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Outline of major topics. Allelic actions and interactions Dominance how it works when there is no real dominance Multiple alleles Lethal alleles. One Gene!. Outline of major topics . Genic interactions Metabolic pathways novel phenotypes epistasis (pleiotropy). More than one gene!.

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outline of major topics
Outline of major topics
  • Allelic actions and interactions
    • Dominance
      • how it works
        • when there is no real dominance
    • Multiple alleles
    • Lethal alleles

One Gene!

outline of major topics2
Outline of major topics ...
  • Genic interactions
    • Metabolic pathways
      • novel phenotypes
      • epistasis
      • (pleiotropy)

More than one gene!

outline of major topics3
Outline of major topics ...
  • Factors affecting phenotype
    • Genetic factors
      • Strange genes
    • Environmental factors
allelic actions dominance
Allelic actions: dominance
  • In general, dominant allele codes for production of functional protein
    • P --> purple pigment
  • In general, recessive allele codes for no functional protein
    • p --> no pigment
allelic actions dominance5
Allelic actions: dominance ...

PP has two alleles coding for purple pigment

Pp has one allele coding for purple pigment, one

allele coding for nothing

pp has no alleles coding for pigment

allelic actions dominance6
Allelic actions: dominance ...
  • In many cases, a simple dominant/recessive relationship does not hold
    • incomplete or co-dominance
incomplete or co dominance
Incomplete or Co-dominance
  • If both alleles code for a contrasting functional protein, heterozygote shows a phenotype completely different from either homozygote!
  • Two different upper case letters used to symbolize the alleles or one letter w/ superscripts
incomplete or co dominance8
Incomplete or Co-dominance ...
  • Flower color in carnations
    • R codes for a red pigment
    • W codes for a white pigment
      • RR gives a red flower
      • WW gives a white flower
      • RW gives a pink flower
incomplete or co dominance9




Incomplete or Co-dominance ...

Phenotypic ratios = genotypic ratios

Pink X Pink --> 1/4 Red, 1/2 Pink, 1/4 White





incomplete or co dominance10
Incomplete or Co-dominance ...

Andalusian fowl:

black X black --> black

white X white --> white


black X white --> gray

gray X gray --> ?

incomplete or co dominance11
Incomplete or Co-dominance ...
  • heterozygote shows a BLEND: alleles are incompletely dominant
  • heterozygote shows BOTH: alleles are co-dominant
incomplete or co dominance12
Incomplete or Co-dominance ...

MN Blood grouping (Landsteiner and Levine)

two alleles:

M - produces M antigens

N - produces N antigens

incomplete or co dominance13













Incomplete or Co-dominance ...






Type M; has only M antigens

(Anti-N antibodies)

Type MN; has both antigens

(no antibodies)

Type N; has only N antigens

(Anti-M antibodies)

incomplete or co dominance14
Incomplete or Co-dominance ...
  • Incomplete dominance = co-dominance
  • Just label differently depending on whether phenotype shows BLENDING
  • Mechanism generally the same; both alleles code for a working protein
multiple alleles
Multiple alleles
  • May be > just two choices of alleles at one locus
  • Population level phenomenon; individuals have, at most, two different alleles (heterozygous)
multiple alleles16
Multiple alleles ...

ABO Blood Groupings

Four possible blood types (phenotypes):





Controlled by one gene

abo blood groupings
ABO Blood Groupings ...

Three alleles, with co-dominance as well as

as dominant/recessive relationship

Alleles: A, B, O

(or IA IB IO)

Phenotype (blood type) is a result of the proteins

coded for by each allele

abo blood groupings18
ABO Blood Groupings ...

A codes for A antigen

B codes for B antigen

O codes for no antigen

abo blood groupings19
ABO Blood Groupings ...



Type A: A antigens; anti-B antibodies



Type B: B antigens; anti-A antibodies


Type AB: A & B antigens; no antibodies

Type O: no antigens; anti-A and

anti-B antibodies


abo blood groupings20
ABO Blood Groupings ...
  • O is recessive to both A and B
  • A and B are co-dominant
  • Blood type AB is considered the “universal recipient”
    • no antibodies produced
  • Blood type O is considered the “universal donor”
    • no antigens produced
Reminders ...
  • Dominance is not an inherent property of an allele
  • An allele may be dominant to a second allele, but co-dominant, incompletely dominant, or even recessive to a third
coat color in rabbits
Coat color in rabbits

Four alleles:

c+ cch ch ca

agouti chinchilla Himalayan albino

c+ is dominant to all others

cch is incompletely dominant to ch and ca

ch is dominant to ca

coat color in rabbits23
Coat color in rabbits ...

c+ __  agouti

cch cch  chinchilla

cch ca  light gray

cch ch  light gray w/dark tips

ch ch or chca  Himalayan

ca ca  albino

multiple alleles24
Multiple alleles ...
  • Many, if not most, genes do have multiple possible alleles
    • Rhesus blood group (+ or -)
      • actually at least 18 alleles
    • One white eye locus in Drosophila
      • over 100 alleles
lethal alleles
Lethal alleles
  • First described by Cuenot in 1905
    • coat color in mice
      • strain with yellow coats
        • yellow dominant to wild-type agouti
        • could not produce true-breeding (homozygous) yellows
lethal alleles26
Lethal alleles ...
  • Each time Cuenot crossed two yellow mice, 1/3 of the offspring were agouti
  • Backcrossed yellow mice to agouti, saw all yellow mice were heterozygotes
    • Impossible to produce homozygous yellow mice!
lethal alleles27
Lethal alleles ...
  • Castle and Little offered explanation:
    • yellow allele dominant w/ respect to coat color
    • yellow allele also a recessive lethal allele
      • homozygous yellows die as embryos
lethal alleles28

c+ cy



c+c+ cyc+

cyc+ cycy

Lethal alleles ...

Yellow X Yellow

cy c+ cy c+

cycy do not live to birth

Apparent ratio:

2/3 yellow : 1/3 agouti

lethal alleles29
Lethal alleles ...
  • Mechanisms of lethality
    • recessive lethals: not coding for some necessary product
    • Hemophilia: dominant allele --> clotting factor recessive allele -> no clotting factor
    • Sickle cell anemia: S --> normal hemoglobin s --> abnormal hemoglobin
lethal alleles30
Lethal alleles ...
  • Mechanisms of lethality …
    • Dominant lethals: very rare. Gene product itself causes death
    • Huntington’s disease: H --> type of neurotoxin h --> no neurotoxin
    • Retinoblastoma: R --> allows tumor formation r --> no tumor formation
lethal alleles31
Lethal alleles ...
  • Once again, note that “dominance” in itself means nothing.
    • Refers only to phenotype that appears in the heterozygote
      • Lethals may be dominant with respect to one phenotype and recessive with respect to the lethal action
genic interactions
Genic interactions
  • Two or more genes acting on one trait
    • Novel phenotypes
    • Epistasis
  • One gene acting on more than one trait
    • Pleiotropy

***All due to metabolic pathways***

genic interactions novel phenotypes
Genic interactions: Novel Phenotypes

Eye color in Drosophila

P: red eyed x white eyed

F1: red eyed

F2: red eyed

white eyed





brown eyed

scarlet eyed

genic interactions novel phenotypes34
Genic interactions: Novel Phenotypes

S = wild-type (red)

s = scarlet

B = wild-type (red)

b = brown

S __ B __ -->wild-type (red)

S __ bb -->brown eyes

ss B __ -->scarlet eyes

ss bb -->white eyes

genic interactions novel phenotypes35


Genic interactions: Novel Phenotypes

The S allele controls the production of BROWN pigment

The B allele controls the production of SCARLET pigment

Cpd. Y ---enzyme S---> BROWN

Cpd. X ---enzyme B---> SCARLET

genic interactions novel phenotypes36
Genic interactions: Novel Phenotypes

Poultry: comb shape

Two genes, w/ 2 alleles each:

pea vs. single, and rose vs. single

Considered separately:

AA, Aa = pea BB, Bb = rose

aa = single bb = single

genic interactions novel phenotypes poultry comb shapes
Genic interactions: Novel PhenotypesPoultry comb shapes ...

The genotype at each of 2 loci plays a role in

determining comb shape:

A__ B__ = walnut

aa B__ = rose

A__ bb = pea

aa bb = single

genic interactions epistasis
Genic interactions: Epistasis
  • One gene masks a second gene
  • Due to a linear, instead of branched, metabolic pathway
genic interactions epistasis coat color in mice
Genic interactions: EpistasisCoat color in mice

B = agouti

b = black

A = non-albino

a = albino

A__ B__ = agouti

A__ bb = black

aa __ __ = albino

genic interactions epistasis coat color in mice40





enzyme A



enzyme B

Genic interactions: EpistasisCoat color in mice ...
genic interactions epistasis coat color in mice41

AB agouti

  • Ab black
  • (3 aB + 1 ab) white
Genic interactions: EpistasisCoat color in mice ...

Aa Bb X Aa Bb:

epistasis coat color in labs
Epistasis: Coat color in Labs

B = black

b = chocolate

E = black or chocolate

e = yellow





E__ B__

E__ bbee B __

ee bb

genic interactions epistasis43

Aa Bb x Aa Bb

Genic interactions: Epistasis

Fruit color in summer squash

aa bb = green

aa B__ = yellow

A__ __ __ = white

how can we distinguish allelic interactions from genic interactions
How can we distinguish allelic interactions from genic interactions?
  • While the F2 ratios will not follow basic Mendelian rules, they still provide information
  • Allelic interaction (one gene) will lead to a deviant 3:1
    • 1:2:1 most likely
  • Genic interaction (2 genes) will lead to a deviant 9:3:3:1
    • 9:3:4 12:3:1 9:6:1 9:7
    • 3 genes: deviant 27:9:9:9:3:3:3:1
  • One gene may affect more than one trait
  • This is again due to the metabolic pathways involved
  • PKU - phenylketonuria
    • P codes for phenylalanine hydroxylase
      • enzyme metabolizes phenylalanine
    • p does not code for functional enzyme
      • phenylalanine cannot be metabolized
    • primary result: mental retardation
    • also affects head size, skin/hair/eye color, “mousy” odor, peculiarities of gait, stance, sitting posture, eczema, epilepsy
pleiotropy pku metabolic pathway
Pleiotropy: PKU metabolic pathway


phenylpyruvic acid (PKU)


phenylalanine hydroxylase




other factors affecting phenotype
Other factors affecting phenotype
  • Overview
    • Genetic factors
      • Incomplete penetrance
      • Variable expressivity
      • Sex limited and sex influenced traits
    • Environmental factors
      • Nutrition, light, temperature, etc.....
incomplete penetrance and variable expressivity
Incomplete Penetrance and Variable Expressivity
  • Incomplete penetrance
    • Identical genotypes differ in phenotype
    • Penetrance = % individuals w/ genotype expressing the associated phenotype
  • Variable expressivity
    • Variation in the degree of expression of the phenotype
incomplete penetrance
Incomplete Penetrance

Eye shape in Drosophila

Lobe locus: dominant allele which reduces

the size of the eye

LL Ll ll

reduced eye

normal eye

incomplete penetrance51
Incomplete Penetrance

However, only 75% of the flies carrying the

L allele actually have reduced eyes.

25% of the LL andLl individuals have eyes which

appear perfectly normal; just like ll flies.

The penetrance of the lobe allele is said

to be 75%.

variable expressivity
Variable Expressivity
  • Neurofibromatosis - autosomal dominant
    • 1/3 virtually symptom free; presence of café-au-lait spots & benign skin tumors
    • 1/3 symptom free most of the time; episodes of acute illness
    • 1/3 severely affected; symptoms severe enough to cause death
sex limited traits
Sex Limited Traits
  • Traits which appear exclusively in one sex
    • All genotypes possible in both sexes
    • One phenotype limited to one sex
  • Generally regulated by a hormone level threshold
  • Any secondary sexual characteristic is sex limited
sex limited traits54
Sex Limited Traits

Horns in some breeds of sheep:

H+ = production of horns

H- = no horns

H+H+ H+H- H-H-

males: horns horns no horns

females: no horns no horns no horns

There must be a sufficiently high level of testosterone for horns

to be produced. Only males have enough!

sex influenced traits


males: A A B

females: A B B

Sex Influenced Traits
  • Sex determines which allele is dominant
    • male and female heterozygotes differ in phenotype!
sex influenced traits56
Sex Influenced Traits

Horns in Suffolk sheep:

h+h+h+ h-h-h-

males: horns horns no horns

females: horns no horns no horns

Again, related to hormonal thresholds.

sex influenced traits57
Sex Influenced Traits

Male pattern baldness:

Testosterone threshold.

B = bald N = not bald


males: bald bald not bald

females: thinning not bald not bald

environmental effects on phenotype
Environmental effects on phenotype
  • The environment may greatly influence the expression of genotypes
    • light
    • temperature
    • nutrition
    • cosmetics
    • drugs
environmental effects on phenotype59
Environmental effects on phenotype
  • Light
    • Corn: sunred allele - homozygous plants bright red when grown in full sun
      • Appear green if all but red light screened
    • Humans: freckles (dominant) - phenotype is a combination of presence of allele and exposure to sunlight
environmental effects on phenotype60
Environmental effects on phenotype
  • Temperature
    • Primroses: red flowers if reared at room temperature; white flowers if >86o F
    • Rabbits (Himalayan allele) & Siamese cats: darker extremities due to lower temperature
    • Many Drosophila melanogaster wing mutations
environmental effects on phenotype61
Environmental effects on phenotype
  • Temperature, continued…
    • Sea turtles: sex is determined by temperature at which the egg is incubated
    • Warmer = female; cooler = male
environmental effects on phenotype62
Environmental effects on phenotype
  • Nutrition
    • PKU: reduce protein intake, eliminate phenotype
    • Diabetes: reduce sugar intake or provide insulin, eliminate or reduce phenotype
    • Significant height increase in first generation Americans
environmental effects on phenotype63
Environmental effects on phenotype
  • Drugs
    • Hemophilia: supply blood clotting factor, eliminate phenotype
    • Cocaine: may induce schizophrenia
environmental effects on phenotype64
Environmental effects on phenotype
  • Environmental factors may produce mimics of genetic traits
    • phenocopies
      • thalidomide
        • 1950s treatment for morning sickness
        • produced phenocopy of phocomelia, “paddle limbs”
        • drug in use again for cancer, Hansen’s disease (leprosy)
environmental effects on phenotype65
Environmental effects on phenotype
  • May modify effects of genotypes for good or bad
  • May mask genes or mimic genes
  • These phenotypic changes WILL NOT be passed on to offspring. Genotype not changed, just phenotype.