1. Announcements
1. You should be finishing “Monk in the garden” and starting to answer questions for writing assignment. We will discuss this next week.
2. Dr. Brad Swanson is guest lecturing on conservation genetics on Monday. This material will be on Exam 2.
3. “Problem set 2” answers due today at start of class.
If you’re in Group A for presentations, I encourage you to discuss your main source/reference with me. Plan ahead! Group A presents in 11 or 12 days.
Pay special attention to “Insights and Solutions” - Ch .4, p
6. Study this weekend! Your exam next week is Thursday and Friday. Avoid lines, go early in the day on Thursday. Give yourself enough time to finish the exam.

4. Review of last lecture 1. Incomplete dominance vs. Codominance
2. Multiple alleles of a gene -studied in a population
ex. Blood group alleles, A, B, AB, O
Lethal alleles - recessive or dominant
-can lead to modified Mendelian ratios such as 2:1 genotypic and phenotypic ratios in the F1
4. Modification of the 9:3:3:1 ratio: examining inheritance of two gene pairs that have different modes of inheritance

5. Modified Dihybrid Cross: First Consider Each Trait on its Own
Ex. 2 humans heterozygous for albinism and are blood type AB

6. Modified Dihybrid Cross: Next Consider Both Traits Together

7. Outline of Lecture 8
I. Epistasis - genetic interaction affecting expression of one trait
II. Novel phenotypes due to gene interaction
III. Complementation analysis
X-linkage and color blindness
V. Is phenotype always completely due to genotype?

8. I. Epistasis
Two or more separate genes (not separate alleles at one genetic locus) interact to control a phenotypic character.
If one gene locus prevents the expression of a second gene, the first locus is epistatic to the second, and the second is hypostatic to the first.

9. Example 1: H/h and AB pathway
A antigen
A allele
HH or Hh
Precursor H substance
H Substance hh
B allele
B antigen

10. Epistasis example 2: Coat color in mice
Wt coat color is agouti - A (dominant to black); Nonagouti (black) coat color - a
Pigmentation expression - B (dominant to albino); No pigmentation (albino) - b
If individual is bb, then is albino regardless of allele at a locus - due to gene interaction.

11. Agouti Markings
A- hair made with bands of black pigment and yellow pigment.
Aa hair all black.

12. Coat color example of epistasis, cont.
AABB (agouti) x aabb (albino)
AaBb (all agouti)
Genotype Phenotype F2 ratio Final phenotypic ratio
AaBb x AaBb
A-B- agouti 9/16 9/16
A-bb albino 3/16 4/16
aaB- black 3/16 3/16
aabb albino 1/16
Due to gene interaction, we see a 9:3:4 F2 ratio. The b locus is epistatic to the a locus.

13. Example of dominant epistasis, a 12:3:1 ratio
Inheritance of fruit color in summer squash: two loci together control color and a dominant allele at one locus can mask the expression of the alleles at the second locus.
A white
aaB- yellow
aabb green
A is dominant to a, and the a locus is epistatic to the b locus.
Therefore, if AaBb is crossed to AaBb, the F2 is as follows:
A-B- white 9/16
A-bb white 3/16
aaB- yellow 3/16
aabb green 1/16
12/16 white
3/16 yellow
1/16 green

14. Novel phenotypes and F2 ratios due to gene interaction
Summer squash fruit shape inheritance:
AABB - disc shape x aabb - long shape
F1 = AaBb - disc shape
F2 = A-B- disc 9/16 9/16 disc
A-bb sphere 3/16 6/16 sphere
aaB- sphere 3/16
aabb long 1/16 1/16 long

15. Summary of modified F2 ratios - was Mendel just wrong?
No, none of these cases has violated the principles of segregation and independent assortment - just added complexity.

16. Practice Problem
In a plant, a tall variety was crossed with a dwarf variety. All F1 plants were tall. When two F1 plants were interbred, 9/16 of the F2 were tall and 7/16 were dwarf.
Explain the inheritance of height by a) indicating the number of gene pairs involved and b) by designating which genotypes yield tall and which yield dwarf.
When studying a single character, a ratio expressed in 16 parts suggests that two gene pairs are “interacting” during the expression of the phenotype.
A 9:7 ratio implies a dihybrid condition with epistasis.

17. 3. From any dihybrid cross of double heterozygotes, we see the following genotypic ratio:
A-B- 9/16
A-bb 3/16
aaB- 3/16
aabb 1/16
tall
dwarf
4. In our problem, we see a 9:7 phenotypic ratio. If the 3:3:1 groups above were lumped together, a 9:7 ratio emerges.
Assign tall to any plant with both A-B- and dwarf to any plant that is homozygous recessive for either or both the recessive alleles.

18. Mutations do not complement, so in same complememtation group.
III. Complementation Analysis Reveals Whether 2 Mutations are at the Same Locus
The mutations do complement each other, so they are in different complementation groups.
Mutations do not complement, so
in same complememtation group.

19. IV. X Linkage Affects males and females differently
Example: red-green color blindness
Other examples in humans:
Hemophilia
Duchenne’s muscular distrophy
“3” or “8”??

20. X-linkage means genes on X chromosome
Sex chromosomes are “unlike” in many species; all others are autosomes.
In humans and Drosophila, males contain XY and females contain XX.
Y chromosome contains region of pairing homology with X: pseudoautosomal region.
Y contains few genes; X contains many.

21. Color Blindness Pedigree
Males are hemizygous
for this gene locus; only
females can be carriers.

22. Hemophilia in the European Royal Families
Disorder is lethal or debilitating prior to reproductive maturation.
X-linked recessive: heterozygous females are carriers with 50% chance of passing trait to sons only.
Arose from spontaneous mutation in Queen Victoria?
Hemizygous males were affected in Russian, German, Spanish royal families, trait not passed on in British royal family.

23. Is phenotype always due to genotype?
Penetrance vs. Expressivity
If 70% of the individuals show the mutant phenotype, then the penetrance is 70%.
Expressivity reflects the range of expression of the mutant genotype.
Variable penetrance or expressivity?

24. Temperature effects on phenotype
Temperature can affect coat coloration.
Siamese cat fur in the extremities is darker due to cooler temperatures. The enzyme making darker pigment doesn’t work well at the higher temperatures in the rest of the body.
Temperature also affects primrose flower color and fur of Himalayan rabbits.

25. Influence of chemicals on phenotype
PKU: phenotype is mental retardation due to metabolic disorder; severity is affected by diet and whether phenylalanine is restricted in the diet.
Phenocopies: non-hereditary phenotypic modification that mimics a phenotype caused by a known gene mutation. example: deafness can be caused in a developing baby if a mother is infected with rubella during the first 12 weeks of pregnancy; also caused by homozygous, recessive mutation

26. Summary of section V
Variation in most of the genetic traits that we have considered so far is determined predominantly by differences in genotype.
For many traits, however, the traits are influenced by both genes and the environment.
Nature vs. Nurture debates for human behaviors:
Ex. Considerations for the weekend/ alcoholism:
genes influence how susceptible you are to alcohol abuse but they do not force you to drink alcohol!