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LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
Chapter 14
Mendel and the Gene Idea
Lectures by
Erin Barley
Kathleen Fitzpatrick
© 2011 Pearson Education, Inc.
Overview: Drawing from the Deck of
Genes
• The “blending” hypothesis
– genetic material from the two parents blends
together
• The “particulate” hypothesis
– parents pass on discrete heritable units
– Traits reappear after several generations
• Mendel used experiments with garden peas
Figure 14.1
Concept 14.1: Mendel identified two
laws of inheritance
• Discovered the
basic principles of
heredity by breeding
garden peas
Mendel’s Experimental, Quantitative
Approach
• Advantages of pea plants for genetic study
There are many varieties of heritable
features(characters)
Character variants (Traits)
Controlled Mating
Individual sex organs
Cross-pollination
Figure 14.2
TECHNIQUE
1
2
Parental
generation
(P)
3
Stamens
Carpel
4
RESULTS
First filial
generation
offspring
(F1)
5
What Mendel Used
• Mendel chose to track only those characters
that occurred in two distinct alternative forms
• True-breeding (plants that produce offspring
of the same variety when they self-pollinate)
Terms of the Experiment
• Hybridization: mating two contrasting, truebreeding varieties
• P generation: true-breeding parents
• F1 generation: hybrid offspring of the P
generation
• F2 generation: F1 individuals self-pollinate or
cross- pollinate with other F1 hybrids
The Law of Segregation
• Crossing contrasting true-breeding white
and purple pea plants: all of the F1 hybrids
were purple
• Crossing the F1 hybrids: many of the F2
plants had purple flowers, but some had
white
• Three to one ratio purple to white flowers in
the F2 generation
Figure 14.3-3
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had purple flowers
Self- or cross-pollination
F2 Generation
705 purpleflowered
plants
224 white
flowered
plants
Ideas that stemmed from observations
• Only the purple flower factor was affecting
flower color in the F1 hybrids
• purple flower color: dominant trait
• white flower color: recessive trait
• Six other pea plant characters had same
patterns
• “Heritable factor”: we now call a gene
Mendel’s Model
• Alternative versions of genes: account for
variations in inherited characters
• Alternative versions of a gene are now called
alleles
• Each gene resides at a specific locus
Figure 14.4
Allele for purple flowers
Locus for flower-color gene
Pair of
homologous
chromosomes
Allele for white flowers
Mendel’s Model
• Organisms inherits two alleles, one from each
parent
• Dominant alleles determine the organism’s
appearance
• Recessive alleles have no noticeable effect
on appearance
• Law of segregation: two alleles for a
heritable character separate during gamete
formation
Punnett Square
• Diagram for predicting the results of a genetic
cross between individuals of known genetic
makeup
• A capital letter represents a dominant allele
• A lowercase letter represents a recessive
allele
Figure 14.5-3
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
pp
PP
p
Gametes:
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
1/
2 p
2 P
Sperm from F1 (Pp) plant
F2 Generation
P
Eggs from
F1 (Pp) plant
p
3
P
p
PP
Pp
Pp
pp
:1
Useful Genetic Vocabulary
• Homozygous: An organism with two identical
alleles for a character
• Heterozygous: An organism that has two
different alleles for a gene
– Unlike homozygotes, heterozygotes are not
true-breeding
• Phenotype: physical appearance
• Genotype: genetic makeup
Figure 14.6
3
Phenotype
Genotype
Purple
PP
(homozygous)
Purple
Pp
(heterozygous)
1
2
1
Purple
Pp
(heterozygous)
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
1
The Testcross
• How can we tell the genotype of an individual
with the dominant phenotype?
• Could be either homozygous dominant or
heterozygous
Figure 14.7
TECHNIQUE
Dominant phenotype,
unknown genotype:
PP or Pp?
Predictions
If purple-flowered
parent is PP
Sperm
p
p
Recessive phenotype,
known genotype:
pp
or
If purple-flowered
parent is Pp
Sperm
p
p
P
Pp
Eggs
P
Pp
Eggs
P
p
Pp
Pp
Pp
Pp
pp
pp
RESULTS
or
All offspring purple
1/
2
offspring purple and
1/ offspring white
2
The Law of Independent Assortment
• Followed a single character
• Monohybrids: individuals that are
heterozygous for one character
• A cross between such heterozygotes is called
a monohybrid cross
• Dihybrids: Two true-breeding parents
differing in two characters produces
• A dihybrid cross, a cross between F1
dihybrids
Figure 14.8
EXPERIMENT
YYRR
P Generation
yyrr
yr
Gametes YR
F1 Generation
Predictions
YyRr
Hypothesis of
dependent assortment
Hypothesis of
independent assortment
Sperm
or
Predicted
offspring of
F2 generation
1/
Sperm
1/
2
YR
1/
2
2
YR
YyRr
YYRR
Eggs
1/
2
1/
4
YR
4
Yr
4
yR
4
yr
Eggs
yr
YyRr
3/
yyrr
1/
4
YR
1/
4
1/
Yr
4
yR
1/
4
yr
yr
1/
1/
4
1/
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
4
Phenotypic ratio 3:1
1/
9/
16
3/
16
3/
16
1/
16
Phenotypic ratio 9:3:3:1
RESULTS
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
Law of independent assortment
• Pairs of alleles
segregate
independently of
each other pair of
alleles during
gamete formation
Figure 14.9
Rr
Segregation of
alleles into eggs

Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
2
Eggs
4
r
2
r
R
R
1/
1/
r
2
R
R
1/
1/
1/
4
r
r
R
r
1/
4
1/
4
The Multiplication and Addition Rules
Applied to Monohybrid Crosses
• probability that two or more independent
events will occur together is the product of
their individual probabilities
• Each gamete has a ½ chance of carrying the
dominant allele and a ½ chance of carrying
the recessive allele
The addition rule
• Probability that any one of two or more
exclusive events will occur is calculated by
adding together their individual probabilities
• The rule of addition can be used to figure out
the probability that an F2 plant from a
monohybrid cross will be heterozygous rather
than homozygous
Probability of YYRR  1/4 (probability of YY)  1/4 (RR)  1/16
Probability of YyRR  1/2 (Yy)
ppyyRr
ppYyrr
Ppyyrr
PPyyrr
ppyyrr
 1/4 (RR)  1/8
1/ (yy)  1/ (Rr)
(probability
of
pp)

4
2
2
1/  1/  1/
4
2
2
1/  1/  1/
2
2
2
1/  1/  1/
4
2
2
1/  1/  1/
4
2
2
1/
Chance of at least two recessive traits
 1/16
 1/16
 2/16
 1/16
 1/16
 6/16 or 3/8
Extending Mendelian Genetics for a
Single Gene
• Inheritance of characters by a single gene
may deviate from simple Mendelian patterns
in the following situations:
– When alleles are not completely dominant or
recessive
– When a gene has more than two alleles
– When a gene produces multiple phenotypes
Degrees of Dominance
• Complete dominance heterozygote and
dominant homozygote phenotypes are
identical
• Incomplete dominance F1 hybrids are
somewhere between the phenotypes of the
two parental varieties
• Codominance two dominant alleles affect the
phenotype in separate, distinguishable ways
Figure 14.10-3
P Generation
White
CWCW
Red
CRCR
CR
Gametes
CW
F1 Generation
Pink
CRCW
1/
Gametes 1/2 CR
2
CW
Sperm
F2 Generation
1/
2
CR
1/
2
CW
Eggs
1/
2
CR
1/
2
CW
CRCR CRCW
CRCW CWCW
Dominance and Phenotype
• A dominant allele does not subdue a
recessive allele; alleles don’t interact that way
• Alleles are simply variations in a gene’s
nucleotide sequence
Frequency of Dominant Alleles
• Dominant alleles are not necessarily more
common
• 1/400 babies in the US is born with extra
fingers or toes
Multiple Alleles
• Four phenotypes of
the ABO blood
group in humans are
determined by three
alleles for the
enzyme (I) that
attaches A or B
carbohydrates to red
blood cells: IA, IB,
and i.
Figure 14.11
(a) The three alleles for the ABO blood groups and their
carbohydrates
IA
Allele
Carbohydrate
IB
i
none
B
A
(b) Blood group genotypes and phenotypes
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
A
B
AB
O
Red blood cell
appearance
Phenotype
(blood group)
Pleiotropy
• Most genes have multiple phenotypic effects,
a property called pleiotropy
• For example, pleiotropic alleles are
responsible for the multiple symptoms of
certain hereditary diseases, such as cystic
fibrosis and sickle-cell disease
Epistasis
• A gene at one locus alters the phenotypic
expression of a gene at a second locus
• Coat color of certain animals depend on two
genes
• One gene determines the pigment color (with
alleles B for black and b for brown)
• The other gene (with alleles C for color and c
for no color) determines whether the pigment
will be deposited in the hair
Figure 14.12
BbEe
Eggs
1/
4 BE
1/
4 bE
1/
4 Be
1/
4
be
Sperm
1/ BE
4
1/
BbEe
4 bE
1/
4 Be
1/
4 be
BBEE
BbEE
BBEe
BbEe
BbEE
bbEE
BbEe
bbEe
BBEe
BbEe
BBee
Bbee
BbEe
bbEe
Bbee
bbee
9
: 3
: 4
Polygenic Inheritance
• Quantitative characters: vary in the
population along a continuum
• Quantitative variation usually indicates
polygenic inheritance
– an additive effect of two or more genes on a
single phenotype
• Skin color in humans is an example of
polygenic inheritance
Figure 14.13
AaBbCc
AaBbCc
Sperm
1/
1/
8
8
1/
1/
Eggs
8
1/
1/
8
8
1/
8
1/
1/
8
8
8
8
1/
8
1/
8
1/
1/
8
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
The effect of environment on
phenotype
Integrating a Mendelian View of
Heredity and Variation
• An organism’s phenotype includes its
physical appearance, internal anatomy,
physiology, and behavior
• An organism’s phenotype reflects its overall
genotype and unique environmental history
Pedigree Analysis
• A pedigree is a family tree that describes the
interrelationships of parents and children
across generations
• Inheritance patterns of particular traits can be
traced and described using pedigrees
Figure 14.15
Key
Male
1st
generation
Affected
female
Affected
male
Female
Mating
1st
generation
Ww
ww
ww
Ww
2nd
generation
Ww
ww
3rd
generation
WW
or
Ww
Widow’s
peak
ff
ff
(a) Is a widow’s peak a dominant or
recessive trait?
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd
generation
ww
No widow’s
peak
ff
Ff
2nd
generation
FF or Ff
Ww ww ww Ww
Ff
Offspring
Attached
earlobe
Free
earlobe
b) Is an attached earlobe a dominant
or recessive trait?
Pedigrees
• Can also be used to make predictions about
future offspring
• Can use the multiplication and addition rules
to predict the probability of specific
phenotypes
Recessively Inherited Disorders
• Many genetic disorders are inherited in a
recessive manner
• These range from relatively mild to lifethreatening
• Recessively inherited disorders show up only
in individuals homozygous for the allele
• Carriers are heterozygous individuals who
carry the recessive allele
Figure 14.16
Parents
Normal
Aa
Normal
Aa
Sperm
A
a
A
AA
Normal
Aa
Normal
(carrier)
a
Aa
Normal
(carrier)
aa
Albino
Eggs
Dominantly Inherited Disorders
• Dominant alleles
that cause a lethal
disease are rare and
arise by mutation
• Achondroplasia is a
form of dwarfism
caused by a rare
dominant allele
Figure 14.17
Parents
Dwarf
Dd
Normal
dd
Sperm
D
d
d
Dd
Dwarf
dd
Normal
d
Dd
Dwarf
dd
Normal
Eggs
Multifactorial Disorders
• Heart disease,
diabetes,
alcoholism, mental
illnesses, and
cancer have both
genetic and
environmental
components
Fetal Testing
• In amniocentesis, the liquid that bathes the
fetus is removed and tested
• In chorionic villus sampling (CVS), a
sample of the placenta is removed and tested
• Other techniques, such as ultrasound and
fetoscopy, allow fetal health to be assessed
visually in utero
Figure 14.19
(a) Amniocentesis
1
(b) Chorionic villus sampling (CVS)
Ultrasound monitor
Amniotic
fluid
withdrawn
Ultrasound
monitor
Fetus
1
Placenta
Chorionic villi
Fetus
Placenta
Uterus
Cervix
Cervix
Uterus
Suction
tube
inserted
through
cervix
Centrifugation
Fluid
Fetal
cells
Several hours
2
Several
weeks
Biochemical
and genetic
tests
Several
hours
Fetal cells
2
Several hours
Several weeks
3
Karyotyping
Newborn Screening
• Some genetic
disorders can be
detected at birth by
simple tests that are
now routinely
performed in most
hospitals in the
United States