1

•  Mendel  iden(fied  (2nd)  Law  of  Independent  Assortment    

o  by  following  two  characters  at  the  same  (me  

•  Crossing  two  true-­‐breeding  parents  differing  in  two  characters    

o  Produces  dihybrids  in  the  F1  genera(on  

u  Heterozygous  for  both  characters  

•  A  dihybrid  cross    

o  determines  whether  two  characters  are  transmiEed  to  offspring  as  a  package  or  independently  

The  Law  of  Independent  Assortment  

EXPERIMENT

RESULTS

P Generation

F1 Generation

Predictions

Gametes

Hypothesis of dependent assortment

YYRR yyrr

YR yr

YyRr

×

Hypothesis of independent assortment

or Predicted offspring of F2 generation

Sperm

Sperm

YR

YR

yr

yr

Yr

YR

yR

Yr

yR yr

YR YYRR

YYRR YyRr

YyRr

YyRr

YyRr

YyRr

YyRr

YYRr

YYRr

YyRR

YyRR

YYrr Yyrr

Yyrr

yyRR yyRr

yyRr yyrr

yyrr

Phenotypic ratio 3:1

Eggs Eggs

Phenotypic ratio 9:3:3:1

1/2 1/2

1/2

1/2

1/4

yr

1/4 1/4

1/4 1/4

1/4

1/4

1/4

1/4 3/4

9/16 3/16 3/16 1/16

Phenotypic ratio approximately 9:3:3:1 315 108 101 32

•  Law  of  independent  assortment  

o  Each  pair  of  alleles  segregates  

independently    

u  of  any  other  pair  of  

alleles  during  gamete  

forma(on  

o  Applies  only  to  genes  on  

different,  nonhomologous  

chromosomes  

•  Genes  located  near  each  other  on  

the  same  chromosome  tend  to  be  

inherited  together  

The  Law  of  Independent  Assortment  

The  laws  of  probability  govern  Mendelian  inheritance  

•  Mendel’s  laws  of  segrega(on  and  independent  assortment    

o  Reflected  in  the  rules  of  probability  

•  When  tossing  a  coin  

o  Outcome  of  one  toss  has  no  impact  on  the  outcome  of  the  next  toss  

•  In  the  same  way  

o  Alleles  of  one  gene  segregate  into  gametes  independently  of  another  

gene’s  alleles  

2

Rr Rr × Segregation of

alleles into eggs

Sperm

R

R R R

R

R r r r

r

r

r 1/2

1/2

1/2

1/2

Segregation of alleles into sperm

Eggs 1/4 1/4

1/4 1/4

•  Rule  of  mul=plica=on  states    

o  The  probability  that  two  or  more  independent  events  will  occur  together    

u  Is  the  product  of  their  individual  probabili(es  

u  P  one(XY)  =  P(X)  *  P(Y)  

•  Probability  in  an  F1  monohybrid  cross  can  be  determined    

o  Using  the  mul(plica(on  rule    

•  Segrega(on  in  a  heterozygous  plant  is  like  flipping  a  coin:  

o  Each  gamete  has  a  chance  of  carrying  the  dominant  allele  and  a        chance  of  carrying  the  recessive  allele  

The  Mul=plica=on  and  Addi=on  Rules  

•  Rule  of  addi=on  

o  The  probability  that  any  one  of  two  or  

more  exclusive  events  will  occur    

u  Is  calculated  by  adding  together  

their  individual  probabili(es  

u  Pan(XY)  =  P(XY1)  +  P(XY2)  +  P(XY3  )  +  

P(XYn)…  

•  The  rule  of  addi(on  can  be  used  to  figure  out  

the  probability  that    

o  An  F2  plant  from  a  monohybrid  cross    

u  will  be  heterozygous  rather  than  

homozygous  

The  Mul=plica=on  and  Addi=on  Rules  

Solving  Gene=cs  Problems  with  the  Rules  of  Probability  •  Predic(ng  the  outcome  of  crosses  involving  mul(ple  characters  

o  Apply  the  mul(plica(on  and  addi(on  rules  

o  Dihybrid  (or  greater)  cross    

u  Equivalent  to  two  or  more  independent  monohybrid  crosses    

q  Occurring  simultaneously  

•  Calcula(ng  the  chances  for  various  genotypes  

o  Considered  each  character  separately  

u  Mul(ply  individual  probabili(es  together  

3

Complex  Inheritance  PaCerns  

•  Other  paEerns  of  inheritance  

o  Rela(onship  between  genotype  and  phenotype    

u  Rarely  as  simple  as  in  the  pea  plant  characters  Mendel  studied  

o  Many  heritable  characters  are  not  determined  by  only  one  gene  with  

two  alleles  

u  However,  the  basic  principles  of  segrega(on  and  independent  

assortment  apply    

q  Even  to  more  complex  paEerns  of  inheritance  

Extending  Mendelian  Gene=cs  for  a  Single  Gene  

•  Inheritance  of  characters  by  a  single  gene    

o  may  deviate  from  simple  Mendelian  paEerns  in  the  following  situa(ons:  

u  When  alleles  are  not  completely  dominant  or  recessive  

u  Incomplete  dominance  

u  Codominance  

u  When  a  gene  has  more  than  two  alleles  

u  Mul(ple  alleles  

u  When  a  gene  produces  mul(ple  phenotypes  

u  Pleiotropy  

Degrees  of  Dominance    

•  Complete  dominance  

o  Phenotypes  of  the  heterozygote  and  dominant  homozygote  are  

iden(cal  

•  Incomplete  dominance  

o  Intermediate  phenotype    

u  Somewhere  between  the  phenotypes  of  the  two  parental  varie(es  

•  Codominance  

o  Two  dominant  alleles  affect  the  phenotype    

u  In  separate,  dis(nguishable  ways  

4

Fig. 14-10-1

Red

P Generation

Gametes

White CRCR CWCW

CR CW

Fig. 14-10-2

Red

P Generation

Gametes

White CRCR CWCW

CR CW

F1 Generation Pink CRCW

CR CW Gametes 1/2 1/2

Fig. 14-10-3

Red

P Generation

Gametes

White CRCR CWCW

CR CW

F1 Generation Pink CRCW

CR CW Gametes 1/2 1/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR CRCW

CRCW CWCW

1/2 1/2

1/2

1/2

5

•  A  dominant  allele  does  not  subdue  a  recessive  allele  

o  Alleles  don’t  interact  

o  Protein  products  of  both  (typically)  expressed  

o  Ac(vity  of  dominant  protein  masks  ac(vity  of  recessive  protein  

•  Alleles    

o  Are  simply  varia(ons  in  a  gene’s  nucleo(de  sequence  

•  For  any  character  

o  Dominance/recessiveness  rela(onships  of  alleles    

u  Depend  on  the  level  at  which  we  examine  the  phenotype  

The  Rela=on  Between  Dominance  and  Phenotype  

•  Tay-­‐Sachs  disease    

o  Fatal  

u  A  dysfunc(onal  enzyme  causes  an  accumula(on  of  lipids  in  the  brain  

o  At  the  organismal  level  

u  The  allele  is  recessive  

o  At  the  biochemical  level  

u  The  phenotype  (i.e.,  the  enzyme  ac(vity  level)  is  incompletely  dominant  

o  At  the  molecular  level  

u  The  alleles  are  codominant  

The  Rela=on  Between  Dominance  and  Phenotype  

•  Dominant  alleles  

o  Not  necessarily  more  common  in  popula(ons  than  recessive  alleles  

•  For  example  

o  One  baby  out  of  400  in  the  United  States  is  born  with  extra  fingers  or  toes  

u  Dominant  to  the  allele  for  the  more  common  trait  of  five  digits  per  

appendage  

•  In  this  example,  the  recessive  allele  is  far  more  prevalent    

o  Than  the  popula(on’s  dominant  allele  

Frequency  of  Dominant  Alleles  

6

Mul=ple  Alleles  •  Most  genes    

o  Exist  in  popula(ons  in  more  than  two  allelic  

forms  

o  ABO  blood  group    

u  determined  by  three  alleles  for  the  

enzyme  (I)  that  aEaches  A  or  B  

carbohydrates  to  red  blood  cells  

q  IA,  IB,  and  I  

u  Enzyme  encoded  by  IA  allele  adds  the  A    

u  Enzyme  encoded  by  IB  allele  adds  the  B  

u  Enzyme  encoded  by  i  allele  adds  neither  

IA

IB

i

A B

none (a) The three alleles for the ABO blood groups and their associated carbohydrates

Allele Carbohydrate

Genotype Red blood cell

appearance Phenotype

(blood group)

IAIA or IA i A

B IBIB or IB i

IAIB AB

ii O

(b) Blood group genotypes and phenotypes

Pleiotropy  •  Pleiotropy    

o  One  gene  with  mul(ple  phenotypic  effects  

o  Pleiotropic  alleles  are  responsible  for  the  mul(ple  symptoms  of  certain  hereditary  diseases  

u  Such  as  cys(c  fibrosis  and  sickle-­‐cell  disease  

Clumping of cells and clogging of

small blood vessels

Pneumonia and other infections

Accumulation of sickled cells in spleen

Pain and fever

Rheumatism

Heart failure

Damage to other organs

Brain damage

Spleen damage

Kidney failure

Anemia

Paralysis Impaired mental

function

Physical weakness

Breakdown of red blood cells

Individual homozygous for sickle-cell allele

Sickle cells

Sickle-cell (abnormal) hemoglobin

Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped

Epistasis  •  Epistasis  

o  A  gene  at  one  locus  alters  the  phenotypic  expression  of  a  gene  at  a  second  locus  

q  In  mice  and  many  other  mammals,  coat  color  depends  on  two  genes  

q  One  gene  determines  the  pigment  color    

²  With  alleles  B  for  black  and  b  for  brown  

q  The  other  gene  determines  whether  the  pigment  will  be  deposited  in  the  hair    

²  With  alleles  C  for  color  and  c  for  no  color  

BbCc BbCc

Sperm

Eggs BC bC Bc bc

BC

bC

Bc

bc

BBCC

1/4 1/4 1/4 1/4

1/4

1/4

1/4

1/4

BbCC BBCc BbCc

BbCC bbCC BbCc bbCc

BBCc BbCc

BbCc bbCc

BBcc Bbcc

Bbcc bbcc

9 : 3 : 4

×

7

Polygenic  Inheritance  

•  Polygenic  Inheritance  

•  An  addi(ve  effect  of  two  or  

more  genes    

•  on  a  single  phenotype  

•  Quan(ta(ve  characters  

o  Those  that  vary  in  the  

popula(on  along  a  

con(nuum  

o  Skin/eye  color,  height  in  

humans  

Eggs

Sperm

Phenotypes: Number of dark-skin alleles: 0 1 2 3 4 5 6

1/64 6/64 15/64 20/64 15/64 6/64 1/64

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8

AaBbCc AaBbCc

×

The  Environmental  Impact  on  Phenotype  •  Phenotype  for  a  character  may  depend  on  environment  as  well  as  genotype  

o  Norm  of  reac=on  

u  Phenotypic  range  of  a  genotype  influenced  by  the  environment  

•  For  example  

o  Hydrangea  flowers  of  the  same  genotype    

u  Range  from  blue-­‐violet  to  pink  

q  Depending  on  soil  acidity  

•  Norms  of  reac(on  

o  Generally  broadest  for  polygenic  

characters  

•  Such  characters  are  called  

mul=factorial    

o  Because  gene(c  and  

environmental  factors  

collec(vely  influence  phenotype  

The  Environmental  Impact  on  Phenotype  

8

Pedigree  Analysis  •  Pedigree  

o  A  family  tree  that  describes  the  interrela(onships  of  parents  and  children  across  genera(ons  

•  Inheritance  paEerns  of  par(cular  traits    

o  Can  be  traced  and  described  using  pedigrees  

Queen Victoria

Albert

Alice Louis

Alexandra Czar Nicholas II of Russia

Alexis

You  should  now  be  able  to:  

1.  Define  the  following  terms:  true  breeding,  hybridiza(on,  monohybrid  cross,  

P  genera(on,  F1  genera(on,  F2  genera(on  

2.  Dis(nguish  between  the  following  pairs  of  terms:  dominant  and  recessive;  

heterozygous  and  homozygous;  genotype  and  phenotype    

3.  Use  a  PunneE  square  to  predict  the  results  of  a  cross  and  to  state  the  

phenotypic  and  genotypic  ra(os  of  the  F2  genera(on    

4.  Explain  how  phenotypic  expression  in  the  heterozygote  differs  with  

complete  dominance,  incomplete  dominance,  and  codominance  

5.  Define  and  give  examples  of  pleiotropy,  epistasis,  and  polygenic  inheritance  

6.  Understand  and  be  able  to  interpret  a  pedigree