Modifications to Mendelian Genetics

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Lecture 2 Modifications to Mendelian
Genetics Complexities in relating phenotypes to
genotypes
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Outlines

• incomplete dominance and codominance
• multiple alleles (human blood types)
• lethal alleles
• epistasis (gene interaction)
• flower color in sweetpea
• coat color in Labrador retrievers
• fruit color in summer squash
• testing genetic models with chi-square test

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Incomplete dominance or codominance

• Crosses between true-breeding strains can
  produce hybrids with phenotypes different from
  both parents (hybrid form)
• F1 hybrids that differ from both parents
  express an intermediate phenotype. Neither
  allele is dominant or recessive to the other
• Phenotype ratios are same as genotype ratio.
• Genotype of F1 121 -------- Phenotype
  121

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Definition of dominance vs recessiveness
Codominance incomplete dominance !
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Snapdragons flower color (incomplete dominance)
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Multiple alleles (human blood type)

• One gene has more than two alleles.
• The human ABO gene has three alleles (IA, IB,
  i).
• Two alleles (IA, IB) are codominant to each
  other, and these two alleles are dominant to the
  i allele.

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Biology of the ABO gene
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No of genotypes?
No of phenotypes?
Transfusion rule
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Human ABO blood type as a paternity test
marker One woman with A type sued another man
with O type for the support of her child with B
type. Is this really a child of that man?
A (IAIA, IAi) x O (i i)
either A (IAIA, IAi) or O (i i)
child possible not B
(IBIB, IBi)
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Lethal alleles (yellow coat mice)

• AA normal color survival
• AAY normal color
• yellow coat
• AYAY die in utero
• -- modified ratio
• 21 at F2 generation
• Dominance vs recessive?

11
Epistasis (gene interaction)

• Interaction of two or more genes at the
  phenotype level to result in a modified F2 ratio
  9331.
• An exception to Mendels principle of
  independent expression
• Transmission of genes from parents to offspring
  follows the independent assortment rule.

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Complementation
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Complementation
AaBb x AaBb
dihybrid cross Use branching diagram
method A gene B
gene
3/4 B_ ------- A_B_ 9/16 purple
3/4 A_
1/4 bb
-------- A_bb 3/16 white ---------gt
3/4
B_ -------- aaB_ 3/16 white
1/4 aa
1/4 bb
-------- aabb 1/16 white
9/16
7/16
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Testcross of complementing two genes
Not the expected 11 ratio!
AaBb x aabb dihybrid
cross 1/4 AaBb
------- A_B_ purple
1/4 Aabb
-------- A_bb white
1/4 aaBb -------- aaB_ white

1/4 aabb -------- aabb white
1/4
3/4
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Recessive epistasis

• coat color in Labrador retrievers

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Recessive epistasis
The recessive allele of the first gene block the
expression of the second gene regardless of the
second genes alleles.
Black BBEE x bbee yellow
F1 BbEe (Black)
F2 9 (Black) 3 (Brown) 4 (Yellow) 9/16 B_E_
Black 3/16 bbE_
brown 3/16 B_ee yellow 1/16 bbee
yellow
E(enzyme) B(enzyme) Precusor (yellow)
-----gt brown pig. -------gt black pig.
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Punnet square explanation
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Homework for the nextweek

• 3-1, 3-2, 3-3, 3-5, 3-7, 3-19, 3-21