Transcript Modified Mendelian Ratios II

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.
4. 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.
5. Pay special attention to “Insights and Solutions” - Ch .4, p. 104-106.
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.
BIO 326- Genetics Fall 2002
Writing assignment on ТThe Monk in the GardenУ
by Robin Marantz Henig
Choose one of the following th ree topic areas to write a 2-4 page, doublespaced, typed essay. This assignment is worth 75 pts (out o f a possible 1000).
You will be graded on: how w ell did you answer the q uestions? (40); do you
have good logical flow and organization of your essay? (15); do you have good
use of references? (10); and is your s pelli ng and grammar correct? (10)
Due date: in lab either 10/1 or 10/2
1) Disc uss s everal of the reasons that we now ack nowledge Mendel as the
father of genetics. You should mention some of the practical iss ues that resulted
in MendelХsexperiments s uccessfully unlocking the secrets of heredity. You
should also discuss some of the reasons his ТgeniusУwas not widely k nown unt il
35 years after he presented his findings on the Тlaws of inher itanceУ. Lastly,
present an argument on whether or no t Mendel w as truly a ТgeniusУ. Support
your s tance with specific examples form the book.
2) W hich of t he three rediscoverers do you most respect and why? Cite
examples from the book to explain your answer. What aspects of Тbeing a
scientistУ surprised or interested you as y ou l earned about th e personalities of
Mendel and his rediscoverers? Cite specific examples from the book and explain
why y ou w ere surprised or interested.
3) H ow c ould DarwinХstheories of evolution and MendelХstheories of inhe ritance
have i nfluenced one another? IF they had crossed paths, who might have
benefited more from k nowing the otherХs work? Explain why. How did theo ries
of evolutionary c hange influence each of the thre e rediscoverers?
Oral Presentations - BIO 326 Genetics Fall 2002
Group A: Sept. 24, 25 Р in lab; Group B: Oct. 29, 30 Р in lab
Assignment: worth 75 pts of total 1000
1) Choose any genetics-related topic and title your presentation
2) Research th e topic using at least one source outside your text and the
internet. An example is an article from a respected magazine such as
Time or N ewsweek, or from a scientific journal such as Scientific
American, Ge netics, or Trends i n Genetics. You may also use the te xt
and internet to supplement your main s ource(s).
3) Choose 1-2 vis ual aids to use in your p resentation- overheads or
powerpoint slides are acceptable forms to use.
4) Organize your 5-10 minute ta lk suc h tha t y ou 1) introduce your topic
and explain why you chose it; 2) share what you learned; and 3)
explain how i t relates to a topic from class.
5) Practice it for a friend; know how long it will go; be prepared.
6) On the day of your talk, turn in a title for y our talk wi th the outside
source(s) y ou used.
7) Take questions from your classmates at the end of your talk.
Grading: appropriate visual aid (15 pts); introduction of topic (10 pts);
explanation of what you l earned (15 pts); relationship to class topics (10
pts); answering questions (10 pts); appropriate source/reference (15 pts).
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
3. 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
Modified Dihybrid Cross: First
Consider Each Trait on its Own
Ex. 2 humans heterozygous for albinism and are blood type AB
Modified Dihybrid Cross: Next
Consider Both Traits Together
Outline of Lecture 8
I. Epistasis - genetic interaction affecting expression of
one trait
II. Novel phenotypes due to gene interaction
III. Complementation analysis
IV. X-linkage and color blindness
V. Is phenotype always completely due to genotype?
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.
Example 1: H/h and AB pathway
A antigen
A allele
HH or Hh
Precursor H substance
H Substance
hh
B allele
B antigen
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.
Agouti Markings
• A- hair made with
bands of black pigment
and yellow pigment.
• Aa hair all black.
Coat color example of epistasis, cont.
AABB (agouti) x aabb (albino)
Genotype Phenotype
AaBb x AaBb
A-BA-bb
aaBaabb
agouti
albino
black
albino
AaBb (all agouti)
F2 ratio Final phenotypic ratio
9/16
3/16
3/16
1/16
9/16
4/16
3/16
Due to gene interaction, we see a 9:3:4 F2 ratio. The b
locus is epistatic to the a locus.
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-BA-bb
aaBaabb
white
white
yellow
green
9/16
3/16
3/16
1/16
12/16 white
3/16 yellow
1/16 green
II. 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-BA-bb
aaBaabb
disc
sphere
sphere
long
9/16
3/16
3/16
1/16
9/16 disc
6/16 sphere
1/16 long
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.
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.
1. When studying a single character, a ratio expressed in 16
parts suggests that two gene pairs are “interacting” during
the expression of the phenotype.
2. A 9:7 ratio implies a dihybrid condition with epistasis.
3. From any dihybrid cross of double heterozygotes, we see
the following genotypic ratio:
A-BA-bb
aaBaabb
9/16
3/16
3/16
1/16
tall
dwarf
dwarf
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.
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.
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”??
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.
Color Blindness Pedigree
Males are hemizygous
for this gene locus; only
females can be carriers.
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.
V. 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?
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.
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
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!