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 .
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.
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.
1. Incomplete dominance vs. Codominance
-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
Ex. 2 humans heterozygous for albinism and are blood type AB
I. Epistasis - genetic interaction affecting expression of one trait
II. Novel phenotypes due to gene interaction
III. Complementation analysis
V. Is phenotype always completely due to genotype?
HH or Hh
Precursor H substance
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.
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.
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 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
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
No, none of these cases has violated the principles of segregation and independent assortment - just added complexity.
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.
3. From any dihybrid cross of double heterozygotes, we see the following genotypic ratio:
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.
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.
“3” or “8”??
Males are hemizygous
for this gene locus; only
females can be carriers.
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?
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.
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
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!