Genetics…
What Mendel never knew!
Incomplete dominance
• Book definition: When one allele is not
completely dominant over another allele
• The heterozygous indidvidual produces a
phenotype different than either type of
homozygous (dom/rec)
• Aa looks different than AA or aa
• Results in a “mixture” of phenotypes
• Ex: A new color; not seen in either parent
Look at the F2 generation…what is
the ratio of phenotypes?
Codominance
• Book definition: when both alleles
contribute to the phenotype of the
individual.
• Neither allele “covers up” the other
• In the phenotype both characteristics are
seen…flowers with red AND white petals.
• In the heterozygous individual you will see
both parental phenotypes.
Codominance: Chickens with both
black and white feathers.
Multiple alleles
• Book definition: more than two alleles (>2)
for a given gene determine a trait
• HOWEVER,NO INDIVIDUAL HAS MORE
THAN TWO ALLELES
• For some reason in the population there
exist more than two gene choices
• Weird interactions go on between the
alleles; some are dominant over others,
some are codominant, etc.
Multiple alleles: human blood type
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Three alleles: A, B, & O
A is dominant over O
B is dominant over O
Therefore O is recessive to both A & B
A and B are codominant
Remember: no one person has more than
two of the thee alleles.
Rabbit fur color: multiple alleles
(four alleles)
• C, Cch, Ch, c
Polygenic traits
• Book definition: a trait controlled by more
than one gene pair.
• Not just a pair of alleles (Rr), but at least
two (RrTt) that work together on one
trait.
• These alleles or gene pairs don’t result in
an either/or phenotype, but a range of
phenotypes…the smallest to the biggest.
• Phenotypes of many different forms
• Examples: human height, skin color, etc
Skin color
Human height
Other polygenic traits
• Any trait with a “bell curve”: two extreme
phenotypes and everything in between…
• Birth weight
• Eye color
• Others????
Pleiotropic:
• NOT IN OUR BOOK…
• Definition: when a pair of alleles has
multiple effects.
• Because the one pair of genes codes for a
protein that may not work correctly
…the effects are seen throughout the body.
Bottom line…
Not all inheritance follows the
simple rules of dominance and
recessive
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Incomplete dominance
Codominance
Multiple alleles
Pleiotropic
polygenic