1. Complexity of gene expression
Epistasis, epigenetics, and sex-linked traits, plieotropy, gene linkage

2. Warm-up
Mouse coat color is controlled by two genes, one for the color, and another that determines whether the color gene will be expressed or not.
B is the dominant allele for coat color; a mouse with BB or Bb, if their genes are expressed, will have an agouti (a type of grey) coat. A mouse with bb geneotype, if it is expressed, will have a black coat.
C is the dominant allele for the gene that controls the color gene. If a mouse has the genotype CC or Cc, they will have whatever coat color is specified by their coat color genotype (agouti or black). A mouse with the genotype cc will be albino (white) because the cc genes will shut off the coat color gene.
Two true-breeding mice were crossed, one with genotype BBCC and one with genotype bbcc. Find the phenotypic ratio of the F2 generation resulting from this cross.

3. Warm-up: Outcome
Describe how the albinism gene caused a change in the phenotypic ratio of the F2 generation.

4. Genetics of Complex Traits
Epistasis
Epigenetics
Gene Expression: SRY
Pleiotropy
Gene Linkage

5. Epistasis
Epistasis occurs when the expression of a gene at one place in the genome (locus) alters the expression of a gene at another locus.
Albinism is an example of epistasis: Inheritance of the aa genotype means that the coat color gene is shut off.

6. Epigenetics
Epigenetics is a relatively new consideration in the study of genetics.
The mechanism by which the albinism gene switches off of the coat color gene is one example of epigenetics.
There is also evidence environmental and behavioral factors can turn genes on and off via epigenetics.

7. NOVA Video We’ll watch a video clip from the NOVA program.
While you watch, pay attention to:
What is epigenetics?
How does it work?
How is it related to the molecular genetics we’ve studied?
How are the biotech methods we’ve studied used in studying epigenetics?

8. Gene Expression: SRY
Another example of how genes may turn other genes on or off is the SRY gene – the Sex-determining Region of Y.
SRY is on the Y chromosome, and turns on genes that lead to the development of testes in male fetuses.
In the absence of the SRY gene, fetal gonads develop into ovaries.
SRY becomes active during fetal development and produces a protein that is a transcription factor.
What are transcription factors?

9. Gene Expression: SRY
The SRY protein attaches to specific regions of the DNA, bending it to new shapes.
In this way the protein controls the activity of other genes.

10. Pleiotropy
Pleiotropy occurs when a gene has more than one effect on the phenotype of the organism.
How is sickle cell anemia an example of pleiotropy?
How is cystic fibrosis an example of pleiotropy?

11. Gene Linkage
When genes are located together on the same chromosome, they generally appear together in the phenotype.
They are said to be linked.

12. Gene Linkage: Example
Example: Wing size and body color are on the same chromosome in fruit flies – The gene for gray bodies is on the same chromosome as normal wings, and the gene for black bodies is on the same chromosome as the gene for vestigial wings.
If a true-breeding gray fly with normal wings is crossed with a true-breeding black fly with vestigial wings, all the offspring are gray with normal wings.
If I do a test cross of the F1 offspring, what is the phenotypic ratio I should expect in the testcross offspring, if these genes are linked?

13. Gene Linkage: Example
Here are the actual numbers of offspring Thomas Hunt Morgan got for the testcross:
965 gray with normal wings
944 black with vestigial wings
206 gray with vestigial wings
185 black with normal wings
What happened?