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Beyond Mendel’s Laws
of Inheritance (14 continued) and 15
AP Biology
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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Incomplete dominance
 Heterozygote shows an intermediate
phenotype

example:
 RR = red flowers RR
 rr = white flowers WW
 Rr = pink flowers RW
 make 50% less color
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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
1:2:1
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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Figure 8.11 ABO Blood Reactions
Are Important in Transfusions
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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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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
Extending Mendelian Genetics for Two or
More Genes
 Some traits may be determined by two or
more genes
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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
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–
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
Figure 14.13
AaBbCc
AaBbCc
Sperm
1/
1/
8
8
1/
1/
1/
1/
8
8
1/
8
1/
1/
8
8
8
8
1/
8
/8
1/
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8
8
1/
Eggs 1
1/
8
1/
8
1/
8
Phenotypes:
Number of
dark-skin alleles:
1/
64
0
6/
64
1
15/
64
2
20/
64
3
15/
64
4
6/
64
5
1/
64
6
Figure 14.UN03
Relationship among
alleles of a single gene
Complete dominance
of one allele
Description
Heterozygous phenotype
same as that of homozygous dominant
Incomplete dominance Heterozygous phenotype
intermediate between
of either allele
the two homozygous
phenotypes
Codominance
Both phenotypes
expressed in
heterozygotes
Example
PP
Pp
CRCR
CRCW CWCW
IAIB
Multiple alleles
In the whole population,
some genes have more
than two alleles
Pleiotropy
One gene is able to affect Sickle-cell disease
multiple phenotypic
characters
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ABO blood group alleles
IA, IB, i
Figure 14.UN04
Relationship among
two or more genes
Epistasis
Description
The phenotypic
expression of one
gene affects that
of another
Example
BbEe
BE
BbEe
bE
Be
be
BE
bE
Be
be
9
Polygenic inheritance
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A single phenotypic
character is affected
by two or more genes
AaBbCc
:3
:4
AaBbCc
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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Discovery of sex linkage
P
F1
true-breeding
red-eye female
X
true-breeding
white-eye male
100%
red eye offspring
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
x
rr
Rr
r
Rr
R
r
R
Rr
Rr
R
RR
Rr
R
Rr
Rr
r
Rr
rr
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100% red eyes
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
XR XR
Xr
XR
XR
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XR Xr
XR Xr
x
XrY
Y
XRY
XRY
100% red eyes

XR
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
 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
 153 million bp

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Chronic granulomatous disease
Retinitis pigmentosa-3
Norrie disease
Retinitis pigmentosa-2
Ichthyosis, X-linked
Placental steroid sulfatase deficiency
Kallmann syndrome
Chondrodysplasia punctata,
X-linked recessive
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
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?
< 70 genes on
Y chromosome
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sex-linked recessive
Hemophilia
H Xh x X
HY
HH
XHh
XH
female / eggs
male / sperm
XH
XH
Y
XH XH
XH Y
XH Xh
Xh
XH
Xh
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XH Xh
XhY
carrier
disease
XHY
Y
Concept 15.3: Linked genes tend to be
inherited together because they are located
near each other on the same chromosome
 Each chromosome has hundreds or
thousands of genes (except the Y
chromosome)
 Genes located on the same chromosome
that tend to be inherited together are
called linked genes
AP Biology
© 2011 Pearson Education, Inc.
Figure 15.9-4
EXPERIMENT
P Generation (homozygous)
Double mutant
(black body,
vestigial wings)
Wild type
(gray body, normal wings)
b b vg vg
b b vg vg
F1 dihybrid
(wild type)
Double mutant
TESTCROSS
b b vg vg
b b vg vg
Testcross
offspring
Eggs b vg
b vg
Wild type
Black(gray-normal) vestigial
b vg
b vg
Grayvestigial
Blacknormal
b b vg vg
b b vg vg
b vg
Sperm
b b vg vg
b b vg vg
PREDICTED RATIOS
If genes are located on different chromosomes:
1
:
1
:
1
:
1
If genes are located on the same chromosome and
parental alleles are always inherited together:
1
:
1
:
0
:
0
965
:
944
:
206
:
185
RESULTS
AP
Biology
Figure 15.10
Black body, vestigial wings
(double mutant)
Gray body, normal wings
(F1 dihybrid)
Testcross
parents
b vg
b vg
b vg
b vg
Replication
of chromosomes
Meiosis I
Replication
of chromosomes
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
Meiosis I and II
b vg
b vg
b vg
Meiosis II
Recombinant
chromosomes
bvg
b vg
b vg
b vg
Eggs
Testcross
offspring
965
Wild type
(gray-normal)
944
Blackvestigial
185
Blacknormal
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
Parental-type offspring
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206
Grayvestigial
Recombinant offspring
391 recombinants
Recombination

 100  17%
frequency
2,300 total offspring
b vg
Sperm
Figure 15.11
Mapping the Distance Between Genes Using Recombination Data:
RESULTS
Recombination
frequencies
9%
Chromosome
17%
b
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9.5%
cn
vg
Alterations of Chromosome Structure
Breakage of a
chromosome
can lead to four
types of
changes in
chromosome
structure
Normal chromosome 9
Normal chromosome 22
Reciprocal translocation
(a) Deletion
A B C D E
F G
H
A deletion removes a chromosomal segment.
A B C
E
F G
H
(b) Duplication
A B C D E
F G
H
A duplication repeats a segment.
A B C
B C D E
F G
H
(c) Inversion
A B C D E
F G
H
An inversion reverses a segment within a
chromosome.
A D C B E
F G
H
(d) Translocation
Translocated chromosome 9
Translocated chromosome 22
(Philadelphia chromosome)
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A B C D E
F G
H
M N O
P Q
R
A translocation moves a segment from one
chromosome to a nonhomologous chromosome.
M N O C D E
F
H
A BG P Q
R
Figure 1 : Chromosome
translocations.
a) An idiogram of a
reciprocal translocation
between chromosomes
12 and 17. b) An
ideogram of a
Robertsonian
translocation between
chromosomes 14 and 21.
Braude, P. et al.
Preimplantation genetic
diagnosis. Nature Reviews
Genetics 3:(12) 941-953,
doi:10.1038/nrg953 (2002).
All rights reserved.
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Environmental effects
 Phenotype is controlled by
both environment & genes
Human skin color is influenced
by both genetics &
environmental conditions
Basic pH
Acidic pH
Color of Hydrangea flowers
APinfluenced
Biology
is
by soil pH
Coat color in arctic
fox influenced by
heat sensitive alleles
MELANOCYTES
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How melanin is produced
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Bacterial Conjugation and Recombination
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Gene Transfer by Plasmids
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GHOSTS
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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
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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
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
X-inactivation
 Female mammals inherit 2 X chromosomes

one X becomes inactivated during
embryonic development
 condenses into compact object = Barr body
 which X becomes Barr body is random
 patchwork trait = “mosaic”
patches of black
XH 
XH Xh
tricolor cats
can only be
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female
Xh
patches of orange