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
• 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
Pea Plants as Subjects for Genetic Study
Fig. 14-2
TECHNIQUE
RESULTS
Parentalgeneration(P) Stamens
Carpel
1
2
3
4
Firstfilialgener-ationoffspring(F1)
5
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
• These plants produce offspring of the same
variety when they self-pollinate
• Mendel then cross-pollinated 2 contrasting true-
breeding pea varieties in a typical breeding experiment
• Crossing of 2 true-breeding varieties is
called hybridization
• 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
Fig. 14-3-3
EXPERIMENT
P Generation
(true-breeding
parents) Purpleflowers
Whiteflowers
F1 Generation
(hybrids)All plants hadpurple flowers
F2 Generation
705 purple-floweredplants
224 white-floweredplants
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
• Mendel discovered a ratio of about
3 purple:1 white in F2 generation
• He reasoned that the heritable
factor for white flowers was not
destroyed in F1 generation but
was somehow masked when
purple flower factor was present
• Purple flower color is dominant trait
• White flower color is recessive trait
Table 14-1
Patterns of Inheritance in Other Pea Plant Characters
• Mendel observed the same pattern of inheritance
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
Fig. 14-4
Allele for purple flowers
Homologouspair ofchromosomes
Locus for flower-color gene
Allele for white flowers
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
• Alleles arise due to slight variations in nucleotide sequence at these
loci
Fig. 14-4
Allele for purple flowers
Homologouspair ofchromosomes
Locus for flower-color gene
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
• Alternatively, the two alleles at a locus may differ
• Ex) F1 hybrids
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
Fig. 14-5-3
P Generation
Appearance:Genetic makeup:
Gametes:
Purple flowers White flowersPP
P
pp
p
F1 Generation
Gametes:
Genetic makeup:Appearance: Purple flowers
Pp
P p1/21/2
F2 Generation
Sperm
Eggs
P
P
PP Pp
p
p
Pp pp
3 1
Punnett Squares
• Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2
generation of his numerous crosses
• Possible combinations of sperm and egg
can be shown using a Punnett square
• Diagram for predicting the results of
a genetic cross between individuals
of known genetic makeup
• Capital letter represents a
dominant allele
• Lowercase letter represents a
recessive allele
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
Fig. 14-6
Phenotype
Purple
Purple3
Purple
Genotype
1 White
Ratio 3:1
(homozygous)
(homozygous)
(heterozygous)
(heterozygous)
PP
Pp
Pp
pp
Ratio 1:2:1
1
1
2
Genotype vs. Phenotype
• An organism’s traits do not always reveal its genetic composition
• Due to the different effects of dominant and recessive alleles
• Therefore, we distinguish
between an organism’s:
• Phenotype = physical
appearance
• Genotype = genetic
makeup
• Ex) PP and Pp plants have
the same phenotype (purple) but different genotypes
Fig. 14-7
TECHNIQUE
RESULTS
Dominant phenotype,unknown genotype:
PP or Pp?
Predictions
Recessive phenotype,known genotype:
pp
If PP If Ppor
Sperm Spermp p p p
P
P
P
p
Eggs Eggs
Pp
Pp Pp
Pp
Pp Pp
pp pp
or
All offspring purple 1/2 offspring purple and1/2 offspring white
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
• A testcross can be carried out to determine
genetic makeup of this mystery individual
– In this cross, the mystery individual is bred
with homozygous recessive individual
– If any offspring display the recessive phenotype,
the mystery parent must be heterozygous
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
Fig. 14-8
EXPERIMENT
RESULTS
P Generation
F1 Generation
Predictions
Gametes
Hypothesis of
dependent
assortment
YYRR yyrr
YR yr
YyRr
Hypothesis of
independent
assortment
orPredicted
offspring of
F2 generationSperm
Sperm
YR
YR
yr
yr
Yr
YR
yR
Yr
yR
yr
YR
YYRR
YYRR YyRr
YyRr
YyRr
YyRr
YyRr
YyRr
YYRr
YYRr
YyRR
YyRR
YYrr Yyrr
Yyrr
yyRR yyRr
yyRr yyrr
yyrr
Phenotypic ratio 3:1
EggsEggs
Phenotypic ratio 9:3:3:1
1/21/2
1/2
1/2
1/4
yr
1/41/4
1/4 1/4
1/4
1/4
1/4
1/43/4
9/163/16
3/161/16
Phenotypic ratio approximately 9:3:3:1315 108 101 32
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
alleles during gamete formation
• Ex) Inheritance of pea color will
not affect inheritance for
pea shape
• This law applies only to genes on
different, nonhomologous chromosomes
• Genes located near each other on
the same chromosome tend to be
inherited together
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
Fig. 14-9
Rr Rr
Segregation ofalleles into eggs
Sperm
R
R
R R
R
R r
rr
r
r
r1/2
1/2
1/2
1/2
Segregation ofalleles into sperm
Eggs
1/41/4
1/41/4
• 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
½ X ½ = ¼
– Probability in an F1 monohybrid cross can be
determined using the multiplication rule
• Ex) Probability of obtaining a white flowered
pea plant (pp) from the cross
Pp x Pp = ½ X ½ = ¼
The Multiplication and Addition Rules Applied to Monohybrid Crosses
1
2
1
2
Fig. 14-9
Rr Rr
Segregation ofalleles into eggs
Sperm
R
R
R R
R
R r
rr
r
r
r1/2
1/2
1/2
1/2
Segregation ofalleles into sperm
Eggs
1/41/4
1/41/4
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
• Dominant allele can come from egg OR
sperm (not both) in a heterozygote
– Ex) Probability of obtaining an F2
heterozygote from the cross
Rr x Rr is ¼ + ¼ = ½
Fig. 14-UN1
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
– 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
• 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
Complex Patterns of Inheritance
Fig. 14-10-3
Red
P Generation
Gametes
WhiteCRCR CWCW
CR CW
F1 GenerationPinkCRCW
CR CWGametes1/2
1/2
F2 Generation
Sperm
Eggs
CR
CR
CW
CW
CRCR CRCW
CRCW CWCW
1/21/2
1/2
1/2
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
• The phenotype of F1 hybrids is somewhere
between the phenotypes of the two parental
varieties
– Ex) Red snapdragon X White
snapdragon = Pink snapdragons
– In codominance, two dominant alleles affect the
phenotype in separate, distinguishable ways
• Ex) Human blood type
• 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)
Relation Between Dominance and Phenotype
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
Fig. 14-11
IA
IB
i
A
B
none
(a) The three alleles for the ABO blood groupsand their associated carbohydrates
Allele Carbohydrate
GenotypeRed blood cell
appearancePhenotype
(blood group)
IAIA or IA i A
BIBIB or IB i
IAIB AB
ii O
(b) Blood group genotypes and phenotypes
Multiple Alleles
• Most genes exist in more than two allelic forms
– Ex) The 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.
• The enzyme encoded by the
IA allele adds the A carbohydrate
• The enzyme encoded by the
IB allele adds the B carbohydrate
• The enzyme encoded by the
i allele adds neither
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
Fig. 14-12
BbCc BbCc
Sperm
Eggs
BC bC Bc bc
BC
bC
Bc
bc
BBCC
1/41/4
1/41/4
1/4
1/4
1/4
1/4
BbCC BBCc BbCc
BbCC bbCC BbCc bbCc
BBCc BbCc
BbCc bbCc
BBcc Bbcc
Bbcc bbcc
9 : 3 : 4
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 :
– B = black
– b = brown
• The other gene determines whether
the pigment will be deposited
in the hair
– C = color
– 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)
Fig. 14-13
Eggs
Sperm
Phenotypes:
Number ofdark-skin alleles: 0 1 2 3 4 5 6
1/646/64
15/6420/64
15/646/64
1/64
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/81/8
1/81/8
1/81/8
1/81/8
AaBbCc AaBbCc
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
• Controlled by at least 3 separately inherited
genes
• Black circles represent dark-skin
alleles (A,B,C)
• White circles represent light-skin
alleles (a,b,c)
• One dark-skin allele contributes one
“unit” of darkness to phenotype
Fig. 14-14
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
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)
3rd generation (two sisters)
Widow’s peak No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
Ww ww
Ww Ww ww ww
ww
ww Ww
Ww
ww WW
Ww
or
Fig. 14-15a
Key
Male
Female
Affectedmale
Affectedfemale
Mating
Offspring, inbirth order(first-born on left)
Fig. 14-15c
Attached earlobe
1st generation (grandparents)
2nd generation (parents, aunts, and uncles)
3rd generation (two sisters)
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?
Ff Ff
Ff Ff Ff
ff Ff
ff ff ff
ff
FF or
or FF
Ff
Fig. 14-16
Parents
Normal Normal
Sperm
Eggs
NormalNormal(carrier)
Normal(carrier)
Albino
Aa Aa
A
AAA
Aa
a
Aaaa
a
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
• Life-threatening: cystic fibrosis
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
Fig. 14-17
Eggs
Parents
Dwarf Normal
Normal
Normal
Dwarf
Dwarf
Sperm
Dd dd
dD
Dd dd
ddDd
d
d
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
– Ex) Achondroplasia is a form of
dwarfism caused by a rare
dominant allele
• Occurs in 1/25,000
people
• 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
Huntington’s Disease
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
Fig. 14-18
Amniotic fluidwithdrawn
Fetus
Placenta
Uterus Cervix
Centrifugation
Fluid
Fetalcells
Severalhours
Severalweeks
Severalweeks
(a) Amniocentesis (b) Chorionic villus sampling (CVS)
Severalhours
Severalhours
Fetalcells
Bio-chemical
tests
Karyotyping
Placenta Chorionicvilli
Fetus
Suction tubeinserted
throughcervix
Fetal Testing - Amniocentesis
• Tests performed in conjunction with a technique called amniocentesis can determine
if a developing fetus has a genetic disorder
– In amniocentesis, the liquid
that bathes the fetus is
removed and tested
– Can usually be performed
during the 14th -16th week of
pregnancy
– Some disorders can be detected
from the presence of certain
chemicals in the fluid
– Tests for other disorders require
that the amniotic fluid be
cultured to produce cells
• In chorionic villus sampling (CVS), a sample of the placenta is removed and
tested
– Physician inserts narrow tube
through cervix and into the
uterus and suctions out a
tiny sample
– The cells of the chorionic villi of
the placenta (the portion
sampled) are derived from fetus
• These cells have the same genotype as the new individual
• These cells also divide rapidly, allowing for quicker karyotyping and
analysis
– Can be performed as early as the 8th-10th week of pregnancy
Fig. 14-18
Amniotic fluidwithdrawn
Fetus
Placenta
Uterus Cervix
Centrifugation
Fluid
Fetalcells
Severalhours
Severalweeks
Severalweeks
(a) Amniocentesis (b) Chorionic villus sampling (CVS)
Severalhours
Severalhours
Fetalcells
Bio-chemical
tests
Karyotyping
Placenta Chorionicvilli
Fetus
Suction tubeinserted
throughcervix
Fetal Testing: Chorionic Villus Sampling
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