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11-3 Exploring Mendelian Genetics
11–3 Other Inheritance Patterns
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11–3 Exploring Mendelian Genetics
Beyond Dominant and
Recessive Alleles
What inheritance patterns exist aside from
simple dominance?
•Not all alleles show simple dominant-recessive
patterns. One allele may be not be completely
dominant over another.
•Most genes have more than 2 alleles (more than 2
contrasting forms).
•Many traits are controlled by more than one gene.
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11–3 Exploring Mendelian Genetics
Beyond Dominant and
Recessive Alleles
Some alleles are neither dominant nor
recessive.
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11–3 Exploring Mendelian Genetics
Beyond Dominant and
Recessive Alleles
Incomplete Dominance
When one allele is not completely dominant over
another it is called incomplete dominance.
In incomplete dominance, the heterozygous
phenotype is between the two homozygous
phenotypes. The offspring is a “blend” of its
parents.
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11–3 Exploring Mendelian Genetics
Beyond Dominant and
Recessive Alleles
RR
A cross between
red (RR) and
white (WW) four
o’clock plants
produces pinkcolored flowers
(RW).
WW
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11–3 Exploring Mendelian Genetics
Beyond Dominant and
Recessive Alleles
Codominance
In codominance, both alleles contribute to the
phenotype. The traits of both parents are seen.
In certain varieties of chicken, the allele for black
feathers is codominant with the allele for white
feathers.
Heterozygous chickens are speckled with both
black and white feathers. The black and white
colors do not blend to form a new color, but appear
separately.
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11–3 Exploring Mendelian Genetics
Beyond Dominant and
Recessive Alleles
Multiple Alleles
Many genes exist in several different forms, and
are therefore said to have multiple alleles.
An individual can’t have more than two alleles in
their cells. However, more than two possible
alleles for each trait can exist in a population.
A rabbit's coat color is determined by a single gene
that has at least four different alleles.
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11–3 Exploring Mendelian Genetics
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11–3 Exploring Mendelian Genetics
Beyond Dominant and
Recessive Alleles
Polygenic Traits
Many traits are produced by the interaction of
several genes.
Traits controlled by two or more genes are said
to be polygenic traits.
Skin color in humans is a polygenic trait
controlled by more than four different genes.
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11–3 Exploring Mendelian Genetics
Genetics and the
Environment
Genetics and the Environment
Characteristics of any organism are not
determined just by the genes that are inherited.
Genes provide a plan for development, but how
that plan unfolds depends also on the
environment.
Environmental conditions can affect gene
expression and influence genetically determined
traits.
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11–3 Exploring Mendelian Genetics
For example, consider the Western white butterfly.
Western white butterflies that hatch in the summer
have different color patterns on their wings than
those hatching in the spring.
Scientific studies revealed that butterflies hatching in
springtime had greater levels of pigment in their
wings than those hatching in the summer.
In other words, the environment in which the
butterflies develop influences the expression of their
genes for wing coloration.
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11–3 Exploring Mendelian Genetics
In order to fly effectively, the body temperature of the
Western white butterfly needs to be 28–40°C.
More pigmentation allows a butterfly to reach the
warm body temperature faster.
Similarly, in the hot summer months, less
pigmentation prevents the butterflies from
overheating.
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