Chapter 14.
Mendel & Genetics
AP Biology
2005-2006
1
Gregor Mendel
 Modern genetics began in the
mid-1800s in an abbey garden,
where a monk named Gregor
Mendel documented
inheritance in peas
used experimental method
 used quantitative analysis

 collected data & counted them

AP Biology
excellent example of scientific
method
2005-2006
2
Mendel’s work
 Bred pea plants
cross-pollinated true breeding parents (P)
 raised seed & then
observed traits (F1)

 filial

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allowed offspring
to cross-pollinate
& observed next
generation (F2)
2005-2006
3
Mendel collected data for 7 pea traits
AP Biology
2005-2006
4
Looking closer at Mendel’s work
P
true-breeding
true-breeding
X
purple-flower peas
white-flower peas
F1
100%
purple-flower peas
generation
(hybrids)
100%
self-pollinate
F2
75%
25%
purple-flower peas white-flower peas
3:1
generation
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2005-2006
5
What did Mendel’s findings mean?
 Traits come in alternative versions
purple vs. white flower color
 alleles

 different alleles vary in the sequence of
nucleotides at the specific locus of a gene
purple-flower allele &
white-flower allele are
2 DNA variations at
flower-color locus
different versions of
gene on homologous
chromosomes
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2005-2006
6
Traits are inherited as discrete units
 For each characteristic, an organism
inherits 2 alleles, 1 from each parent

diploid organism
 inherits 2 sets of chromosomes,
1 from each parent
 homologous chromosomes
 like having 2 editions of encyclopedia
 Encyclopedia Britannica
 Encyclopedia Americana
What are the
advantages of
being diploid?
AP Biology
2005-2006
7
What did Mendel’s findings mean?
 Some traits mask others

purple & white flower colors are
separate traits that do not blend
 purple x white ≠ light purple
 purple masked white

dominant allele
 fully expressed

recessive allele
 no noticeable effect
 the gene makes a
non-functional protein
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2005-2006
8
Genotype vs. phenotype
 difference between how an organism
“looks” & its genetics

phenotype
 description of an organism’s trait

genotype
 description of an organism’s genetic
makeup
P
Explain Mendel’s results using
…dominant & recessive
…phenotype & gentotype
F1
AP Biology
2005-2006
9
Making crosses
 using representative letters
flower color alleles  P or p
 true-breeding purple-flower peas  PP
 true-breeding white-flower peas  pp

PP x pp
Pp
AP Biology
2005-2006
10
Looking closer at Mendel’s work
P
true-breeding
true-breeding
X
purple-flower peas
white-flower peas
PP
pp
100%
purple-flower peas
F1
generation
(hybrids)
phenotype
100%
Pp Pp Pp Pp
self-pollinate
F2
generation
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75%
purple-flower peas
?
?
?
25%
white-flower peas
?
3:1
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11
Punnett squares
Pp x Pp
%
genotype
male / sperm
P
p
PP
%
phenotype
25%
75%
Pp
female / eggs
50%
P
p
PP
Pp
Pp
pp
Pp
pp
25% 25%
1:2:1
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3:1
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12
Genotypes
 Homozygous = same alleles = PP, pp
 Heterozygous = different alleles = Pp
homozygous
dominant
homozygous
recessive
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2005-2006
13
Phenotype vs. genotype
 2 organisms can have the same
phenotype but have different genotypes
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purple PP
homozygous dominant
purple Pp
heterozygous
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14
Dominant phenotypes
 It is not possible to determine the
genotype of an organism with a
dominant phenotype by looking at it.
PP?
Pp?
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So how
do you figure out
the genotype?
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15
Test cross
 Cross-breed the dominant phenotype
— unknown genotype — with a
homozygous recessive (pp) to
determine the identity of the unknown
allele
x
is it
PP or Pp?
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pp
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16
Test cross
x
x
PP
P
pp
p
p
Pp
Pp
Pp
p
P
100%
P
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Pp
Pp
p
pp
p
Pp
Pp
50%:50%
1:1
pp
pp
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Mendel’s laws of heredity (#1)
 Law of segregation
PP
when gametes are produced
during meiosis, homologous
chromosomes separate from
each other
 each allele for a trait is
packaged into a separate
gamete
P

P
p
pp
p
P
Pp
p
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2005-2006
18
Law of Segregation
 What meiotic event
creates the
law of segregation?
And Mendel
didn’t even know
DNA or genes
existed!
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Meiosis 1
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Monohybrid cross
 Some of Mendel’s experiments
followed the inheritance of single
characters
flower color
 seed color
 monohybrid crosses

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2005-2006
20
Dihybrid cross
 Other of Mendel’s
experiments followed
the inheritance of 2
different characters
seed color and
seed shape
 dihybrid crosses

This helped Mendel
understand other
genetic “rules”
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2005-2006
21
Dihybrid cross
P
true-breeding
yellow, round peas
Y = yellow
R = round
generation
(hybrids)
F2
x
YYRR
yyrr
y = green
r = wrinkled
yellow, round peas
F1
self-pollinate
true-breeding
green, wrinkled peas
100%
YyRr
9/16
yellow
round
peas
3/16
green
round
peas
3/16
1/16
yellow
green
wrinkled wrinkled
peas
peas
9:3:3:1
generation
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2005-2006
22
What’s going on here?
 How are the alleles on different
chromosomes handed out?

together or separately?
YyRr
YR
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yr
YyRr
YR
Yr
Which system
explains the
data?
yR
yr
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Dihybrid cross
YyRr x YyRr
YR
Yr
yR
yr
YR YYRR YYRr YyRR YyRr
Yr
YyRr
Yyrr
yR YyRR YyRr yyRR
yyRr
yr
AP Biology
YYRr
YyRr
YYrr
Yyrr
yyRr
yyrr
9/16
yellow
round
3/16
green
round
3/16
yellow
wrinkled
1/16
green
wrinkled
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Can you
think
of an
exception
to this?
Mendel’s laws of heredity (#2)
 Law of independent assortment

each pair of alleles segregates into
gametes independently
 4 classes of gametes are produced
in equal amounts
 YR, Yr, yR, yr
 only true for genes on separate chromosomes
YyRr
Yr
Yr
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yR
yR
YR
YR
yr
yr
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25
Law of Independent Assortment
 What meiotic event
creates the
law of independent assortment?
Remember…
Mendel didn’t
even know DNA
—or genes—
existed!
AP Biology
Meiosis 1
2005-2006
26
The
chromosomal
basis of
Mendel’s
laws…
Trace the genetic
events through
meiosis, gamete
formation &
fertilization to
offspring
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One Option
The Other
Option
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27
Review: Mendel’s laws of heredity
 Law of segregation

monohybrid cross
 single trait

each allele segregates into separate gametes
 established by Meiosis 1
 Law of independent assortment

dihybrid (or more) cross
 2 or more traits

each pair of alleles for genes on separate
chromosomes segregates into gametes
independently
 established by Meiosis 1
AP Biology
2005-2006
28
Probability & Genetics
AP Biology
2006-2007
Genetics & Probability
 Mendel’s laws:
segregation
 independent assortment

reflect same laws of
probability that apply to
tossing coins or rolling dice
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Probability & genetics
 Calculating probability of
making a specific gamete
is just like calculating the
probability in flipping a
coin
probability of tossing
heads?
 probability making a B
gamete?
B
100%
BB
B

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B
50%
Bb
b
Probability & genetics
 Outcome of 1 toss has no
impact on the outcome of the
next toss
probability of tossing heads
each time? 50%
 probability making a B gamete
each time? 50%

B
Bb
b
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Probability
 Likelihood that a specific event will
occur
 Probability= # of one kind of possible outcome total
# of all possible outcomes
AP Biology
2005-2006
Rule of multiplication (“AND”)
 Chance that 2 or more independent
events will occur together

probability that 2 coins tossed at the
same time will land heads up
1/2 x 1/2 = 1/4

probability of Pp x Pp  pp
1/2 x 1/2 = 1/4
P
Pp
p
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Multiplication
 Question: In a Mendelian cross between pea

•
•
•
plants that are heterozygous for flower color (Pp),
what is the probability that the offspring will be
homozygous recessive?
Answer:
Probability that an egg from the F1 (Pp) will
receive a p allele = ½
Probability that a sperm from the F1 will receive a
p allele = ½
Overall probability that 2 recessive alleles will
unite at fertilization: ½ x ½ = ¼
AP Biology
2005-2006
Calculating probability in crosses
Use rule of multiplication to predict crosses
YyRr x YyRr
x
Yy
Yy
Rr
yyrr
x
?%
1/16
yy
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rr
1/4
x
1/4
Rr
Apply the Rule of Multiplication
AABbccDdEEFf
x
AaBbccDdeeFf
AabbccDdEeFF
Got it?
Try this!
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AA x Aa  Aa
Bb x Bb  bb
cc x cc  cc
Dd x Dd  Dd
EE x ee  Ee
Ff x Ff  FF
1/2
1/4
1
1/2
1
1/4
1/64
Rule of addition “OR”
 Chance that an event can occur
2 or more different ways
 sum of the separate probabilities
 probability of Bb x Bb  Bb
sperm
egg
offspring
B
b
Bb
1/2 x 1/2 =
b
B
1/2 x 1/2 =
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1/4
Bb
1/4
1/4
+ 1/4
1/2
Addition
 Question: In a Mendelian cross between pea plants that
are heterozygous for flower color (Pp), what is the
probability that the offspring will being a heterozygote?
 Answer:
• There are 2 ways in which a heterozygote may be
produced: the dominant allele may be in the egg and the
recessive allele in the sperm, or the dominant allele may
be in the sperm and the recessive allele in the egg.


Probability that the dominant allele will be in the egg with the
recessive in the sperm is ½ x ½ = ¼
Probability that the dominant allele will be in the sperm with the
recessive in the egg is ½ x ½ = ¼
 Therefore, the overall probability that a heterozygote
offspring
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will be produced is ¼ + ¼ = ½
2005-2006
Calculating probability
Pp x Pp
sperm
egg
offspring
P
P
PP
P
p
1/2 x 1/2
=
1/4
male / sperm
P
p
female / eggs
1/2 x 1/2
P
PP
Pp
p
Pp
pp
p
=
p
1/2 x 1/2
AP Biology
=
P
1/2 x 1/2
p
Pp
1/4
+
1/4
1/2
pp
=
1/4
Chi-square test
 Test to see if your data supports
your hypothesis
 Compare “observed” vs. “expected” data
is variance from expected due to
“random chance”?
 or is there another factor influencing data?

 null hypothesis
 degrees of freedom
 statistical significance
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Any Questions??
AP Biology
2006-2007
Mendel chose peas wisely
 Pea plants are good for genetic research

available in many varieties with distinct
heritable features with different variations
 flower color, seed color, seed shape, etc.

Mendel had strict control over which
plants mated with which
 each pea plant has male & female structures
 pea plants can self-fertilize
 Mendel could also cross-pollinate plants:
moving pollen from one plant to another
AP Biology
2005-2006
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Mendel chose peas luckily
 Pea plants are good for genetic research

relatively simple genetically
 most characters are controlled by
a single gene
 each gene has only 2 alleles,
one of which is completely
dominant over the other
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2005-2006
45
Any Questions??
AP Biology
2005-2006
46
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
AP Biology
2005-2006
Exceptions to Medelian Genetics
1. Incomplete Dominance
2. Codominance
3. Epistasis
4. Pleiotrophy
5. Polygenetic Inheritance
6. Multiple Alleles
7. Sex-linked traits
8. Environmental Effects
AP Biology
2005-2006
The Spectrum of Dominance
Diseases/Disorders are not just the result of the
presence of the dominant allele
•Tay-Sachs disease: homozygous recessive
•Polydactyly: dominant allele that is more
common than the allele for 5 digits (399/400
people are recessive to this disorder)
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2005-2006
1) Incomplete dominance
 Heterozygotes show an intermediate
phenotype
RR = red flowers
 rr = white flowers
 Rr = pink flowers

 make 50% less color
AP Biology
2005-2006
Incomplete dominance
P
X
true-breeding
red flowers
true-breeding
white flowers
100% pink flowers
F1
100%
generation
(hybrids)
self-pollinate
25%
red
50%
pink
25%
white
1:2:1
F2
generation
AP Biology
2005-2006
Incomplete dominance
CRCW x C RCW
%
genotype
female / eggs
male / sperm
CR
CW
CR
CW
CRCR
CRCW
CRCR
CRCW
25% 25%
50% 50%
CRCW
CRC W
C WC W
C WC W
25% 25%
1:2:1
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%
phenotype
1:2:1
2005-2006
2) Codominance
When both alleles affect the phenotype in
separate, distinguishable ways
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2005-2006
3) Epistasis
 Many genes
controlling one
phenotype or the gene
at one locus
controlling the
expression of a gene
at another locus

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Consider the
biochemical cascade
ABCD. If a single
mutation occurs in A,B,
or C, the product D will
be effected.
2005-2006
4) Pleiotrophy
 One gene controlling many phenotypes
(essentially the opposite of epistasis)
The agouti gene in mice is linked to coat
color, obesity, and certain tumors.
 Cystic fibrosis
 Sickle cell disease

AP Biology
2005-2006
5) Polygenetic Inheritance
 Many genes
controlling one
phenotype with an
additive effect

Laborador Retreiver
colors are polygenic
AABBCC = Black Lab
AaBbCc = Lighter
Aabbcc= Lightest
AP Biology
2005-2006
6) Multiple Alleles
2+ alleles control a trait
 ABO blood groups
 3 alleles

 IA, IB, i
 both IA & IB are dominant to i allele
 IA & IB alleles are co-dominant to each other

determines presences of
oligosaccharides on the
surface of red blood cells
AP Biology
2005-2006
Blood type Video
genotype
phenotype
phenotype
status
IA IA
IA i type A
type A
oligosaccharides on
surface of RBC
IB IB
IB i
type B
type B
oligosaccharides on
surface of RBC
__
type AB
both type A & type B
oligosaccharides on
surface of RBC
universal
recipient
type O
no oligosaccharides
on surface of RBC
universal
donor
IA IB
ii
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__
2005-2006
Blood compatibility
1901 | 1930
 Matching compatible blood groups
critical for blood transfusions
A person produces antibodies against
oligosaccharides in foreign blood
 wrong blood type


Karl Landsteiner
(1868-1943)
 donor’s blood has A or B oligosaccharide
that is foreign to recipient
 antibodies in recipient’s blood bind to
foreign molecules
 cause donated blood cells to clump together
 can kill the recipient
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2005-2006
A “foreign” antigen will trigger
your immune system to create
antibodies.
Since antibodies are proteins,
they have a specific shape & will
attach onto the antigen for which
they were made.
There are bacteria in the
atmosphere that have a very
similar shape to our “A” and
“B” antigens & we are exposed
to them as soon as we are born.
Therefore, each person can
have antibodies against an
antigen they didn’t inherit.
These antibodies are floating
around in our blood with our
RBC’s.
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2005-2006
Blood donation
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2005-2006
The “Rh Issue”… Mom = Rh-
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Baby #1 = Rh+
9) Sex-linked traits
 The genes of interest are located on the
sex chromosomes.
 Sex linked traits are carried on the X
chromosome as no genes shared by
both male and female can be carried on
the Y chromosome.

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Hemophilia
2005-2006
10) Environmental Factors
 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
2005-2006
14.4
AP Biology
2005-2006
Pedigree Analysis
 Information about a family’s history for
a particular trait assembled into a tree
showing the interrelationships of
parents and children across the
generations
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2005-2006
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2005-2006
Genetic Disorders
 Recessive
Cystic Fibrosis
 Sickle Cell Disease

 Dominant
Achondroplasia
 Huntington’s Disease

AP Biology
2005-2006
Genetic Testing
 Amniocentesis- 14-16
weeks of pregnancy,
needle into uterus to
obtain amniotic fluid
 Chorionic Villus
Sampling- CVS- 8 weeks,
narrow tube through
cervix into uterus to
obtain a small sample of
placenta tissue
 Ultrasound- imaging
technique to examine
anatomical abnormalities
AP Biology
2005-2006