Beyond  Dominant  &  Recessive  Alleles

Incomplete  Dominance  &  Codominance

Beyond  Dominant  and  Recessive  Alleles

•Some  alleles  are  neither  dominant  nor  recessive,  and  many  traits  are  controlled  by  multiple  alleles  or  multiple  genes.

Incomplete  Dominance

SIncomplete  dominance  -­‐ when  one  allele  is  not  completely  dominant over  another.SHeterozygous  genotype  =  BLEND

SSince  one  allele  is  not  completely  dominant  over  the  other  allele,  we  use  two  different  capital  letters  to  represent  the  alleles

Incomplete  Dominance

•Example:    A  cross  between  red  (RR)  and  white  (WW)  snapdragons  produces  pink  flowers  (RW).

Homozygous

RR

(Red)

Heterozygous

RW

(Pink)

Homozygous

WW

(White)

Codominance

SCodominance -­‐ both  alleles contribute  to  the  phenotype.SHeterozygous  genotype  =  both  traits  appear

SSince  one  allele  is  not  completely  dominant  over  the  other  allele,  and  both  traits  appear  together,  we  use  two  different  capital  letters  to  represent  the  alleles

Codominance

•Example:    A  cross  between  black  (BB)  and  white  (WW)  chickens  produces  chickens  speckled  with  black  and  white  feathers  (BW).  

Multiple  Alleles

•Multiple  alleles  -­‐genes  that  are  controlled  by  more  than  two  alleles.•An  individual  can’t  have  more  than  two  alleles  (Remember  you  get  one  copy  from  each  parent).  •However,  more  than  two  possible  alleles  can  exist  in  a  population.  

•Example:    A  human’s  blood  type  is  determined  by  a  single  gene  that  has  three  different  alleles.

Polygenic  Traits

•Polygenic  traits  -­‐traits  controlled  by  two  or  more  genes.• Skin  color  in  humans  is  a  polygenic  trait  controlled  by  more  than  four  different  genes.

Whiteboard  Practice

• Step  1:    Incomplete  Dominance  or  Codominance??????• Step  2:    You  will  be  given  the  3  phenotypes,  Write  the  corresponding  genotypes.•2  alleles  (1  from  mom,  1  from  dad)•Use  the  1st letter  for  each  trait•Both  letters  are  capitol

• Step  3:    Set  up    and  fill-­‐in  a  punnett  square.    Calculate  percent  probability  for  ALL  3  TRAITS!!!

Let’s  do  the  1st one  together  J

SBirds  can  be  blue,  white,  or  white  with  blue-­‐tipped  feathers

S1.    Incomplete  Dominance  or  Codominance?S2.    Genotypes:blue-­‐white  –blue-­‐tipped  –

3.    Show  a  cross  between  a  white  and  blue-­‐tipped  bird

Practice  Identifying

SA  cat  can  be  black,  tan,  or  tabby  (black  &  tan)

S1.    Incomplete  Dominance  or  Codominance?S2.    Genotypes:black-­‐tan  –tabby  –

3.    Show  a  cross  between  a  heterozygous  tabby  and  tan  cat

Practice  Identifying

SA  Who  can  have  curly  hair,  spiked  hair,  or  wavy

S1.    Incomplete  Dominance  or  Codominance?S2.    Genotypes:curly-­‐spiked  –wavy-­‐

3.    Show  a  cross  between  a  curly  and  wavy  Who

Practice  Identifying

SA  sneech can  be  tall,  short,  or  mediumS1.    Incomplete  Dominance  or  Codominance?S2.    Genotypes:tall-­‐short  –medium  –

3.    Show  a  cross  between  two  heterozygous  sneeches

Practice  Identifying

SA  horse  can  be  black,  white,  or  roan  (black  &  white)

S1.    Incomplete  Dominance  or  Codominance?S2.    Genotypes:black  –white  –roan  -­‐

3.    Show  a  cross  between  a  heterozygous  roan  and  black  horse

Practice  Identifying

SFlowers  can  be  white,  pink,  or  redS1.    Incomplete  Dominance  or  Codominance?S2.    Genotypes:white-­‐pink  –red-­‐

3.    Show  a  cross  between  a  white  and  red  flower