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Beyond Mendel’s Laws
of Inheritance
AP Biology
2006-2007
Extending Mendelian genetics
 Mendel worked with a simple system
peas are genetically simple
 most traits are controlled by a single gene
 each gene has only 2 alleles, 1 of which
is completely dominant to the other

 The relationship between
genotype & phenotype
is rarely that simple
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_____________________________
 Heterozygote shows an intermediate,
blended phenotype

example:
 RR = red flowers _______
 rr = white flowers _______
 Rr = pink flowers _______
 make 50% less color
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RR
RW
WW
Incomplete dominance
 Heterozygote shows an intermediate,
blended phenotype

example:
 RR = red flowers RR
 rr = white flowers WW
 Rr = pink flowers RW
 make 50% less color
AP Biology
RR
RW
WW
Incomplete dominance
P
X
true-breeding
red flowers
true-breeding
white flowers
100% pink flowers
F1
100%
generation
(hybrids)
self-pollinate
25%
red
F2
generation
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50%
pink
25%
white
It’s like
flipping 2
pennies!
1:2:1
_____________________________
 2 alleles affect the phenotype equally &
separately
not blended phenotype
 human ABO blood groups
 3 alleles

 IA, IB, i
 IA & IB alleles are co-dominant
 glycoprotein antigens on RBC
 IAIB = both antigens are produced
 i allele recessive to both
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Co-dominance
 2 alleles affect the phenotype equally &
separately
not blended phenotype
 human ABO blood groups
 3 alleles

 IA, IB, i
 IA & IB alleles are co-dominant
 glycoprotein antigens on RBC
 IAIB = both antigens are produced
 i allele recessive to both
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Genetics of Blood type
phenogenotype
type
A
B
AB
O
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antigen
on RBC
antibodies
in blood
donation
status
IA IA or IA i
_______________
on surface
of RBC
__
IB IB or IB i
_______________
on surface
of RBC
__
IA IB
_______________
_______________
on surface
of RBC
universal
recipient
ii
_______________
on surface
of RBC
universal
donor
Genetics of Blood type
phenogenotype
type
A
B
AB
O
AP Biology
antigen
on RBC
antibodies
in blood
donation
status
IA IA or IA i
type A antigens
on surface
of RBC
anti-B antibodies
__
IB IB or IB i
type B antigens
on surface
of RBC
anti-A antibodies
__
IA IB
both type A &
type B antigens
on surface
of RBC
no antibodies
universal
recipient
ii
no antigens
on surface
of RBC
anti-A & anti-B
antibodies
universal
donor
_____________________________
 Most genes are pleiotropic

one gene affects more than one
phenotypic character
 1 gene affects more than 1 trait
 dwarfism (achondroplasia)
 gigantism (acromegaly)
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Pleiotropy
 Most genes are pleiotropic

one gene affects more than one
phenotypic character
 1 gene affects more than 1 trait
 dwarfism (achondroplasia)
 gigantism (acromegaly)
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Acromegaly: André the Giant
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http://www.youtube.com/watch?v=cBy12LEnUjw
Inheritance pattern of Achondroplasia
A
Aa
x aa
a
a
dwarf
dwarf
a
Aa
A
x Aa
A
a
lethal
dwarf
a
dwarf
dwarf:___%
AP___%
Biology
normal or _:_
__% dwarf:__% normal or _:_
Inheritance pattern of Achondroplasia
Aa
x aa
Aa
x Aa
dominant
inheritance
A
a
a
a
Aa
Aa
dwarf
dwarf
aa
aa
50% dwarf:50%
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normal or 1:1
A
A

a
AA
Aa
Aa
aa
lethal
a
67% dwarf:33% normal or 2:1
_____________________________
 One gene completely masks another gene

coat color in mice = 2 separate genes
 C,c:
B_C_
bbC_
_ _cc
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pigment (C) or
no pigment (c)
 B,b:
more pigment (black=B)
or less (brown=b)
 cc = albino,
no matter B allele
 9:3:3:1 becomes 9:3:4
How would you know that
difference wasn’t random chance?
Chi-square test!
Epistasis
 One gene completely masks another gene

coat color in mice = 2 separate genes
 C,c:
B_C_
bbC_
_ _cc
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pigment (C) or
no pigment (c)
 B,b:
more pigment (black=B)
or less (brown=b)
 cc = albino,
no matter B allele
 9:3:3:1 becomes 9:3:4
How would you know that
difference wasn’t random chance?
Chi-square test!
Epistasis in Labrador retrievers
 2 genes: (E,e) & (B,b)


pigment (E) or no pigment (e)
pigment concentration: black (B) to brown (b)
eebb
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eeB–
E–bb
E–B–
_____________________________
 Some phenotypes determined by
additive effects of 2 or more genes on a
single character
phenotypes on a continuum
 human traits

 skin color
 height
 weight
 eye color
 intelligence
 behaviors
AP Biology
Polygenic inheritance
 Some phenotypes determined by
additive effects of 2 or more genes on a
single character
phenotypes on a continuum
 human traits

 skin color
 height
 weight
 intelligence
 behaviors
AP Biology
Skin color: Albinism
Johnny & Edgar Winter
 However albinism can be
inherited as a single gene trait

aa = albino
albino
Africans
melanin = universal brown color
enzyme
tyrosine
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melanin
albinism
OCA1 albino
AP
Biology
oculocutaneous
albinism (OCA)
Bianca Knowlton
1910 | 1933
_____________________________
 Genes are on ____________________________



as opposed to autosomal chromosomes
first discovered by T.H. Morgan at Columbia U.
Drosophila breeding
 good genetic subject
 prolific
 2 week generations
 4 pairs of chromosomes
 XX=female, XY=male
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Sex linked traits
1910 | 1933
 Genes are on sex chromosomes



as opposed to autosomal chromosomes
first discovered by T.H. Morgan at Columbia U.
Drosophila breeding
 good genetic subject
 prolific
 2 week generations
 4 pairs of chromosomes
 XX=female, XY=male
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Classes of chromosomes
autosomal
chromosomes
sex
chromosomes
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 Thomas Hunt Morgan in The Fly Room!
(Columbia University 1910)
 Fruit Flies (Drosophila melanogaster)
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Discovery of sex linkage
P
F1
true-breeding
red-eye female
X
true-breeding
white-eye male
100%
red eye offspring
Huh!
Sex matters?!
generation
(hybrids)
F2
generation
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100%
red-eye female
50% red-eye male
50% white eye male
What’s up with Morgan’s flies?
x
RR
r
R
Rr
x
rr
Rr
r
Rr
Rr
R
R
r
RR
Rr
Rr
rr
Doesn’t work
that way!
R
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Rr
Rr
100% red eyes
r
3 red : 1 white
Genetics of Sex
 In humans & other mammals, there are 2
sex chromosomes: X & Y

2 X chromosomes
 develop as a female: XX
 gene redundancy,
like autosomal chromosomes

an X & Y chromosome
X
Y
X
XX
XY
X
XX
XY
 develop as a male: XY
 no redundancy
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50% female : 50% male
Let’s reconsider Morgan’s flies…
x
X R XR
Xr
XR
XR
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X R Xr
X R Xr
x
X rY
Y
XRY
XRY
100% red eyes

XR
BINGO!
Xr
XR Xr
XRY
XR
Y
XR XR
XRY
XR Xr
X rY
100% red females
50% red males; 50% white males
Genes on sex chromosomes
 Y chromosome

few genes other than SRY
 sex-determining region
 master regulator for maleness
 turns on genes for production of male hormones
 many effects = pleiotropy!
 X chromosome

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other genes/traits beyond sex
determination
 mutations:
 hemophilia
 Duchenne muscular dystrophy
 color-blindness
Human X chromosome
 Sex-linked
Duchenne muscular dystrophy
Becker muscular dystrophy
usually
means
“X-linked”
 more than
60 diseases
traced to
genes on X
chromosome

Chronic granulomatous disease
Retinitis pigmentosa-3
Norrie disease
Retinitis pigmentosa-2
Hypophosphatemia
Aicardi syndrome
Hypomagnesemia, X-linked
Ocular albinism
Retinoschisis
Adrenal hypoplasia
Glycerol kinase deficiency
Ornithine transcarbamylase
deficiency
Incontinentia pigmenti
Wiskott-Aldrich syndrome
Menkes syndrome
Androgen insensitivity
Sideroblastic anemia
Aarskog-Scott syndrome
PGK deficiency hemolytic anemia
Anhidrotic ectodermal dysplasia
Agammaglobulinemia
Kennedy disease
Pelizaeus-Merzbacher disease
Alport syndrome
Fabry disease
Immunodeficiency, X-linked,
with hyper IgM
Lymphoproliferative syndrome
Albinism-deafness syndrome
Fragile-X syndrome
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Ichthyosis, X-linked
Placental steroid sulfatase deficiency
Kallmann syndrome
Chondrodysplasia punctata,
X-linked recessive
Charcot-Marie-Tooth neuropathy
Choroideremia
Cleft palate, X-linked
Spastic paraplegia, X-linked,
uncomplicated
Deafness with stapes fixation
PRPS-related gout
Lowe syndrome
Lesch-Nyhan syndrome
HPRT-related gout
Hunter syndrome
Hemophilia B
Hemophilia A
G6PD deficiency: favism
Drug-sensitive anemia
Chronic hemolytic anemia
Manic-depressive illness, X-linked
Colorblindness, (several forms)
Dyskeratosis congenita
TKCR syndrome
Adrenoleukodystrophy
Adrenomyeloneuropathy
Emery-Dreifuss muscular dystrophy
Diabetes insipidus, renal
Myotubular myopathy, X-linked
Map of Human Y chromosome?
< 30 genes on
Y chromosome
Sex-determining Region Y (SRY)
Channel Flipping (FLP)
Catching & Throwing (BLZ-1)
Self confidence (BLZ-2)
Devotion to sports (BUD-E)
Addiction to death &
destruction movies (SAW-2)
note: not linked to ability gene
Air guitar (RIF)
Scratching (ITCH-E)
Spitting (P2E)
Inability to express
affection over phone (ME-2)
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linked
Selective hearing loss (HUH)
Total lack of recall for dates (OOPS)
AP Biology
AP Biology
Contact with collagen fibres
in blood vessels
Factor III
Thromboplastin released
from blood vessel walls
Factor XII (inactive)  Factor XII (active)
Factor XI (inactive)  Factor XI (active)
Factor IX (inactive)  Factor IX (active)
Antihemophilic factor B
Factor VIII
Antihemophilic factor A
Factor X (inactive)  Factor X (active)
Ca2+ ions and blood platelets
Vitamin K precursor
© 2007 Paul Billiet ODWS
AP Biology
Factor II (inactive)  Factor II (active)
Prothrombin
Thrombin
Factor I (inactive)  Factor I (active)
Fibrinogen
Fibrin
Hemophilia
 About 85% of haemophiliacs suffer from



classic haemophilia (1 male in 10 000)
cannot produce factor VIII
The rest show Christmas disease where
cannot make factor IX
Genes for hemophilia are sex linked
Hemophiliacs do clot their blood, but very
slowly (there is an alternative pathway via
thromboplastin)
AP Biology
© 2007 Paul Billiet ODWS
Hemophilia
sex-linked recessive
H Xh x X
HY
HH
XHh
male / sperm
female / eggs
XH
XH Xh
Y
XH
XH Y
Xh
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carrier
disease
Hemophilia
sex-linked recessive
H Xh x X
HY
HH
XHh
XH
female / eggs
male / sperm
XH
XH
Y
XH X H
XH Y
XH Xh
Xh
XH
Xh
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XH X h
XhY
carrier
disease
XH Y
Y
Daltonism = Red-Green Colorblindness
Normal
vision
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Colour blind simulation
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The retina
Optic
nerve
fibres
Rod
cell
Cone
cell
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LIGHT
Ganglion layer
Bipolar cells
(neurones)
Synapse layer
Nuclear layer
Inner segments packed
with mitochondria
Rod and cone
outer segments
PHOTORECEPTION
VISION
COLOUR
MONOCHROME
PHOTORECEPTOR CONES:
red sensitive 560nm
green sensitive 530nm
blue sensitive 420nm
RODS: max. sensitivity
505nm
DISTRIBUTION
Concentrated in the fovea
Widely spread over whole
retina, absent from fovea
PIGMENTS
3 proteins controlled by 3 genes.
RHODOPSIN = Retinol
Red and green pigments sex linked (Vit A) + Opsin (a protein).
Blue pigment autosomal (Chr.7)
Also called visual purple
BLEACHING
Slow
Fast (very sensitive)
REGENERATION
Slow (after images in bright light,
complementary colours)
Fast
USE
Daylight vision
Light adaptation 5 min
Night vision
Dark adaptation 20 min or
wear red goggles!
© 2007 Paul Billiet ODWS
AP
Biology
____________________________
 Female mammals inherit 2 X chromosomes

one X becomes inactivated during
embryonic development
 condenses into compact object = _____________
 which X becomes Barr body is random
 patchwork trait = “mosaic”
XH 
XH Xh
Xh
AP Biology
X-inactivation
 Female mammals inherit 2 X chromosomes

one X becomes inactivated during
embryonic development
 condenses into compact object = Barr body
Meow.
 which X becomes Barr body is random
 patchwork trait = “mosaic”
patches of black
XH 
XH Xh
tricolor cats
can only be
AP Biology
female
Xh
patches of orange
Fig. 15-8
Early embryo:
Two cell
populations
in adult cat:
Active X
X chromosomes
Allele for
orange fur
Allele for
black fur
Cell division and
X chromosome
inactivation
Black fur
AP Biology
Active X
Barr Body – inactive X
Orange fur visible in interphase
nucleus
Inactive X
Male pattern baldness
 Sex influenced trait

autosomal trait influenced by sex hormones
 age effect as well = onset after 30 years old

dominant in males & recessive in females
 B_ = bald in males; bb = bald in females
AP Biology
Environmental effects
 Phenotype is controlled by
both environment & genes
Human skin color is influenced
by both genetics &
environmental conditions
Coat color in arctic
fox influenced by
heat sensitive alleles
Color of Hydrangea flowers
APinfluenced
Biology
is
by soil pH
Any Questions?
AP Biology
2006-2007
Colorblind individuals see a yellow square.
Color normal individuals see a yellow square and a faint brown circle.
AP Biology
Colorblind individuals see a number 17.
Color normal individuals see a number 15.
AP Biology