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Non-Mendelian Genetics
4B Incomplete & Codominance
Multiple Alleles
Non-Mendelian Genetics
 Stop for first section of notes
Begin Incomplete and Codominance
Notes
 Non-Mendelian
If red flower were dominant over white flower,
what color would the F1 generation be?
Why are the
offspring a
different color
from either
parent?
When neither allele is dominant.
Incomplete Dominance:
-The heterozygous phenotype
(i.e. Rr) is between the two
homozygous phenotypes
-Neither allele is dominant over
another
-Example: a red
flower crosses with
a white flower to Genotype
RR (homozygous)
make pink flower. rr (homozygous)
Rr (heterozygous)
Phenotype
Red
White
Pink
Incomplete Dominance Practice
Problem
A species of mice are controlled by incomplete dominance.
The two alleles are black (B) and white (W). The
heterozygous genotype is expressed as gray fur.
Give the phenotypes:
BB:
WW:
BW:
Incomplete Dominance Practice
Problem
Cross a gray male mouse with a white female mouse.
List the probabilities of having the
following offspring phenotypes:
Black:
Gray:
White:
How are these two phenotypes
different?
Incomplete dominance
Neither allele is dominant
(blended)
Codominance
Both alleles are dominant
(mixed)
Codominance:
Both alleles are clearly
expressed in the phenotype of
a heterozygous.
Some chickens:
 Allele for black (B) and
white (W) feathers are
codominant.
 Heterozygous chickens are
speckled with black and
Codominance Practice Problem
A species of chickens are controlled by codominance. The
two alleles are black (B) and white (W). The heterozygous
genotype is expressed as black and white speckled
chickens.
Give the phenotypes:
BB:
WW:
BW:
Codominance Practice Problem
Cross a speckled male with a speckled female.
List the probabilities of having the
following offspring phenotypes:
Black:
White:
Speckled:
Beyond Dominant and Recessive Alleles
There are important exceptions
to many of Mendel’s principles
– Not all genes show simple
patterns of inheritance
– Majority of genes have more
than 2 alleles
– Many important traits are
controlled by more than one
gene
Multiple alleles: genes with more
than two alleles
We’ve been working with two alleles
(R/r)
Human Blood type has more than two
alleles
Blood Types
Can the environment affect the
exression of traits?
 Genes provide the blueprint for an
organisms traits
 Environmental conditions can affect
how that develops
Pontia occidentalis
 Western white butterfly
Born in spring, wings are darker
 Born in summer, wings are lighter
Spring vs Summer
Why?
 If the butterflies are born in the spring,
temps are colder. They use the darker
colors (absorb heat) to maintain body
temp
 If butterflies are born in the summer,
temps are warmer, don’t need darker
colors, have lighter colors to avoid
overheating