Beyond Mendel’s Laws
of Inheritance
AP Biology
2006-2007
Rules of Probability
 Addition Rule- probability that any one
of two or more mutually exclusive
events will occur is calculated by
adding the individual probabilities.
Example- the chance of getting heads or
tails on a coin toss
heads- ½ tails – ½ ½ +1/2 = 1
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Rules of Probability
Multiplication- probability that two or
more independent events will occur
together
Ex- What is the chance that two coins
tossed together will both land heads
up?
probability of 1 head- ½
probability of 2 heads ½ x ½ = 1/4
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Probability Problems
 Chance of rolling a three on a die? 1/6
 Chance of getting any other number other



than three? 5/6
Chance of rolling a 1 and then a 2 and then
4?
1/6 x 1/6 x 1/6 = 1/216
Parents are PP and pp- chance that offspring
will be Pp? 1 x 1 = 1
Parents are PpYy x Ppyy chance that
offspring will be ppYy? Pp= ½ x ½ Yy= ½ x 1
total=1/8
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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
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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
Codominance
 There are two or more alleles that are
dominant in a phenotype.
 Both alleles are expressed in
heterozygous condition.

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Ex: Red – RR, white – WW, red and
white - RW
Multiple Alleles
 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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Polygenic inheritance
 Some phenotypes determined by
additive effects of 2 or more genes on a
single characteristic
phenotypes on a continuum
 human traits

 skin color
 height
 weight
 intelligence
 behaviors
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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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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
Pedigree Analysis
 Used to
predict
inheritance
based on past
phenotypes
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Autosomal Recessive Inheritancecystic fibrosis
Generation
skipped
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Autosomal Dominant InheritanceHuntington Disease
Generations are
not skipped
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Human disorders
 The family
pedigree
 Recessive
disorders:
•Cystic
fibrosis •TaySachs
•Sickle-cell
 Dominant
disorders:
-Huntington’s
AP Biology
Practice Question
Sickle cell anemia is caused by the mutation of a single
base pair in the gene for hemoglobin. This mutation
results in hemoglobin molecules that form long chains in
low-oxygen settings and stretch the blood cells into their
characteristic sickled shape. What would the result be if
a different base pair in the sickle cell hemoglobin gene
were to become mutated?
A. The effects of sickle cell would be increased
B. The effects of sickle cell would be reduced
C. The new mutation would increase resistance to
malaria
D. It is impossible to predict what the effect would
be without knowing more about the mutation
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Practice Problem
In the cross AaBBcc x aaBbCc, what is the
probability of an offspring that is aaBbcc?
A 1/2
B 1/4
C 1/8
D 1/16
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Practice Problem
What type of inheritance pattern is
illustrated above?
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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
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
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
 X chromosome

traits other than sex determination
 mutations: (all are recessive)
 hemophilia
 Duchenne muscular dystrophy
 color-blindness
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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
Sex Linked Traits
 Usually found on


the X chromosome
Only if she is
homozygous
recessive, will she
exhibit the trait
Expressed more
with males since
they have only one
X
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Sex Linked diseases in humans
 Duchenne muscular


dystrophy
Hemophilia
Colorblindness
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Hemophilia in the Russian Family
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Linked Genes
 Are physically close to each other and
get inherited together even with
crossing over
 The greater the distance between the
genes, the greater the chance of
separation during crossing over
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Map Units
 Morgan and Sturtevant developed the
genetic map that shows a list of
ordered loci on a chromosome
 The distance between the genes is
called a map unit
 A recombinant frequency of 1% = 1
map unit
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Example
 If the black body gene (b) is 17% or 17
map units from the vestigal wing gene
(vg) and the cinnabar eye gene (cn) is
9.75% or 9.75 map units from the b
gene, what are the order of the genes
on the chromosome?
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Cytogenic Maps
Locates genes with respect to
chromosomal features as stained
bands
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Alternative Systems
Nondisjunction
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Down Syndrome
Remember
this
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Aneuploidy- too many or too few
 Plants- are actually less serious
 Animals (one too many or too few
chromosomes), are very serious or
fatal due to gene imbalances
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XO Turner Syndrome- Monosomy
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XXY- Klinefelter Syndrome
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XYY- Jacob’s Syndrome
 Tall, thin, larger build
 More prone to acne
 Increased testosterone levels
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Genomic Imprinting
 Some traits vary depending



on which parent passes it
on
The section of DNA has a
methyl group (-CH3) added
on
Certain cancers can be
stimulated this way
Fragile X syndrome can
come from father
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Mitochondrial Defects
 These defects originate in the




mitochondrial DNA. Some examples
are:
Some forms of Diabetes
Some forms of Deafness
Multiple sclerosis
Some forms of epilepsy
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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
Human Genetic Diseases to Know
 Huntington’s (AD)
 Colorblindness

(SR)
Hemophilia (SR)
AP Biology
Cystic Fibrosis (AR)
Sickle cell anemia
(AR)
PKU (AR)
Tay Sachs (AR)
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