1. 11-3 Exploring Mendelian Genetics
11–3 Other Inheritance Patterns
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2. 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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3. Beyond Dominant and Recessive Alleles
Some alleles are neither dominant nor recessive.
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4. 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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5. Beyond Dominant and Recessive AllelesRR
A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW). WW
Some alleles are neither dominant nor recessive. In four o’clock plants, for example, the alleles for red and white flowers show incomplete dominance. Heterozygous (RW) plants have pink flowers—a mix of red and white coloring.
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6. 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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7. 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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9. 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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10. 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. For example, consider the Western white butterfly
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.

12. 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.