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Chapter 14
Mendel and the Gene Idea
AP Biology
Overview: Drawing from the Deck of Genes
•
The “blending” hypothesis was the most widely favored explanation of heredity in the
1800s
•
Idea that genetic material from the two parents blends together (like blue and
yellow paint blend to make green)
•
Predicts that a freely mating population will give rise to a uniform
population of individuals over many generations
•
This hypothesis contradicts many everyday observations and the results of
breeding experiments with plants and animals
•
•
Ex)Traits reappear after skipping a generation
The “particulate” hypothesis is an alternative to the blending model
•
Idea that parents pass on discrete heritable units (genes)
•
Gregor Mendel documented a particulate mechanism through his
experiments with garden peas
Concept 14.1:
Mendel used the scientific approach
to identify two laws of inheritance
Pea Plants as Subjects for Genetic Study
•
There are many advantages of using pea plants for genetic study:
–
There are many varieties with distinct heritable features, or characters (ex:
flower color);
•
–
Character variants (such as purple or white flowers) are called traits
Large numbers of offspring are produced during each mating in a short
generation time
–
Mating of plants can be controlled
•
Each pea plant has sperm-producing organs (stamens) and egg-producing
organs (carpels)
–
Cross-pollination (fertilization between different plants) can be
achieved by dusting one plant with pollen from another
–
Self-pollination can also be forced
Mendel’s Experiment
•
Mendel chose to track only those characters that varied between 2 distinct
alternatives
•
•
Ex) Purple vs. white flowers
He also used varieties that had been allowed to self-fertilize over many generations,
producing true-breeding populations Fig. 14-2
TECHNIQUE
•
•
These plants produce offspring of the same
variety when they self-pollinate
1
Mendel then cross-pollinated 2 contrasting truebreeding pea varieties in a typical breeding experiment
•
Crossing of 2 true-breeding varieties is
called hybridization
•
•
•
2
Parental
generation
(P)
Stamens
Carpel
3
True-breeding parents are referred to
as P (parental) generation
Hybrid offspring are known as F1
(1st filial) generation
Allowing F1 hybrids to self-pollinate
produces an F2 (2nd filial) generation
4
RESULTS
First
filial
generation
offspring
(F1)
5
The Law of Segregation
•
In his experiments, Mendel crossed contrasting, true-breeding white and purple
flowered pea plants
•
•
All of the F1 hybrids were purple
When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but
some had white
•
Fig. 14-3-3
Mendel discovered a ratio of about
3 purple:1 white in F2 generation
•
EXPERIMENT
P Generation
(true-breeding
parents)

Purple
flowers
White
flowers
He reasoned that the heritable
factor for white flowers was not
destroyed in F1 generation but
F1 Generation
(hybrids)
All plants had
purple flowers
was somehow masked when
purple flower factor was present
F2 Generation
•
Purple flower color is dominant trait
•
White flower color is recessive trait
705 purple-flowered 224 white-flowered
plants
plants
Patterns of Inheritance in Other Pea Plant Characters
• Mendel observed the same pattern of inheritance
Table 14-1
in six other pea plant
characters, each
represented by two traits
• What Mendel called a
“heritable factor” is what
we now call a gene
Mendel’s Model
• Mendel developed a hypothesis to explain the
3:1 inheritance pattern he observed in F2
offspring
• Four related concepts make up this model
• These concepts can be related to what we
now know about genes and chromosomes
Alleles
•
The first concept is that alternative versions of genes account for variations
in inherited characters
• Ex) the gene for flower color in pea plants exists in two versions,
one for purple flowers and the other for white flowers
•
•
These alternative versions of a gene are now called alleles
Each gene resides at a specific locus on a specific chromosome
Fig. 14-4
•
Alleles arise due to slight variations in nucleotide sequence at these
loci
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers
Inheritance of Alleles
•
The second concept is that an organism inherits 2 alleles for each character,
one from each parent
•
Mendel made this deduction without knowing about the role of
chromosomes
• The two alleles at a locus on a chromosome may be identical
• Ex) True-breeding plants of Mendel’s P generation
Fig. 14-4
• Alternatively, the two alleles at a locus may differ
Allele for purple flowers
• Ex) F1 hybrids
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers
Dominant vs. Recessive Alleles
• The third concept is that if the two alleles at a locus differ, then:
•
One allele (the dominant allele) determines the
organism’s appearance
•
The other allele (the recessive allele) has no noticeable
effect on appearance
• Ex) In the case of flower color, the F1 plants had purple
flowers because the allele for that trait is dominant
The Law of Segregation
•
The fourth concept states that the two alleles for a heritable character separate
(segregate) during gamete formation and end up in different gametes
•
•
Known as the law of segregation
Ex) Egg or sperm gets only one of the two alleles that are present in the
somatic cells of the organism
•
Segregation of alleles corresponds to the distribution of homologous
chromosomes to different gametes in meiosis
•
If organism has identical alleles for a character (true-breeding), than
that allele will be present in all gametes
•
If different alleles are present (F1 hybrids), 50% of gametes receive
dominant allele and 50% receive recessive allele
Punnett Squares
•
Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2
Fig. 14-5-3
generation of his numerous crosses
P Generation
•
Possible combinations of sperm and egg
can be shown using a Punnett square
•
Purple flowers White flowers
Appearance:
PP
pp
Genetic makeup:
p
P
Gametes:
Diagram for predicting the results of
a genetic cross between individuals
F1 Generation
of known genetic makeup
Appearance:
Genetic makeup:
•
Capital letter represents a
dominant allele
•
Lowercase letter represents a
Gametes:
Purple flowers
Pp
1/
2
Sperm
F2 Generation
P
p
PP
Pp
Pp
pp
P
Eggs
recessive allele
1/
2
P
p
3
1
p
Useful Genetic Vocabulary
•
An organism with two identical alleles for a character is said to be homozygous for
the gene controlling that character
•
Homozygous organisms are true-breeding because all their gametes contain
the same allele
•
•
May be homozygous dominant (PP – purple)
•
May be homozygous recessive (pp-white)
An organism that has two different alleles for a gene is said to be heterozygous for
the gene controlling that character
•
Unlike homozygotes, heterozygotes are not true-breeding
•
•
Produce gametes with different alleles (P or p)
Heterozygotes display the dominant trait
Genotype vs. Phenotype
•
An organism’s traits do not always reveal its genetic composition
•
Due to the different effects of dominant and recessive alleles
Fig. 14-6
• Therefore, we distinguish
Phenotype
Genotype
Purple
PP
(homozygous)
Purple
Pp
(heterozygous)
between an organism’s:
• Phenotype = physical
3
1
2
appearance
Purple
Pp
(heterozygous)
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
• Genotype = genetic
makeup
• Ex) PP and Pp plants have
1
the same phenotype (purple) but different genotypes
1
The Testcross
•
Individuals displaying the dominant phenotype can have one of 2 genotypes
–
Such an individual must have one dominant allele, but the individual could be
either homozygous dominant or heterozygous
Fig. 14-7
•
A testcross can be carried out to determine
TECHNIQUE
genetic makeup of this mystery individual
–
In this cross, the mystery individual is bred
with homozygous recessive individual
–

Dominant phenotype, Recessive phenotype,
unknown genotype:
known genotype:
PP or Pp?
pp
Predictions
If PP
Sperm
p
p
If any offspring display the recessive phenotype,
the mystery parent must be heterozygous
P
Pp
Pp
Pp
Pp
Eggs
If Pp
Sperm
p
p
or
P
Eggs
P
Pp
Pp
pp
pp
p
RESULTS
or
All offspring purple
1/2
offspring purple and
2 offspring white
1/
Monohybrid vs. Dihybrid Crosses
•
Mendel derived the law of segregation by following a single character (ex: flower
color)
–
The F1 offspring produced in this cross were monohybrids, individuals that are
heterozygous for one character
•
•
A cross between such heterozygotes is called a monohybrid cross
Mendel identified a 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
•
Crossing between F1 dihybrids (dihybrid cross) can determine whether two
characters are transmitted to offspring as a package or independently
The Law of Independent Assortment
•
Using a dihybrid cross, Mendel developed the law of independent assortment
•
States that each pair of alleles
segregates independently of each other pair of
Fig. 14-8
alleles during gamete formation
EXPERIMENT
YYRR
P Generation
•
pea shape
•
This law applies only to genes on

Gametes YR
Ex) Inheritance of pea color will
not affect inheritance for
yyrr
F1 Generation
1/
2
Hypothesis of
independent
assortment
1/
4
Sperm
1/ YR
1/
2
2 yr
YR
YYRR
YyRr
Eggs
1/
2
•
Sperm
or
Predicted
offspring of
F2 generation
different, nonhomologous chromosomes
YyRr
Hypothesis of
dependent
assortment
Predictions
yr
YyRr
3/
4
the same chromosome tend to be
YR
1/
4
Yr
Eggs
yr
Genes located near each other on
1/
4
yyrr
1/
4
yR
1/
4
Yr
yR
1/
4
yr
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
1/
4
Phenotypic ratio 3:1
1/
4
yr
9/
16
inherited together
YR
1/
4
3/
16
3/
16
yyrr
1/
16
Phenotypic ratio 9:3:3:1
RESULTS
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
Concept Check 14.1
•
1) A pea plant heterozygous for inflated pods (Ii) is crossed with a plant
homozygous for constricted pods (ii). Draw a Punnett square for this cross.
Assume pollen comes from the ii plant.
•
2) Pea plants heterozygous for flower position an stem length (AaTt) are
allowed to self-pollinate, and 400 of the resulting seeds are planted. Draw a
Punnett square for this cross. How many offspring would be predicted to
have terminal flowers and be dwarf (see Table 14.1, pp. 265)?
•
3) List the different gametes that could be made by a pea plant
heterozygous for seed color, seed shape, and pod shape (YyRrIi; see Table
14.1). How large a Punnett square would you need to predict the offspring
of a self-pollination of this “trihybrid?”
Concept 14.2:
The laws of probability govern
Mendelian inheritance
The Rules of Probability
•
Mendel’s laws of segregation and independent assortment reflect the rules of
probability
•
•
Probability scale ranges from 0-1
•
An event that is certain to occur has a probability of 1
•
An event that will never occur has a probability of 0
One independent event has no impact on the outcome of the next independent event
•
Ex) When tossing a coin, the outcome of one toss has no impact on the
outcome of the next toss
•
In the same way, the alleles of one gene segregate into gametes independently
of another gene’s alleles
The Multiplication and Addition Rules Applied to
Monohybrid Crosses
•
The multiplication rule: the probability that two or more independent events will
occur together is the product of their individual probabilities
•
Ex) Probability of coming up with 2 heads on 2 separate coin tosses is
Fig. 14-9
½X½=¼
–
Rr
Segregation of
alleles into sperm
Segregation of
alleles into eggs
Probability in an F1 monohybrid cross can be
Sperm
determined using the multiplication rule
•
Rr

1/
Ex) Probability of obtaining a white flowered
1/
2
R
2
R
pea plant (pp) from the cross
R
1/
Eggs
1
1/
2
r
2
R
R
Pp x Pp = ½ X ½ = ¼
1/
r
1
4
r
1/
4
r
2
2
r
r
R
1/
4
1/
4
The Addition Rule
•
Addition rule: the probability that any one of two or more exclusive events will occur
is calculated by adding together their individual probabilities
–
Can be used to figure out the probability that an F2 plant from a monohybrid
cross will be heterozygous rather than homozygous
•
Fig. 14-9
Dominant allele can come from egg OR
sperm (not both) in a heterozygote
–
Rr
Segregation of
alleles into sperm
Segregation of
alleles into eggs
Ex) Probability of obtaining an F2
Sperm
1/
heterozygote from the cross
Rr x Rr is ¼ + ¼
Rr

R
2
R
1/
2
R
R
1/
Eggs
4
r
1/
2
1/
=½
r
2
R
r
1/
4
r
r
R
r
1/
4
1/
4
Solving Complex Genetics Problems with the Rules of
Probability
•
Multiplication and addition rules can be used to predict the outcome of
crosses involving multiple characters
–
A dihybrid or other multicharacter cross is equivalent to two or more
independent monohybrid crosses occurring simultaneously
Fig. 14-UN1
–
In calculating the chances for various genotypes, each character is
considered separately, and then the individual probabilities are
multiplied together
Concept Check 14.2
•
1) For any gene with a dominant allele C and a recessive allele c, what
proportions of the offspring from a CC X Cc cross are expected to be
homozygous dominant, homozygous recessive, and heterozygous?
•
2) An organism with the genotype BbDD is mated to one with the genotype
BBDd. Assuming independent assortment of these 2 genes, write the
genotypes of all possible offspring from this cross and use the rules of
probability to calculate the chance of each genotype occurring.
•
3) Three characters (flower color, seed color, and pod shape) are
considered in a cross between 2 pea plants (PpYyIi x ppYyii). What fraction
of offspring would be predicted to be homozygous recessive for at least 2 of
the 3 characters?
Concept 14.3:
Inheritance patterns are often more
complex than predicted by simple
Mendelian genetics
Complex Patterns of Inheritance
•
The relationship between genotype and phenotype is rarely as simple as in the pea
plant characters Mendel studied
•
Many heritable characters are not determined by only one gene with two
alleles
•
However, the basic principles of segregation and independent assortment
apply even to more complex patterns of inheritance
•
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
•
Alleles can show different degrees of dominance and recessiveness in relation to
each other
–
Complete dominance occurs when phenotypes of the heterozygote and
dominant homozygote are identical
–
In incomplete dominance, neither allele
is completely dominant
Fig. 14-10-3
•
P Generation
The phenotype of F1 hybrids is somewhere
Red
CRCR
White
CWCW
between the phenotypes of the two parental
varieties
–
Ex) Red snapdragon X White
CR
Gametes
CW
Pink
CRCW
F1 Generation
snapdragon = Pink snapdragons
–
Gametes 1/2 CR
In codominance, two dominant alleles affect the
1
/2 CW
Sperm
phenotype in separate, distinguishable ways
1/
2
CR
1/
2
CW
F2 Generation
•
Ex) Human blood type
1/
2
CR
Eggs
1/
2
CRCR
CRCW
CRCW
CWCW
CW
Relation Between Dominance and Phenotype
•
A dominant allele does not subdue a recessive allele; alleles don’t interact
•
Alleles are simply variations in a gene’s nucleotide sequence
•
•
Ex) Pea seed shape: one dominant allele results in enough enzyme to
make adequate amounts of branched starch
For any character, dominance/recessiveness relationships of alleles depend on the
level at which we examine the phenotype
–
Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation
of lipids in the brain
•
At the organismal level, the allele is recessive (need 2 recessive alleles to
inherit disease)
•
At the biochemical level, the phenotype (i.e., the enzyme activity level) is
incompletely dominant (1/2 the normal enzyme activity is sufficient to
prevent lipid accumulation in brain)
•
At the molecular level, the alleles are codominant (heterozygotes produce
equal numbers of normal and dysfunctional enzymes)
Frequency of Dominant Alleles
• Dominant alleles are not necessarily more
common in populations than recessive alleles
• Ex) Some cases of polydactyly (having extra
fingers or toes) are caused by presence of
dominant allele
• Only one baby out of 400 in the United
States is born with extra fingers or toes
Multiple Alleles
•
Most genes exist in more than two allelic forms
–
Ex) The four phenotypes of the ABO blood group in humans are
14-11
determined by three allelesFig.for
the enzyme (I)
that attaches A or B carbohydrates to
red blood cells: IA, IB, and i.
Allele
IA
IB
B
i
none
(a) The three alleles for the ABO blood groups
and their associated carbohydrates
• The enzyme encoded by the
IA allele adds the A carbohydrate
Carbohydrate
A
Genotype
Red blood cell
appearance
Phenotype
(blood group)
IAIA or IA i
A
IBIB or IB i
B
• The enzyme encoded by the
IB
allele adds the B carbohydrate
• The enzyme encoded by the
i allele adds neither
IAIB
AB
ii
O
(b) Blood group genotypes and phenotypes
Pleiotropy, Epistasis, and Polygenic Inheritance
•
Most genes have multiple phenotypic effects, a property called pleiotropy
–
Ex) Responsible for the multiple symptoms of certain hereditary
diseases, such as cystic fibrosis and sickle-cell disease
•
Some traits may also be determined by two or more genes
–
Include 2 different situations:
• Epistasis
• Polygenic inheritance
Epistasis
•
In epistasis, a gene at one locus alters the phenotypic expression of a gene at a
second locus
–
Ex) In mice and many other mammals, coat color depends on two genes
•
One gene determines the pigment color :
Fig. 14-12
–
–
•
B = black
b = brown
The other gene determines whether
Eggs
the pigment will be deposited
1/
4 BC
in the hair
– C = color
–
•
BbCc
c = no color
The gene for pigment deposition
(C/c) is said to be epistatic to
the gene that codes for pigment
color (B/b)

BbCc
Sperm
1/
4 BC
1/
4 bC
1/
4 Bc
1/
4 bc
BBCC
BbCC
BBCc
BbCc
BbCC
bbCC
BbCc
bbCc
BBCc
BbCc
BBcc
Bbcc
BbCc
bbCc
Bbcc
bbcc
1/
4 bC
1/
4 Bc
1/
4 bc
9
: 3
: 4
Polygenic Inheritance
•
Quantitative characters are those that 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
Fig. 14-13
•

Controlled by at least 3 separately inherited
genes
AaBbCc
Sperm
1/
8
•
1/
8
1/
8
1/
8
1/
8
1/
8
1/
8
White circles represent light-skin
Eggs
1/
8
1/
8
1/
8
One dark-skin allele contributes one
“unit” of darkness to phenotype
1/
8
1/
8
alleles (a,b,c)
•
1/
8
1/
8
Black circles represent dark-skin
alleles (A,B,C)
•
AaBbCc
1/
8
1/
8
Phenotypes:
1/
64
Number of
dark-skin alleles: 0
6/
64
15/
64
20/
64
15/
64
6/
64
1/
64
1
2
3
4
5
6
The Environmental Impact on Phenotype
•
The phenotype for a character may also depend on environment as well as genotype
–
Genotypes are generally not associated with a rigidly defined phenotype but
rather a range of phenotypes due to environmental influences
•
The norm of reaction is the phenotypic range of a genotype influenced by
the environment
–
Norms of reactions are usually broadest for polygenic characters
–
Such characters are called multifactorial because genetic and
environmental factors collectively influence
Fig. 14-14
phenotype
•
Ex) Hydrangea flowers
of the same genotype
range from blue-violet to
pink, depending on soil
acidity
Concept Check 14.3
• 1) Incomplete dominance and epistasis are both terms that define
genetic relationships. What is the most basic distinction between
these terms?
• 2) If a man with type AB blood marries a woman with type O blood,
what blood types would you expect in their children?
• 3) A rooster with gray feathers is mated with a hen of the same
phenotype. Among their offspring, 15 chicks are gray, 6 are black,
and 8 are white. What is the simplest explanation for the inheritance
of these colors in chickens? What phenotypes would you expect in
the offspring of a cross between a gray rooster and a black hen?
Concept 14.4:
Many human traits follow
Mendelian patterns of inheritance
Human Genetics
• Humans are not good subjects for genetic research
–
Generation time is too long (~20 years)
–
Parents produce relatively few offspring
–
Breeding experiments are unacceptable
• The study of human genetics continues to advance
despite these constraints
Pedigree Analysis
•
Geneticists must analyze results of matings that have already occurred
–
Information about a family’s history for a particular trait is assembled into a
pedigree
•
A pedigree is a family tree that describes the interrelationships of parents
and children across generations
– Inheritance patterns of particular traits can thus be traced and
described using pedigrees
•
Pedigrees can also be used to make predictions about future offspring
–
Can help calculate probability that a child will have a particular genotype and
phenotype
–
We can also use the multiplication and addition rules to predict the probability
of specific phenotypes
Fig. 14-15b
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
Ww
ww
Ww ww ww Ww
ww
Ww
Ww
ww
3rd generation
(two sisters)
Fig. 14-15a
WW
or
Widow’s peakWw
ww
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
Key
Male
Female
Affected
male
Affected
female
Mating
Offspring, in
birth order
(first-born on left)
Fig. 14-15c
1st generation
(grandparents)
Ff
2nd generation
(parents, aunts,
and uncles)
FF or Ff ff
Ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd generation
(two sisters)
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?
Recessively Inherited Disorders
•
Many genetic disorders are inherited in a recessive manner
–
These disorders range in severity:
• Relatively mild: albinism (lack of skin pigmentation)
– Increases susceptibility to skin cancers and vision problems
Fig. 14-16
• Life-threatening: cystic fibrosis
Parents
Normal
Aa

Normal
Aa
Sperm
A
a
A
AA
Normal
Aa
Normal
(carrier)
a
Aa
Normal
(carrier)
aa
Albino
Eggs
The Behavior of Recessive Alleles
•
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
•
Most people with recessive disorders are born to parents that are carriers of the
disorder
–
If a recessive allele that causes a disease is rare, then the chance of two
carriers meeting and mating is low
–
Consanguineous matings (i.e., matings between close relatives) increase the
chance of mating between two carriers of the same rare allele
•
People with recent common ancestors are more likely to carry the same
recessive alleles
–
•
Indicated in pedigrees by double lines
Most societies and cultures have laws or taboos against marriages
between close relatives
Cystic Fibrosis
•
Cystic fibrosis (CF) is the most common lethal genetic disease in the United States
–
1/25 (4%) people of European descent are carriers of CF allele
•
–
Strikes one out of every 2,500 people of European descent
Normal allele for this gene codes for a membrane protein that is involved in
transport of chloride ions between cells and extracellular fluid
•
The cystic fibrosis allele results in defective or absent chloride transport
channels in plasma membranes
•
Results in abnormally high concentration of extracellular chloride
–
Causes mucus buildup in some internal organs and abnormal
absorption of nutrients in the small intestine
– If untreated, most children infected with CF die before the age of 5
Sickle-Cell Disease
•
Sickle-cell disease is the most common inherited disorder among African
Americans
–
Affects one out of 400 African-Americans (1/10 are carriers)
•
Frequency of recessive allele can be explained by the observation that a
single copy of sickle-cell allele reduces frequency and severity of malaria
attacks
–
Caused by the substitution of a single amino acid in the hemoglobin protein in
red blood cells
•
Sickled cells may clump and clog small blood vessels
•
Symptoms include physical weakness, pain, organ damage, and even
paralysis
Dominantly Inherited Disorders
•
Some human disorders are caused by dominant alleles
–
Dominant alleles that cause a lethal disease are rare and arise by
mutation
• Lethal dominant alleles often cause death of offspring prior to
maturity and reproduction
Fig. 14-17
–
Ex) Achondroplasia is a form of
Parents
Dwarf
Dd
dwarfism caused by a rare
dominant allele
• Occurs in 1/25,000

Normal
dd
Sperm
D
d
d
Dd
Dwarf
dd
Normal
d
Dd
Dwarf
dd
Normal
Eggs
people
Huntington’s Disease
•
Lethal dominant alleles can escape elimination if they do not become apparent until
more advanced ages
–
Ex) Huntington’s disease is an irreversible and fatal degenerative disease of
the nervous system
•
The disease has no obvious phenotypic effects until the individual is about
35 to 45 years of age
•
Affects 1/10,000 people in the US
•
Geneticists have tracked Huntington’s allele to a locus near the tip of
chromosome 4
–
Sequencing of this gene has allowed development of a test to detect
the presence of this allele
Multifactorial Disorders
•
Many diseases have both genetic and environmental components
–
Known as multifactorial disorders
•
Include heart disease, diabetes, cancer, alcoholism, schizophrenia, and
bipolar disorder
–
Hereditary component is usually polygenic
•
Ex) Many genes affect cardiovascular health, making some individuals
more prone to heart attacks and strokes
•
Little is understood about the genetic contribution to most multifactorial
diseases
–
The best public health strategy seems to be educating people about
the importance of environmental factors and promoting healthy
behaviors
Genetic Testing and Counseling
•
Genetic counselors can provide information to prospective parents
concerned about a family history for a specific disease
–
Using family histories, genetic counselors help couples determine the
odds that their children will have genetic disorders
• Ex: What is the probability that a couple will have a child with a
recessively inherited disorder if both of their brothers died of this
disorder?
– (2/3) x (2/3) x (1/4) = 1/9
• Ex: What is the probability that this same couple will have a child
with the disorder if they already have a child with the disease?
– (1/2) x (1/2) = 1/4
Tests for Identifying Carriers
•
For a growing number of diseases, tests are available that identify carriers and help
define the odds more accurately
–
–
These tests are already available for:
•
Tay-Sachs disease
•
Sickle-cell disease
•
Cystic fibrosis
The tests allow people with family histories of genetic disorders to make
informed decisions about having children
–
These tests also pose potential problems
•
Breaches of confidentiality
•
Stigmatization of carriers
•
Discrimination by health and life insurance companies, as well as
employers
Fetal Testing - Amniocentesis
•
Tests performed in conjunction with a technique called amniocentesis can determine
if a developing fetus has a genetic disorder
Fig. 14-18
–
In amniocentesis, the liquid
that bathes the fetus is
Amniotic fluid
withdrawn
Centrifugation
Fetus
Fetus
removed and tested
Placenta
–
Can usually be performed
during the
14th
-16th
week of
Uterus
Placenta
Cervix
Fluid
Fetal
cells
pregnancy
–
BioSeveral chemical
hours
tests
Several
weeks
Several
hours
Fetal
cells
Some disorders can be detected
from the presence of certain
chemicals in the fluid
–
Chorionic
villi
Suction tube
inserted
through
cervix
Tests for other disorders require
that the amniotic fluid be
cultured to produce cells
Several
weeks
(a) Amniocentesis
Karyotyping
Several
hours
(b) Chorionic villus sampling (CVS)
Fetal Testing: Chorionic Villus Sampling
•
In chorionic villus sampling (CVS), a sample of the placenta is removed and
Fig. 14-18
tested
–
Amniotic fluid
withdrawn
Physician inserts narrow tube
through cervix and into the
Centrifugation
Fetus
Fetus
Placenta
Uterus
–
Placenta
uterus and suctions out a
Fluid
tiny sample
Fetal
cells
BioSeveral chemical
hours
tests
Several
weeks
The cells of the chorionic villi of
the placenta (the portion
sampled) are derived from fetus
Several
weeks
(a) Amniocentesis
Karyotyping
Several
hours
Fetal
cells
Several
hours
(b) Chorionic villus sampling (CVS)
•
These cells have the same genotype as the new individual
•
These cells also divide rapidly, allowing for quicker karyotyping and
analysis
–
Chorionic
villi
Cervix
Suction tube
inserted
through
cervix
Can be performed as early as the 8th-10th week of pregnancy
Fetal Testing: Imaging Techniques
• Other techniques allow fetal health to be assessed
visually in utero
– Ultrasound: sound waves are used to produce an
image of the fetus
• Carries no known risk to mother or fetus
– Fetoscopy: a needle-thin tube containing a
viewing scope and fiber optics (to transmit light) is
inserted into uterus
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
–
Include screening for phenylketonuria (PKU)
•
Recessively inherited disorder affecting one out of every 10,000-15,000
children in the US
•
Affected children cannot properly metabolize the amino acid phenylalanine
–
Phenylalanine and its byproduct (phenylpyruvate) accumulate to toxic
levels in blood, causing mental retardation
–
If deficiency is detected in newborns, a special diet low in
phenylalanine will usually allow normal development and prevent
retardation
Concept Check 14.4
•
1) Beth and Tom each have a sibling with cystic fibrosis, but neither Beth nor
Tom nor any of their parents have the disease. Calculate the probability that
if this couple has a child, the child will have CF. What would be the
probability if a test revealed that Tom is a carrier but Beth is not?
•
2) Joan was born with 6 toes on each foot, a dominant trait called
polydactyly. Two of her 5 siblings and her mother, but not her father, also
have extra digits. What is Joan’s genotype for the number-of-digits
character? Explain your answer. Use D and d to symbolize the alleles for
this character.
•
3) What would you suspect if Peter was born with polydactyly, but neither of
his biological parents had extra digits?
You should now be able to:
1. Define the following terms: true breeding,
hybridization, monohybrid cross, P
generation, F1 generation, F2 generation
2. Distinguish between the following pairs of
terms: dominant and recessive; heterozygous
and homozygous; genotype and phenotype
3. Use a Punnett square to predict the results of
a cross and to state the phenotypic and
genotypic ratios of the F2 generation
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
4. Explain how phenotypic expression in the
heterozygote differs with complete
dominance, incomplete dominance, and
codominance
5. Define and give examples of pleiotropy and
epistasis
6. Explain why lethal dominant genes are much
rarer than lethal recessive genes
7. Explain how carrier recognition, fetal testing,
and newborn screening can be used in
genetic screening and counseling
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings