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LECTURE 10: FROM GENE TO PHENOTYPE I
exam 1: review
chapter 6
questions & concepts
genes & gene products
allele interactions
gene & protein interactions
chi-square applications
CHAPTER 6: QUESTIONS
how do genes influence the organism?
what are gene products?
... & what are they doing?
do alleles determine
a specific phenotype?
how do genes interact?
can we dissect gene interactions using
mutations?
CHAPTER 6: CONCEPTS
if 2 haploid genomes (i.e., the gametes of
diploids) each with 1 recessive mutation are
combined
 mutant phenotype?... the mutations are allelic
(they identify alleles of the same gene) or
 wild type phenotype?... the mutations are not
allelic (they identify alleles of different genes)
CHAPTER 6: CONCEPTS
dominance can be complete or incomplete
some mutations can cause lethality or sterility
expression of some mutations can be dependent on
environment  conditional mutations
most traits are determined by sets of genes that
interact with the environment
modified monohybrid ratios reveal allele interactions
modified dihybrid ratios reveal gene interactions
GENE INTERACTION
genes never do anything by themselves
levels of interaction between alleles of
1. the same gene
2. different genes
GENE INTERACTION
3 ways to study these interactions
1. genetic analysis (ch 6)
2. functional genomics (ch 12)
3. proteomics (ch 12)
GENES & GENE PRODUCTS
1st clue from human “inborn metabolism error”
PKU (phenylketonuria)
autosomal recessive
phenylalanine  tyrosine

phenylpyruvic acid (toxic)
GENES & GENE PRODUCTS
“1 gene - 1 enzyme” hypothesis
Beadle & Tatum (1940s, Nobel Prize)
Neurospora crassa (haploid fungus)
mutants (by irradiation) & analysis
GENES & GENE PRODUCTS
supports growth of all genotypes
GENES & GENE PRODUCTS
GENES & GENE PRODUCTS
GENES & GENE PRODUCTS
3 arginine auxotrophs (arginine metabolism mutants)
mapped to different loci... different genes
growth on medium supplemented with different
related compounds
GENES & GENE PRODUCTS
chemical structure  biochemical pathway (B & T)
enzyme X enzyme Y enzyme Z



precursor  ornithine  citrulline  arginine
GENES & GENE PRODUCTS
results  arginine metabolism biochemical pathway
arg-1+
arg-2+
arg-3+






enzyme X enzyme Y enzyme Z
precursor  ornithine  citrulline  arginine
GENES & GENE PRODUCTS
results  “1 gene - 1 enzyme” hypothesis
more accurately “1 gene - 1 polypeptide”
most genes encode physical structure of proteins
DNA  mRNA  polypeptide
some genes encode functional RNA only, e.g.
tRNA
rRNA
GENES & GENE PRODUCTS
GENES & GENE PRODUCTS
ALLELE INTERACTION
interactions between alleles of one gene
1. dominance / recessiveness
2. semi-dominance = incomplete dominance
3. co-dominance (e.g.: IA & IB of ABO system)
4. multiple alleles (e.g.: IA, IB & i of ABO system)
5. conditional (e.g.: temperature sensitive)
6. lethality
7. sterility
DOMINANT & RECESSIVE ALLELES
mutation recessive  + allele haplosufficient
mutation dominant  + allele haploinsufficient
DOMINANT & RECESSIVE ALLELES
dominant & recessive alleles...
P
F1
F2
red x white

red x red

¾ red + ¼ white
3
:
1
INCOMPLETELY DOMINANT ALLELES
incomplete dominance (= semidominance)...
P
F1
F2
red x white

pink x pink

¼ red + ½ pink + ¼ white
1
:
2
:
1
phenotypes are quantitatively different
INCOMPLETELY DOMINANT ALLELES
are pink flowers more red or more white?
why is this not blending?
CODOMINANT ALLELES
ABO blood type
i recessive to both dominant alleles (IA & IB > i)
GENOTYPE
IAIA
IAi
IBIB
IBi
IAIB
ii
PHENOTYPE
ANTIGENS
ANTIBODIES RECEIVE FROM DONATE TO
A
A
anti-B
A&O
A & AB
B
B
anti-A
B&O
B & AB
AB
O
AB
none
none
anti-A & -B
all
O
AB
all
IAIB is AB, qualitatively different from A or B
MULTIPLE ALLELES
ABO blood type
6 possible genotypes & 4 possible phenotypes
GENOTYPE
IAIA
IAi
IBIB
IBi
IAIB
ii
PHENOTYPE
ANTIGENS
ANTIBODIES RECEIVE FROM DONATE TO
A
A
anti-B
A&O
A & AB
B
B
anti-A
B&O
B & AB
AB
O
AB
none
none
anti-A & -B
all
O
AB
all
influences variation of trait in populations
CONDITIONAL ALLELES
influenced by environment, e.g.:
temperature (hot or cold)
desiccation
nutrient requirement
chemicals
infection
CONDITIONAL ALLELES
e.g.: temperature sensitive shibire (paralyzed) mutant
P
F1
F2
shi+ x shits

shi+/shits x shi+/shits

all active @ 25º
1
shits wild type @  25º = permissive temperature
CONDITIONAL ALLELES
e.g.: temperature sensitive shibire (paralyzed) mutant
P
F1
F2
shi+ x shits

shi+/shits x shi+/shits

¾ active + ¼ paralyzed @ 29º
3
:
1
shits paralyzed @  29º = restrictive temperature
LETHAL ALLELES
homozygotes lethal (can be dominant or recessive)
e.g., yellow (AY) allele in mice
LETHAL ALLELES
homozygotes lethal (can be dominant or recessive)
e.g., yellow (AY) dominant allele in mice
AY/A  x AY/A 

F1 ¼ AY/AY lethal + ½ AY/A yellow + ¼ A/A black
don’t see these
Y Y
P
2/3 AY/A yellow + 1/3 A/A black
A /A
AY/A
AY/A
A/A
LETHAL ALLELES
e.g.: Curly (Cy) mutations in Drosophila
P
F1
Cy–
Cy–
——+ x ——
Cy
Cy+

¾ alive + ¼ dead
3
:
1
2/3 curly + 1/3 wild type
Cy–/Cy– Cy–/Cy+
Cy–/Cy+ Cy+/Cy+
STERILE ALLELES
homozygotes sterile (can be dominant or recessive)
e.g., fruitless (fru) allele in Drosophila
STERILE ALLELES
homozygotes sterile (can be dominant or recessive)
e.g., fruitless (fru) allele in Drosophila
P
F1
F2
fru+/fru  x fru+/fru 

¾ wild type + ¼ fruitless
fru/fru fruitless  x fru+/fru+ wild type 

 do not mate, no progeny
STERILE ALLELES
e.g.: mushroom body miniature B (mbmB) mutations
P
F1
mbmB–
mbmB–
———— x ————
mbmB+
mbmB+

¾ alive + ¼ sterile
3
:
1
mbmB+
mbmB–
homozygous mbmB– live but give no offspring