Download Extensions of Mendel`s Rules

Extensions of Mendel’s Rules
Incomplete dominance
Codominance
Pleiotropy
Environmental effects
Mendel was lucky
• Simple traits - only two phenotypes for
each; complete dominance relationships
• Discrete traits - they were polar (either,
or; yes, no)
• Independent - genes were on different
chromosomes (or very far apart on same)
Beyond Mendel
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Incomplete dominance
Codominance
Gene Interactions (Epistasis)
Quantitative traits
Pedigrees & diseases
Incomplete dominance
Incomplete dominance
• All F1 (heterozygotes)
have intermediate
phenotype
• Parental phenotype
reappears in F2
• Phenotype ratio 1:2:1
• How many phenotypes
do you expect?*
Incomplete
• Each allele contributes to creating at
intermediate phenotype
Codominance
• Heterozygotes have phenotypes of BOTH
parents
– Ex: MN cell membrane proteins
• Each allele contributes to produce a
multi-part phenotype
x
NN
MM
F1
MN
P
Dominance relationships
• Complete & incomplete dominance associated with genes that code for
protein activity or regulation of their
production; different alleles may code for
hyperactivity, no activity, etc.
• Codominance - associated with structural
proteins so heterozygotes have both types
in their cells; both types expressed
• Interactions are complex
Multiple alleles
• More than two alleles for a single gene
– How many alleles per trait did Mendel have?
– β-globin gene in humans (makes hemoglobin,
responsible for O2 transport) has ~ 500
alleles.
• Different combinations produce different
phenotypes (reduced O2 carrying capacity, parasite
resistance, higher affinity for O2)
– Polymorphic: different combinations of alleles
produce more than 2 phenotypes
ABO Blood Group
ABO Blood Group
Multiallelic, polymorphic, and
completely dominant or
codominant
Pleiotropy
• When single genes affect more than one
trait they are said to be pleiotropic
– Marfan syndrome: caused by mutation in
single gene
– Produces multiple phenotypes:
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increased height
disproportionately long limbs & digits
Improper backbone formation
abnormal chest shape
Both Physical & Genetic
environment influence phenotype
• Physical environmental effects
– Individuals with the same genes that
experience different environments have
different phenotypes (twins)
– Health status divergence
• Epistatic effects (genetic environment)
– Presence of other genes influences final
phenotype
Physical environmental
• PKU = inability to break down AA
phenylalanine. It (and its acid) accumulate
and prevent normal CNS development
– 1/15,000 births on average
– If affected person consumes a diet low in
phenylalanine, then they develop normally
– *Mere presence of gene DOES NOT
determine phenotype
Epistasis (genetic environment)
X
9
3
4
Incomplete dominance?
Epistasis (two genes interact)
B = Black
b = brown
C = color
c = no color
Continuous (quantitative)
traits
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Often influenced by many genes
Produce gradient of phenotypes instead of discrete*
Often normally distributed
Ex: human height, skin color
Discrete vs. continuous
Normal distribution
Wheat kernel color
• Ah, a normal distribution
• Is color probably
controlled by a single gene
or multiple?
– Additive effects of genes
• Blending inheritance??*
Trait shows polygenic
inheritance
• 3 genes
• Polygenic inheritance contributes small amount
to value of phenotype
• Homozygous dominant =
lots o’ color
• Heterozygous = medium
color
• Homozygous recessive =
no color