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