Mendelian Genetics Part 2

1
Mendelian Genetics Part 2

• Incomplete dominance, Codominance, Multiple
  alleles and Dihybrid crosses.

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Incomplete dominance

• Neither trait will take dominance and the traits
  will blend.
• Homozygous black (bb) trait shown will be black
  fur.
• Homozygous white (ww) trait shown will be white
  fur.
• Heterozygous (bw) trait will be gray fur.
• Sometimes the genotypes are capitals.

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Genotypes and Phenotypes for Incomplete Dominance.

• Three genotypes possible.
• bbbwww
• Three phenotypes possible as well
• BlackGrayWhite

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Incomplete dominance
c
w
cc cw
cw ww
c
w
cc

• Genotypic ratio would be 1cc2cw1ww
• Phenotypic ratio would also be
• 1chestnut2 palomino1white

cw
ww
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Codominance

• Both traits will take dominance and the traits
  will both show.
• Homozygous red (RR) trait shown will be red
  hair.
• Homozygous white (WW) trait shown will be white
  hair.
• Heterozygous (RW) trait will be red and white
  hair.

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Genotypes and Phenotypes for Codominance

• Three genotypes possible.
• RRRWWW
• Three phenotypes possible as well
• RedRed and WhiteWhite
• A red and white haired horse is called a roan

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Codominance
R
Y
RR RY
RY YY
R
Y

• Genotypic ratio would be 1RR2RY1YY
• Phenotypic ratio would also be
• 1 Red2 Red Yellow1Yellow

8
Multiple Alleles

• Several alleles can be present for the gene
  locus.
• Only two are carried at a time!
• Human hair color is a good example.
• Responsible for the several variations that we
  see in humans.

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Multiple Alleles

• Human hair color has three alleles
• B Brown
• L Blonde (yellow)
• R Red

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Hair genotype and phenotype

• 6 possible genotypes give 6 different phenotypes.
• BB brunette
• LL blonde
• RR red
• LR carrot
• LB light brown
• RB auburn

carrot
brunettes
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Crossing the hair types.

• A auburn female has children with a carrot male.
• What are the genotype and phenotype of the
  children?
• Possible genotypes are LR, BL, RR, and BR. All
  are 25.
• 0BB0LL1RR1LR1LB1BR
• Possible phenotypes are carrot, light brown,
  red and auburn. All are 25.
• 0Brunette0Blonde1Red1Carrot 1Light
  brown1Auburn

The cross is RB x LR
R B


L R
LR
BL
RR
BR
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Incomplete or Codominance?