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Chapter 14
Mendel and the Gene Idea
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: Drawing from the Deck of Genes
• What genetic principles account for the transmission of traits
from parents to offspring?
•Why do you share some but not all characters of each
parent?
• What are the rules of this sharing game?
At one level, the
answers lie in
meiosis.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Is inheritance “blended”?
• One possible explanation of heredity is a
“blending” hypothesis
– The idea that genetic material contributed by
two parents mixes in a manner analogous to
the way blue and yellow paints blend to make
green
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Is inheritance “particulate”?
• An alternative to the blending model is the
“particulate” hypothesis of inheritance: the
gene idea.
– Parents pass on discrete heritable units,
“genes”
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Gregor Mendel
Documented a particulate mechanism of
inheritance through his experiments with
garden peas
Figure 14.1
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Mendel’s experiments
• Concept 14.1: Mendel
used the scientific
approach to identify two
laws of inheritance
• Mendel discovered the
basic principles of heredity
by breeding garden peas
in carefully planned
experiments
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Mendel’s Experimental, Quantitative Approach
• Mendel chose to work with peas
– Because they are available in many varieties
– Because he could strictly control which plants
mated with which
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Crossing Pea Plants
1
APPLICATION By crossing (mating) two true-breeding
varieties of an organism, scientists can study patterns of
inheritance. In this example, Mendel crossed pea plants
that varied in flower color.
TECHNIQUE
Removed stamens
from purple flower
2 Transferred sperm-
bearing pollen from
stamens of white
flower to eggbearing carpel of
purple flower
Parental
generation
(P)
3 Pollinated carpel
Stamens
Carpel (male)
(female)
matured into pod
4 Planted seeds
from pod
When pollen from a white flower fertilizes
TECHNIQUE
RESULTS
eggs of a purple flower, the first-generation hybrids all have purple
flowers. The result is the same for the reciprocal cross, the transfer
of pollen from purple flowers to white flowers.
Figure 14.2
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5 Examined
First
generation
offspring
(F1)
offspring:
all purple
flowers
Some genetic vocabulary
– Character: a heritable
feature, such as flower
color.
– Trait: a variant of a
character, such as purple
or white flowers.
– Gene: the basic unit of
heredity in a living
organism. A section of a
chromosome that codes
for a trait, or
characteristic.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Mendel’s experiments : Controls
• Wisely, Mendel chose to track only those
characters that varied in an “either-or” manner:
- dominant or recessive, no blended
characteristics, no polygenic traits
• Mendel also made sure that:
– He started his experiments with varieties that
were “true-breeding” (purebred lines isolated
through self-pollination)
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In a typical breeding experiment
– Mendel mated two contrasting, true-breeding
varieties, a process called hybridization
• The true-breeding parents
– Are called the P generation
The hybrid offspring of the P
generation
Are called the F1 generation
When F1 individuals self-pollinate
The F2 generation is produced
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The Law of Segregation
• When Mendel
crossed contrasting,
true-breeding white
and purple flowered
pea plants
– ALL of the
offspring were
purple. Why?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
…but when Mendel crossed the F1 plants:
– Many of the F2 plants had purple flowers, but some
had white flowers
– He found a repeatable ratio of about three to one,
purple to white flowers, in the F2 generation
EXPERIMENT True-breeding purple-flowered pea plants and
white-flowered pea plants were crossed (symbolized by ). The
resulting F1 hybrids were allowed to self-pollinate or were crosspollinated with other F1 hybrids. Flower color was then observed
in the F2 generation.
P Generation
(true-breeding
parents)

Purple
flowers
White
flowers
F1 Generation
(hybrids)
RESULTS Both purple-flowered plants and whiteflowered plants appeared in the F2 generation. In Mendel’s
experiment, 705 plants had purple flowers, and 224 had white
flowers, a ratio of about 3 purple : 1 white.
Figure 14.3
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All plants had
purple flowers
F2 Generation
Mendel’s “Factors” (genes)
• Mendel reasoned that
– In the F1 plants, only the purple flower “factor“
was affecting flower color in these hybrids
– Purple flower color was dominant, and white
flower color was recessive
In such cases,
the dominant
allele MASKS
the recessive
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Mendel observed the
same pattern in many
other pea plant
characters 
• Mendel developed a
hypothesis to explain
the 3:1 inheritance
pattern that he observed
among the F2 offspring
• Four related concepts
make up this model…
Table 14.1
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Alleles
• First, alternative versions of genes
– Account for variations in inherited characters,
which are now called alleles
Allele for purple flowers
Locus for flower-color gene
Figure 14.4
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Allele for white flowers
Homologous
pair of
chromosomes
• Second, for each character
– An organism inherits two alleles, one from
each parent
– A genetic locus is actually represented twice
Homologous
chromosomes,
one paternal, one
maternal 
Figure 14.4
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Allele for purple flowers
Locus for flower-color gene
Allele for white flowers
Homologous
pair of
chromosomes
Dominants and Recessives
• Third, if the two alleles at a locus differ
– Then one, the dominant allele, determines
the organism’s appearance
– The other allele, the recessive allele, has no
noticeable effect on the organism’s
appearance (is “masked”)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Vocabulary
Allele: different forms of
a gene (for a trait).
Homozygous: purebred;
identical set of alleles
for a trait
Heterozygous: hybrid;
nonmatching alleles
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Segregation principle
• Fourth, the law of segregation
– The two alleles for a heritable character
separate (segregate) during gamete formation
and end up in different gametes
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• Does Mendel’s segregation model account for
the 3:1 ratio he observed in the F2 generation
of his numerous crosses?
– We can answer this question using a Punnett
square
The Punnett square is a diagram that
is used to predict an outcome of a
particular cross or breeding experiment.
It is named after Reginald C. Punnett,
who devised the approach, and is used
by biologists to determine the probability
of an offspring having a particular
genotype.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Mendel’s law of segregation, probability and
the Punnett square
Each true-breeding plant of the
parental generation has identical
alleles, PP or pp.
Gametes (circles) each contain only
one allele for the flower-color gene.
In this case, every gamete produced
by one parent has the same allele.
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
PP
pp
Gametes:
p
P
Union of the parental gametes
produces F1 hybrids having a Pp
combination. Because the purpleflower allele is dominant, all
these hybrids have purple flowers.
F1 Generation
When the hybrid plants produce
gametes, the two alleles segregate,
half the gametes receiving the P
allele and the other half the p allele.
Gametes:
This box, a Punnett square, shows
all possible combinations of alleles
in offspring that result from an
F1  F1 (Pp  Pp) cross. Each square
represents an equally probable product
of fertilization. For example, the bottom
left box shows the genetic combination
resulting from a p egg fertilized by
a P sperm.

Appearance:
Genetic makeup:
Purple flowers
Pp
1/
1/
2 P
F1 sperm
P
p
PP
Pp
F2 Generation
P
F1 eggs
p
pp
Pp
Figure 14.5
Random combination of the gametes
results in the 3:1 ratio that Mendel
observed in the F2 generation.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
2 p
3
:1
Useful Genetic Vocabulary
• An organism that is homozygous for a
particular gene
– Has a pair of identical alleles for that gene
– Exhibits true-breeding
• An organism that is heterozygous for a
particular gene
– Has a pair of alleles that are different for that
gene (hybrid)
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• An organism’s phenotype
– Is its physical appearance
• An organism’s genotype
– Is its genetic makeup
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Phenotype versus genotype
Phenotype
Purple
3
Purple
Genotype
PP
(homozygous)
1
Pp
(heterozygous)
2
Pp
(heterozygous)
Purple
1
Figure 14.6
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
• In Mendel’s pea plants with purple flowers
– The genotype is not immediately obvious
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1
A testcross…
– Allows us to determine the genotype of an
organism with the dominant phenotype, but
unknown genotype
– Crosses an individual with the dominant
phenotype with an individual that is
homozygous recessive for a trait
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Punnett Squares as probability predictors
• The testcross
APPLICATION An organism that exhibits a dominant trait,
such as purple flowers in pea plants, can be either homozygous for
the dominant allele or heterozygous. To determine the organism’s
genotype, geneticists can perform a testcross.

TECHNIQUE In a testcross, the individual with the
unknown genotype is crossed with a homozygous individual
expressing the recessive trait (white flowers in this example).
By observing the phenotypes of the offspring resulting from this
cross, we can deduce the genotype of the purple-flowered
parent.
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype:
pp
If PP,
then all offspring
purple:
If Pp,
then 2 offspring purple
and 1⁄2 offspring white:
p
1⁄
p
p
p
Pp
Pp
pp
pp
RESULTS
P
P
Pp
Pp
P
p
Pp
Figure 14.7
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Pp
• Probability in a monohybrid cross
– Can be determined using this rule
Rr
Rr

Segregation of
alleles into eggs
Segregation of
alleles into sperm
Sperm
1⁄
R
2
1⁄
Eggs
r
1⁄
2
r
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
1⁄
4
R
1⁄
Figure 14.9
r
R
R
2
r
2
R
R
1⁄
1⁄
4
r
4
r
1⁄
4
The Multiplication and Addition Rules Applied to
Monohybrid Crosses
• Think of the rule of addition as when two events are not
dependent on each other such as:
• ex: whether a coin lands on heads OR a dice lands on 6
= 1/2 + 1/6
• then the rule of multiplication would be when two events
are dependent on each other
• ex: whether a coin lands on heads AND a dice lands on 6
= 1/2 *1/6
Try this: what is the probability of flipping a coin TWICE and getting two
heads in a row?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Solving Complex Genetics Problems with the Rules
of Probability
• We can also apply the rules of probability to
predict the outcome of crosses involving
multiple characters
– Dihybrid crosses
– Trihybrid crosses
• Must remember that alleles will sort
independently when gametes are made!
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Dihybrid Crosses
• Mendel identified his second law of inheritance by
following two characters at the same time
• Crossing two, true-breeding parents differing in two
characters produces dihybrids in the F1 generation,
heterozygous for both characters
• How are two characters transmitted from parents to
offspring?
– As a package?
– Independently?
– Mendel’s laws of segregation and independent
assortment reflect the rules of probability
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• A dihybrid cross
– Illustrates the inheritance of two characters
• Produces four phenotypes in the F2 generation
EXPERIMENT Two true-breeding pea plants—
one with yellow-round seeds and the other with
green-wrinkled seeds—were crossed, producing
dihybrid F1 plants. Self-pollination of the F1 dihybrids,
which are heterozygous for both characters,
produced the F2 generation. The two hypotheses
predict different phenotypic ratios. Note that yellow
color (Y) and round shape (R) are dominant.
P Generation
YYRR
yyrr
Gametes
F1 Generation
YR

Hypothesis of
dependent
assortment
yr
YyRr
Hypothesis of
independent
assortment
Sperm
1⁄ YR
2
RESULTS
CONCLUSION The results support the hypothesis of
independent assortment. The alleles for seed color and seed
shape sort into gametes independently of each other.
Sperm
yr
1⁄
2
Eggs
1
F2 Generation ⁄2 YR YYRR YyRr
(predicted
offspring)
1 ⁄ yr
2
YyRr yyrr
3⁄
4
1⁄
4
1⁄
4
Yr
1⁄
4
yR
1⁄
4
yr
Eggs
1 ⁄ YR
4
1⁄
4
Yr
1⁄
4
yR
1⁄
4
yr
1⁄
4
Phenotypic ratio 3:1
YR
9⁄
16
YYRR YYRr YyRR YyRr
YYrr
YYrr YyRr
Yyrr
YyRR YyRr yyRR yyRr
YyRr
3⁄
16
Yyrr
yyRr
3⁄
16
yyrr
1⁄
16
Phenotypic ratio 9:3:3:1
Figure 14.8
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
Independent Assortment Principle
• Using the information from a dihybrid cross,
Mendel developed the law of independent
assortment
– Each pair of alleles segregates independently
during gamete formation
– Animated version:
http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• A dihybrid or other multicharacter cross
– Is equivalent to two or more independent
monohybrid crosses occurring simultaneously
• In calculating the chances for various
genotypes from such crosses
– Each character first is considered separately
and then the individual probabilities are
multiplied together
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The reality of inheritance
• Concept 14.3: Inheritance patterns are often
more complex than predicted by simple
Mendelian genetics
• The relationship between genotype and
phenotype is rarely simple
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Extending Mendelian Genetics for a Single Gene
• The inheritance of characters by a single gene
– May deviate from simple Mendelian patterns
= “exceptions” to the “rules” (listed in the following
slides)
Try these “drag and drop” Punnett squares
http://www.execulink.com/~ekimmel/mendel1a.htm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The extent of variation
• Not only does meiosis guarantee segregation and
independent assortment of alleles, but crossing over also
mixes up alleles on homologous chromosomes before
distribution
“Therefore, in humans with 23 pairs of chromosomes, a gamete (egg or
sperm) could have 223 or 8,388,604 possible combinations of
chromosomes from that parent. Any couple could have 223 × 223 or
70,368,744,177,644 (70 trillion) different possible children, based just on
the number of chromosomes, not considering the actual genes on those
chromosomes.
Thus, the chance of two siblings being exactly identical would be 1 in 70
trillion.
In addition, something called crossing over, in which the two
homologous chromosomes of a pair exchange equal segments during
synapsis in Meiosis I, can add further variation to an individual’s genetic
make-up.”
http://www.biology.clc.uc.edu/Courses/bio104/meiosis.htm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sex and variation
• Crossing over prior to gamete formation
increases variables
• Independent assortment
http://www.csuchico.edu/~jbell/Biol207/animations/assortment.html
• Crossover animation
http://www.csuchico.edu/~jbell/Biol207/animations/recombination.html
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Spectrum of Dominance
• Complete dominance
– Occurs when the phenotypes of the
heterozygote and dominant homozygote are
identical
PP
pp
Pp
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Incomplete dominance
• The phenotype of F1 hybrids is somewhere between
the phenotypes of the two parental varieties
P Generation
Red
CRCR
Example: Flower colors
RR = red
R’R’ = white
RR’ = pink (intermediate
pigmentation)

Gametes CR
F1 Generation
Gametes
F2 Generation
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CW
Pink
CRCW
Eggs
Figure 14.10
White
CW CW
1⁄
2
CR
1⁄
2
Cw
1⁄
2
1⁄
2
CR
1⁄
2
CR
CR 1⁄2 CR
CR CR CR CW
CR CW CW CW
Sperm
The Spectrum of Dominance
• In codominance
– Two dominant alleles affect the
phenotype in separate,
distinguishable ways
• IAIB = The human blood group type AB is
an example of codominance
• RW = roan coat color in cattle or horses
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Multiple Alleles and Codominance
• Most genes exist in populations
– In more than two allelic forms
• The ABO blood
group in humans
– Is determined by
multiple alleles
Table 14.2
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Frequency of Dominant Alleles
• Dominant alleles
– Are not necessarily more common in
populations than recessive alleles
– It really depends on whether a trait gives an
individual an adaptive advantage, and is
naturally selected.
– Examples: type O blood (ii) recessive present
in majority of population
http://www.bio-medicine.org/biology-definition/Blood_type/#Frequency
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Pleiotropy
•
Pleiotropy describes the genetic effect of a single gene on multiple
phenotypic traits.
•
The underlying mechanism is that the gene codes for a product that is, for
example, used by various cells, or has a signaling function on various
targets.
•
A classic example of pleiotropy is the human disease PKU
(phenylketonuria).
•
This disease can cause mental retardation and reduced hair and skin
pigmentation, and can be caused by any of a large number of mutations in
a single gene that codes for the enzyme (phenylalanine hydroxylase), which
converts the amino acid phenylalanine to tyrosine, another amino acid.
•
Depending on the mutation involved, this results in reduced or zero
conversion of phenylalanine to tyrosine, and phenylalanine concentrations
increase to toxic levels, causing damage at several locations in the body.
•
PKU is totally benign if a diet free from phenylalanine is maintained.
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Pleiotropy – a case study
• In pleiotropy a single gene has multiple
phenotypic effects
–
An example of pleiotropy can be seen in plants that have become
resistant to the herbicide paraquat.
–
Paraquat kills plants by shunting electron flow in the photosystems
of the chloroplasts to oxygen, forming superoxide radicals. These
superoxide radicals cause oxidative damage to the plant cells that
kills the leaves.
–
In Egypt, paraquat has been used on weeds for decades and
paraquat-resistant weeds have appeared. These paraquat resistant
plants also happen to be more resistant to environmental stresses
such as drought.
–
The reason is, the genetic changes in the paraquat resistant plants
lead to higher levels of enzymes that protect these plants from the
oxidative damage caused by paraquat. These enzymes also
protect the plants from oxidative damage caused by drought and
other environmental stresses.
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Gene Linkage
• Sometimes, when genes are located close
together on the same chromosome, they tend
to be inherited together.
• Which of Mendel’s Laws does this defy?
• See linkage animation
http://www.csuchico.edu/~jbell/Biol207/animations/linkage.html
• A “classic” example of this is why cats with
white fur and blue eyes are (more often than
not) also deaf.
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Extending Mendelian Genetics for Two or More Genes
• Many traits (especially in complex organisms)
– May be determined by two or more genes
(polygenic)
• In EPISTASIS
“standing upon”
– the phenomenon where the effects of one
gene are modified by one or several other
genes (which are sometimes called modifier
genes).
– A gene at one locus alters the phenotypic
expression of a gene at a second locus
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
An example of epistasis in mice
Here we see the effect
the bb gene combo
has on the C gene for
the agouti (brownish)
fur color in mice

BbCc
BbCc
Sperm
1⁄
BC
4
1⁄
4
bC
1⁄
4
1⁄
Bc
4
bc
Eggs
1⁄
4
BC
BBCC
BbCC
BBCc
BbCc
4
bC
BbCC
bbCC
BbCc
bbCc
cc combo = white fur
1⁄
C_ combo = black fur,
unless bb “stands
upon” it. Then you get
agouti color.
1⁄
1⁄
4
Bc
BBCc
BbCc
BBcc
4
bc
BbCc
bbCc
Bbcc
9⁄
Figure 14.11
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
16
3⁄
16
Bbcc
4⁄
bbcc
16
Other variations in gene expression
• Incomplete Expressivity is seen in cases where
individuals with the same genotypes may have, often for
unknown reasons, variability in their phenotypes.
–
This is often seen in genetic diseases where one person
with a disease such as diabetes may be very severely
effected while another with the same allele may have a
milder form of the disease.
• Incomplete Penetrance is seen when an individual with a
particular genotype does not express the phenotype. Again
the reasons for this are not clearly understood.
–
For example, known mutations in the gene responsible for
Huntington disease have “95% penetrance”, because 5% of
those with the dominant allele for Huntington disease don't
develop the disease and 95% do.
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Beyond simple inheritance
• Polygenic traits - two or more
sets of alleles govern one trait
Note that:
Environmental
effects can
cause intervening
phenotypes!
– Each dominant allele codes for a
product so these effects are additive
– Results in a continuous variation of
phenotypes
– e.g. skin color ranges from very dark
to very light

AaBbCc
aabbccAabbccAaBbccAaBbCc
AABBCc
AABBCC
AABbCc
20⁄
15⁄
6⁄
64
64
64
1⁄
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
AaBbCc
64
Polygenic inheritance
Number of Men
Multifactorial trait – a trait that is
influenced by both genetic and
environmental factors
e.g. skin color is influenced by sun
exposure
e.g. height can be affected by nutrition
most
are
this
height
few
62
short
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
64
Courtesy University of Connecticut/Peter Morenus, photographer;
few
66
68
Height in Inches
70
72
74
tall
Genes are affected by Environment
• While there is a strong genetic component in human
height, the average height has increased over the past
50 years in developed countries. This is considered to
be due to improved nutrition.
• Likewise, a cotton plant may have the alleles
necessary for high yields but if it doesn't receive
enough water or fertilizer it cannot reach its genetic
potential.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Example:
– A phenotypic range of a particular genotype
may be influenced by the environment
Figure 14.13
Exact color often mirrors the pH of the soil; acidic soils produce
blue flowers, neutral soils produce very pale cream petals, and
alkaline soils results in pink or purple. This is caused by a color
change of the flower pigments in the presence of aluminum ions
which can be taken up into the flower.
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Beyond simple inheritance
Demonstrating environmental influences
on phenotype
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
•
Himalayan rabbit/Siamese cat
coat color influenced by
temperature
•
There is an allele responsible
for melanin production that
appears to be active only at
lower temperatures
•
The extremities have a lower
temperature and thus the ears,
nose paws and tail are dark in
color
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
© H. Reinhard/Arco Images/ Peter Arnold
• Concept 14.4: Many human traits follow
Mendelian patterns of inheritance
• Humans are not convenient subjects for
genetic research
– However, the study of human genetics
continues to advance
• Many human characters
– Vary in the population along a continuum and
are called “quantitative characters”
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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
Ww
ww
Ww ww ww Ww
WW
or
Ww
Widow’s peak
ww
Ww
Ww
ww
First generation
(grandparents)
Second generation
(parents plus aunts
and uncles)
Ff
FF or Ff
Ff
ff
Third
generation
(two sisters)
ww
No Widow’s peak
(a) Dominant trait (widow’s peak)
Figure 14.14 A, B
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Attached earlobe
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
Free earlobe
(b) Recessive trait (attached earlobe)
Pedigree Analysis
•
Human geneticists illustrate the inheritance of a gene within a family by using a
pedigree chart. On such a chart, males are symbolized by a square ( □ ) and
females are symbolized by a circle ( ○ ). People who are affected by a disease
are symbolized by a dark circle or square.
•
The pedigree chart below shows inheritance of the gene that causes albinism.
A and B represent a couple who had five children, including C and E. Only one
of the children, E, was albino. E and her husband had five children, including
G. In the pedigree below write the genotypes of the individuals who are labeled
with letters, using (A) to represent the dominant allele and (a) to represent the
recessive allele. Start by indicating the genotypes of E and F. Then use a
Punnett Square to figure out what the genotypes for C and D must be. Next,
determine the genotypes of A and B. Finally, determine the genotype of G.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Recessively Inherited Autosomal Disorders
• Many genetic disorders
– Are inherited in a recessive manner (aa)
– So, most affected individuals are homozygous (dual
inheritance)
– AA, Aa are normal phenotype, in this case
• Recessively inherited disorders
– Show up only in individuals homozygous for the allele
• Carriers
– Are heterozygous individuals who carry the recessive
allele but are phenotypically normal
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Cystic Fibrosis
• Symptoms of cystic fibrosis include
– Mucus buildup in the some internal organs
– Abnormal absorption of nutrients in the small
intestine
Lung tissue
from a cystic
fibrosis patient,
showing
extensive
destruction as a
result of
obstruction and
infection.
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Phenyketonuria
PKU is an autosomal recessive genetic
disorder that is characterized by an inability
of the body to utilize the essential amino
acid, phenylalanine. Amino acids are the
building blocks for body proteins.
• In 'classic PKU', the enzyme that breaks down phenylalanine
(phenylalanine hydroxylase), is completely or nearly completely
deficient.
• This enzyme normally converts phenylalanine to another amino
acid, tyrosine. Without this enzyme, phenylalanine and its'
breakdown chemicals from other enzyme routes, accumulate in
the blood and body tissues.
• Infants are born “normal” but if not treated, severe brain problems,
such as mental retardation and seizures, will occur.
• Thus, this disease is GREATLY influenced by environmental
factors
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Other autosomal recessive disorders
• Tay-Sachs disease
• Albinism
• Hemochromatosis types 1-3: the most common
genetic disease in Europe.
You can watch this animation:
http://www.koshlandsciencemuseum.org/exhibitdna/inh05activity.jsp
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Mating of Close Relatives
• Matings between relatives are called
“consanguineous” matings
• Can increase the probability of the appearance of
a genetic disease (especially harmful recessive
homozygotes)
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Dominantly Inherited Disorders
• Some human disorders
– Are due to dominant alleles
One example is
achondroplasia
– A form of dwarfism that is
lethal when homozygous for
the dominant allele (DD)
Figure 14.15
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Dominant allele
• Huntington’s disease
– Is a degenerative disease of the nervous
system
– Has no obvious phenotypic effects until about
35 to 40 years of age
Lake Maracaibo, Venezuela
Village pedigree
Figure 14.16
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Multifactorial Disorders
• Many human diseases
– Have both genetic and environment
components
• Examples include
– Heart disease and cancer
– Try this interactive family pedigree (nicotine
addiction)
http://learn.genetics.utah.edu/units/addiction/genetics/pi.cfm
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Genetic Testing and Counseling
• Genetic counselors
– Can provide information to prospective parents
concerned about a family history for a specific
disease
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Counseling Based on Mendelian Genetics and
Probability Rules
• Using family histories
– Genetic counselors help couples determine the
odds that their children will have genetic
disorders
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Tests for Identifying Carriers
• For a growing number of diseases
– Tests are available that identify carriers and
help define the odds more accurately
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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
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• Fetal testing
(b) Chorionic villus sampling (CVS)
(a) Amniocentesis
Amniotic
fluid
withdrawn
A sample of chorionic villus
tissue can be taken as early
as the 8th to 10th week of
pregnancy.
A sample of
amniotic fluid can
be taken starting at
the 14th to 16th
week of pregnancy.
Fetus
Fetus
Suction tube
Inserted through
cervix
Centrifugation
Placenta
Placenta
Uterus
Chorionic viIIi
Cervix
Fluid
Fetal
cells
Fetal
cells
Biochemical tests can be
Performed immediately on
the amniotic fluid or later
on the cultured cells.
Fetal cells must be cultured
for several weeks to obtain
sufficient numbers for
karyotyping.
Biochemical
tests
Several
weeks
Several
hours
Karyotyping
Figure 14.17 A, B
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Karyotyping and biochemical
tests can be performed on
the fetal cells immediately,
providing results within a day
or so.