1. KEY CONCEPT Phenotype is affected by many different factors.

2. Phenotype can depend on interactions of alleles.
In incomplete dominance, neither allele is completely dominant nor completely recessive. Upper case letters with either subscripts or superscripts are used to represent the different alleles.
Heterozygous phenotype is intermediate between the two homozygous phenotypes
Homozygous parental phenotypes not seen in F1 offspring
Phenotype
Genotype
Green
B1B1
Steel blue
B2B2
Royal blue
B1B2
The green betta fish is homozygous for the green color allele.
The royal blue betta fish is heterozygous for the two color alleles.
The steel blue betta fish is homozygous for the blue color allele.

3. Other examples of incomplete dominance
How can you tell that the two color alleles show incomplete dominance?

4. Other examples of incomplete dominance

5. Codominant alleles will both be completely expressed.
Codominant alleles are neither dominant nor recessive.
The ABO blood types result from codominant alleles.
Many genes have more than two alleles.
APPLY How can two people with type B blood have a child with type O blood?

6. Examples of Codominance
Synthesize: How is codominance the same as having no dominant and recessive relationship at all between two alleles?

7. Another example of codominance compared to incomplete dominance
Compare and Contrast: How are codominant alleles and incompletely dominant alleles similar? How are they different?

8. Many genes may interact to produce one trait.
Polygenic traits are produced by two or more genes.
Order of dominance: brown > green > blue.
At least 3 different genes interact to produce the range of human eye colors, such as the examples on the right.

9. Example of polygenic traits
Main Idea: Why might polygenic traits vary more in phenotype than do single gene traits?
Contrast: How do multiple-allele traits differ from polygenic traits?

10. An epistatic gene can interfere with the expression other genes.
Fig. 2.4 Albinism in mammals, such as this hedgehog, is caused by an epistatic gene that blocks the production of pigments. The same epistatic mechanism applies to humans and albinism. They will have very light skin, hair, and eyes regardless of the other gene they inherited.

11. The environment interacts with genotype.
Phenotype is a combination of genotype and environment.
The sex of sea turtles depends on both genes and the environment. Warmer temps cause eggs to develop into females and cooler temps cause eggs to develop into males.

12. Another example of environmental interactions with genotype.
Height is an example of a phenotype strongly affected by the environment.
Fingerprints are polygenic but are also influenced by fetal development. Ridge patterns can be altered during weeks 6-13 of development, as the fetus touches the wall of the amniotic sac with its finger pads
Connect: Sunlight can cause a person’s hair to become lighter in color. Is this an example of an interaction between genes and the environment? Why or why not?