  genetic  combination   • Largely  determined  by  genotype   • Commonly  referred  to  as  a  trait     Prior  to  Mendel,  there  were  several  theories  that  weren’t  widely  accepted  by   the  scientific   community   • One  parent  (thought  to  be  the  male)  contributes  more  to  an  offspring’s  inherited  feature   o Through  reciprocal  crosses,  Mendel  found  that  it  had  nothing  to  do  with  which   parent  it  came  from  but  whether  a  trait  was  dominant/recessive   • Parental  traits  become  mixed  and  forever  changed  in  the  offspring   o Disproved  through  the  reappearance  of  recessive  traits   Monohybrid  Cross:  matings  between  individuals  that  differ  in  only  one  trait   • Cross  fertilize  parental  pure  breeding  generation   • All  progeny  are  monohybrids  known  as  the  first  filial  (F1)  generation  since  they  are  the   first  outcome  from  the  parental  cross   o All  resemble  one  of  the  parental  strains   • Self-­‐fertilize  the  progeny  (interbreed)  so  that  the  second  filial  (F2)  generation  has  no  new   genes  coming  in  from  other  individuals   • In  the  F2  progeny,  the  lost  trait  disappears;  3:1  ratio     Allele:  an  alternative  form  of  a  single  gene   • Discrete  unit  of  inheritance   • Most  human  traits  are  determined  by  genes  with  multiple  alleles   • Homologous  chromosomes:  same  genes,  alleles  may  be  different     Polymorphic:  a  gene  may  have  several  alleles  that  normally  occur  in  a  population   • Doesn’t  mean  that  a  gene  has  multiple  alleles   • Ex.  blood  type   • A  maximum  of  two  alleles  for  one  gene  can  exist  in  a  diploid  individual     Monomorphic:  some  genes  have  only  one  allele  that  is  normally  present  in  a  population   • Other  alleles  are  mutations  so  the  majority  of  humans  carry  the  non -­‐mutated  (normal)  allele   • Ex.  sickle  cell  anemia,  Huntington’s  disease,  cystic  fibrosis,  hemophilia     Mendel’s  Law  of  Segregation:  the  two  alleles  for  each  trait  separate  during  gamete  formation  and   then  unite  at  random  (one  from  each  parent)  at  fertilization   • Describes  how  alleles  of  one  gene  behave     Monohybrid  Cross:  one  gene,  two  alleles  acting  in  a  simple  domina nt/recessive  manner   • Monohybrid:  two  different  alleles  for  a  single  gene  that  determine  a  single  feature   • 3:1  ratio  for  inheritance  (phenotypic)   • 1:2:1  ratio  for  genotypic  inheritance     Dominant  and  recessive  are  terms  used  to  describe  the  phenotypic  effect  of  different  alleles.   • At  the  DNA  level,  different  alleles  differ  in  nucleotide  sequence,  changing  the  amino  acid   sequence  of  amount  of  protein     Test  cross:  a  cross  against  a  homozygous  recessive  phenotype   • If  crossing  with  homozygous  dominant,  the  offspring  a re  identical   • If  crossing  with  heterozygote,  1:1  ratio  in  the  offspring;  recessive  allele  is  uncovered   Mendel’s  results  reflect  basic  probability  rules:   • Law  of  the  Product:  probability  of  two  or  more  independent  events  occurring  together  is   the  product  of  the  probabilities  that  each  event  will  occur  by  itself   o P(A)  x  P(B)   • Law  of  the  Sum:  the  probability  of  either  of  the  two  mutually  exclusive  events  occurring  is   the  sum  of  their  individual  probabilities   o P(A)  +  P(B)     Dihybrid  Cross:  matings  between  individuals  that  differ  in  two  traits   • 9:3:3:1  ratio  for  phenotype   • 1:2:2:4  ratio  for  genotype   • When  the  alleles  act  in  a  simple  dominant/recessive  manner,  new  phenotypic  combinations   appear     Law  of  Independent  Assortment :  describes  how  different  genes  behave   • Deals  with  two  or  more  genes   • During  gamete  formation,  different  pairs  of  alleles  segregate  independently  of  each  other   and  recombine  in  every  possible  way     Multihybrid  Crosses:  matings  between  individuals  that  differ  in  three  or  more  traits   • Break  into  individual  gam etes  and  work  out  the  probability  of  each  combination  occurring   • Use  genotypic  ratios  established  for  heterozygous  crosses     Topic  2:  Mendelian  Extensions     Incomplete  Dominance:  F1  hybrid  resembles  neither  purebred  parent   • Often  an  intermediate  phenotype   • 1:2:1  ratio  for  genotype  and  phenotype   • ex.  in  snapdragons,  A  produces  red  pigment,  A  produces  white  pigment  and  A A  is  pink  in   colour     Codominance:  alternative  traits  are  both  visible  in  the  F1  hybrid   • Often  seen  as  spots  or  stripes  in  animals  and  plants   • 1:2:1  ratio  for  genotype  and  phenotype   • Neither  allele  is  dominant  or  recessive  to  each  other   • ex.  in  blood  typing,  I  and  I  are  codominant  to  each  other  but  dominant  to  i     Reciprocal  crosses:  conducted  between  pure-­‐breeding  lines  representing  all  phenotypes  t o   establish  the  dominance  relationships  between  all  possible  pairs  of  alleles   • Reveals  a  dominance  series   • When  dealing  with  multiple  alleles,  wild  type  is  designated  by    and  all  others  are  mutant   alleles     New  alleles  arise  through   mutation   • Chance  alterations  in  genetic  material  arise  spontaneously  in  nature   • Different  sites  of  mutation  create  different  alleles  but  not  necessarily  different  phenotypes     Allele  Frequency:  the  percentage  of  the  total  number  of  copies  of  a  gene  in  a  population  represented   by  a  particular  allele   • Wild-­‐type  allele:  any  allele  found  at  a  frequency  in  the  population  >1%   • Mutant  allele:  any  allele  found  at  a  frequency  in  the  population  <1%   • Note:  this  is  not  how  it  manifests  itself  in  the  population  (phenotype)  but  how  it  is  carried   (genotype)   • Monomorphic:  only  one  allele  is  found  at  a  frequency  of  >1%  (wild-­‐type)   • Polymorphic:  gene  with  more  than  one  wild-­‐type  allele     Pleiotrophy:  multiple  phenotypic  effects  caused  by  a  single  gene   • Some  alleles  may  result  not  only  in  a  visible  phenotype,   but  may  affect  viability  (lethal   genes)     Multifactoral  Inheritance:   a  phenotype  arising  from  the  action  of  two  or  more  genes  ( polygenic)  or   from  interactions  between  genes  and  the  environment   • Most  common  traits  are  determined  by  more  than  one  gene  (ex.  eye  colour)   • New  phenotypes  can  emerge  from  the  combined  action  of  the  alleles  of  two  genes   • 9:3:3:1  ratio  indicates  that  there  are  two  genes  acting  on  the  trait     Complementary  Gene  Action :  two  or  more  genes  can  work  in  tandem  in  the  same  biochemical   pathway  to  produce  a  particular  trait     Heterogenous  Trait:  a  mutation  at  any  one  of  a  number  of  genes  can  give  rise  to  the  same   phenotype   • “Two  wrongs  make  a  right,”  in  that  the  wild -­‐type  phenotype  can  be  rescued  by   complementation   • 9:3:3:1  ratio  or  a  variation  of  it   • Complementation  testing  takes  true-­‐breeding  lines  of  test  subjects  showing  the  same   mutant  phenotype  and  tries  to  get  the  wild  type  back   • If  mutations  are  in  the  same  gene,  no  complementation     Epistasis:  a  gene  interaction  in  which  the  effects  of  an  allele   at  one  gene  hide  the  effects  of  alleles  at   another  gene   • 9:3:4  phenotypic  ratio  indicates  recessive  epistasis   • 12:3:1  phenotypic  ratio  indicates  dominant  epistasis     Penetrance:  percentage  of  the  population  with  a  particular  genotype  that  demonstrates  the   expected  trait   • Incomplete  penetrance:  everyone  has  the  same  genotype  for  a  particular  trait,  yet  some   show  it  while  others  do  not     Expressivity:  the  degree  or  intensity  with  which  a  particular  genotype  is  expressed  within  a   population   • Variable  expression:  expressed  by  everyone,  but  to  different  degrees     Conditional  Lethality:  an  allele  which  is  lethal  only  manifests  under  certain   conditions/environmental  circumstances   • Permissive  Conditions:  normal,  allow  life   • Restrictive  Conditions :  lethal  alleles  manifest  themselves,  restricting  life     Chance:  occurrences  in  the  lives  of  individuals  can  influence  the  expression  of  some  alleles   • ex.  exposure  to  carcinogens,  radiation     Topic  3:  Pedigree  Analysis     Many  human  traits  run  in  families  and  do  not  show  Mendelian  patterns  of  inheritance   • This  is  because  most  traits  are  influenced  by  more  than  one  gene  (multifactorial)   • Studying  pedigrees  gives  insight  as  to  how  mutant  alleles  causing  abnormalities  are   inherited     Sickle  Cell  Anemia   • Single  nucleotide  substitution  results  in  va line  production  instead  of  glutamic  acid   • Results  in  a  change  in  conformation  of   ß-­‐globin  protein  when  deoxygenated   • Pleiotrophic:  may  result  in  anemia,  blocked  circulation,  increased  malaria  resistance   • Recessive  trait  under  normal  conditions  only     Tay-­‐Sachs  Disease   • Missing  enzyme  leads  to  a  build -­‐up  of  fatty  deposit  in  the  brain  that  destroys  nervous   development   • Manifests  in  children  before  the  age  of  5,  resulting  in  blindness,  paralysis,  mental  retardati     Phenylketonuria  (PKU)   • Mutation  in  enzyme  in  metabo lic  pathway  causes  phenylalanine  buildup   o Converted  to  phenylpyruvic  acid,  interfering  with  early  nervous  system   development   • Manifests  in  children     Albinism   • Recessive  trait   • Missing  enzyme  in  pathway  to  melanin  production  results  in  unpigmented  hair,  skin,  e yes     Huntington’s  Disease   • Dominant  trait   • Mutation  caused  by  extra  nucleotide  repeats  CAG  in  the  gene  add  extra  glutamine  to  protein   o Variable  expression;  more  repeats  =  earlier,  more  severe  symptoms   • Causes  progressive  mental  and  neurological  damage     Cystic  Fibrosis   • Recessive  allele   -­‐ • Mutations  in  protein  responsible  for  Cl  regulation   • Cells  don’t  produce  normal  amounts  of  water,  leading  to  accumulation  of  mucus  in  lungs  and   digestive  organs   o Leads  to  difficulty  breathing,  pneumonia,  digestive  malfunctions     Pedigree  of  a  Dominant  Trait   • Appears  frequently  in  each  generation   • Most  are  detrimental     Pedigree  of  a  Recessive  Trait   • Appears  sporadically,  depending  on  the  mate  being  homozygous  or  heterozygous  for  the   recessive  allele   • Not  seen  in  every  generation     Belgian  Blue   • Mutation  in  myostatin  causes  uncontrolled  muscle  growth   • Incomplete  dominance  in  heterozygotes,  who  have  more  muscle  than  normal  but  not  quite   “double  muscle”     Autosomal  Trait:  conferred  by  a  gene  residing  on  a  chromosome  not  involved  in  sex  determinati on     Sex-­‐Linked  Trait:  conferred  by  a  gene  residing  on  the  X  or  Y  chromosome     Sex-­‐Limited  Traits:  affect  a  structure  or  process  that  is  found  in  one  sex  but  not  the  other   • ex.  bright  plumage  in  male  birds,  milk  production,  horns/antlers     Sex-­‐Influenced  Traits:  show  up  in  both  sexes  but  their  expression  may  differ  between  the  two  sexes   • ex.  patterned  baldness     Set  4:  Chromosomes     Chromosomal  Theory  of  Inheritance:  hereditary  information  is  on  genes,  which  are  located  on   chromosomes   • Egg  and  sperm  contribute  equa lly  to  genetic  endowment  of  offspring  through  fusion  of  their   nuclei     The  X  chromosome  does  more  than  just  determine  sex   • Contains  genes  that  are  essential  for  life  and  metabolic  processes   • Y  chromosome  is  thought  to  be  derived  from  the  X  chromosome     In  humans,  sex  is  determined  by  the  presence  (or  absence)  of  a  Y  chromosome   • Males  =  XY   • Females  =  XX     In  Drosophila,  sex  is  determined  by  the  ratio  of  X  chromosomes  to  autosomes   • XX  =  2X  :  2  autosomes  =  female   • XY:  1X  :  2  autosomes  =  male     Some  species  use  the  ZW  system,  wherein  sex  is  determined  by  environmental  factors:  temperature,   size,  and  dominant  behavior     How  are  genes  transmitted  from  organism  to  organism,  and  how  do  their  numbers  remain   consistent?   • Mitosis:  nuclear  division  that  results  i n  two  daughter  cells  each  containing  identical   numbers  of  chromosomes  to  the  parent  cell   • Meiosis:  nuclear  division  that  results  in  each  egg  and  sperm  containing  half  the  number  of   chromosomes  found  in  other  somatic  cells   o Involved  in  species  propagation     Somatic  Cells:  non-­‐gamete  cells  that  are  always  diploid  and  undergo  mitosis   • Gametes  are  the  only  haploid  cells;  undergo  meiosis     Chromatid:  one  double-­‐stranded  DNA  molecule  complexed  with  protein   • Each  chromosome  is  made  up  of  one  chromatid,  except  for  metaphase  when  there  are   two     Metaphase  Chromosomes   • Condensed  due  to  DNA  supercoiling  and  association  with  histone  proteins,  so  hat  they  are   visible  under  a  microscope  in  a  characteristic  X  shape   • Duplicated  chromatids  (2/chromosome)     Sister  Chromatids:  identical  gene  and  allele  sequences     Non-­‐Sister  Chromatids:  same  genes,  possibly  different  alleles   Karyotype:  digital  arrangement  of  chromosomes  in  a  cell  going  through  meiosis   • Always  uses  metaphase  chromosomes   • Arranged  from  largest  to  smallest,  with  centromere  position  as  secondary   arrangement   o Metacentric  chromosomes:  have  the  centromere  in  the  middle   o Acrocentric  chromosomes :  have  the  centromere  somewhere  other  than  the  center   • Can  reveal  abnormalities  in  chromosome  number   • Treats  X  and  Y  as  a  pair,  even  though  they  don’t  have  the  same  genes     Topic  5:  Mitosis     Mitosis:  the  process  of  nuclear  division  in  cells  that  produces  genetically  identical  daughter  cells   • Happens  with  somatic  cells  to  grow  and  replace  them     Chromatin:  the  complex  of  DNA  and  protein  found  in  a  cell’s  nucleus     G1  Phase:  period  in  the  cell  cycle  when  cells  are  actively  producing  many  of  the  products  specific  to   their  biological  role  in  the  body   • b   • Varies  in  length  depending  on  cell  type   • Some  cells  have  this  as  their  only  phase;  called  G0  phase     Synthesis  (S)  Phase:  synthesis  or  duplication  of  chromosomes  (DNA)  to  produce  identical  sister   chromatids   • Sister  chromatids  remain  joined  at  the  centromere     G2  Phase:  cell  synthesizes  proteins  necessary  for  impending  cell  division  and  mitosis   • Bipartite  chromosomes     Interphase:  the  cell  prepares  for  mitosis  and  cell  division     Prophase:  microtubules  form  the  mitotic  spindle   • Spindle  fiber:  protein  structures  that  eventually  attach  to  the  chromosome   o Centriole:  an  organizing  structure  at  either  pole  that  allows  these  to  radiate  out  in  a   parallel  manner   • Duplicated  chromosomes  condense   • Nuclear  membrane  and  nucleolus  disappear   • Microtubules  of  the  mitotic  spindle  form  the   aster,  which  attaches  to  the  centromere  of   chromosomes     Metaphase:  duplicated  chromosomes  move  to  the  center  of  the  cell  ( metaphase  equatorial  plane )   so  that  each  sister  chromatid  is  facing  one  po
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