chapter 14: mendel and genes

Chapter 14Mendel and the

Gene Idea

Overview: Drawing from the Deck of Genes• What genetic principles account for the passing

of traits from parents to offspring?• The “blending” hypothesis is the idea that

genetic material from the two parents blends together (like blue and yellow paint blend to make green)

• The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes)

• Mendel documented a particulate mechanism through his experiments with garden peas

Mendel’s Experimental, Quantitative Approach

• Advantages of pea plants for genetic study:– There are many varieties with distinct heritable

features, or characters (genes) (such as color); character variations are called traits (alleles)

– 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

Figure 14.2Technique

1

2

Parentalgeneration(P)

Stamens

Carpel3

4

5

Results

First filialgenerationoffspring(F1)

-short generation time-lots of progeny-controlled matings

Mendel Chose Pea Plants as His Experimental Organism

• Hybridization– The mating or crossing between two individuals that

have different characteristics• Purple-flowered plant X white-flowered plant

• Hybrids– The offspring that result from such a mating

• He also used varieties that were “true-breeding” (plants that produce offspring of the same variety when they self-pollinate for 5 generations)

• In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization

• The true-breeding parents are the P generation• The hybrid offspring of the P generation are

called the F1 generation

• When F1 individuals self-pollinate, the F2 generation is produced

The Law of Segregation• When 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 three to one, purple to white flowers, in the F2 generation

Figure 14.3-3

705 purple-floweredplants

224 white-floweredplants

Self- or cross-pollinationAll plants had purple flowers

Purpleflowers

Whiteflowers

P Generation(true-breeding

parents)

F1 Generation(hybrids)

F2 Generation

Experiment

• Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids

• Mendel called the purple flower color a dominant trait and white flower color a recessive trait

• 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

Table 14.1

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

Figure 14.4

Allele forwhite flowers

Allele forpurple flowers

Pair ofhomologouschromosomes

Enzyme that helpssynthesize purplepigment

Absence of enzyme

Enzyme

Locus forflower-color gene

One alleleresults insufficientpigment

C T A A A T C G G T

G A T T T A G C C A

CTAAATCGGT

ATAAATCGGT

A T A A A T C G G T

T T T TA A G C C A

Concept 1. Alternative versions (now called alleles) of inherited characters account for variations. Each allele resides at a specific locus (gene) on a specific chromosome.

Concept 2. Each character of an organism inherits two alleles, one from each parent. Mendel made this deduction without knowing about the role of chromosomes.

Concept 3. If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance. The recessive is masked when the dominant is present.

Concept 4, now known as the law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes. Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism

This segregation of alleles corresponds to the separation of homologous chromosomes to different gametes in anaphase I of meiosis.

• Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses

• The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup

• A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele

Figure 14.5-3

P Generation

F1 Generation

Appearance:Genetic makeup:Gametes:

Purple flowersPP

White flowerspp

Purple flowersPp

P p

P p1 2 1 2

P p

p

PPP Pp

ppPp

Sperm from F1 (Pp) plantF2 Generation

Appearance:Genetic makeup:Gametes:

Eggs fromF1 (Pp) plant

3 : 1

• 1. Write down the genotypes of both parents– Male parent = Tt– Female parent = Tt

• 2. Write down the possible gametes each parent can make.– Male gametes: T or t– Female gametes: T or t

Punnett Squares

• 3. Create an empty Punnett square

• 4. Fill in the Punnett square with the possible genotypes of the offspring

• 5. Determine the relative proportions of genotypes and phenotypes of the offspring– Genotypic ratio

• TT : Tt : tt• 1 : 2 : 1

– Phenotypic ratio• Tall : dwarf

• 3 : 1

Phenotype Genotype

Purple PP(homozygous)

Purple

Purple

White

Ratio 3:1 Ratio 1:2:1

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

3

1 1

1

2

1. Imagine a genetic counselor working with a couple who have just had a child who is suffering from Tay-Sachs disease. Neither parent has Tay-Sachs, nor does anyone in their families. Which of the following statements should this counselor make to this couple?

a. “Because no one in either of your families has Tay-Sachs, you are not likely to have another baby with Tay-Sachs. You can safely have another child.”

b. “Because you have had one child with Tay-Sachs, you must each carry the allele. Any child you have has a 50% chance of having the disease.”

c. “Because you have had one child with Tay-Sachs, you must each carry the allele. Any child you have has a 25% chance of having the disease.”

d. “Because you have had one child with Tay-Sachs, you must both carry the allele. However, since the chance of having an affected child is 25%, you may safely have thee more children without worrying about having another child with Tay-Sachs.”

e. “You must both be tested to see who is a carrier of the Tay-Sachs allele.”

The Testcross• How can we tell the genotype of an individual

with the dominant phenotype?• Such an individual must have one dominant

allele, but the individual could be either homozygous dominant or heterozygous

• The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual

• If any offspring display the recessive phenotype, the mystery parent must be heterozygous

Figure 14.7

Dominant phenotype,unknown genotype:

PP or Pp?

Recessive phenotype,known genotype:

pp

Predictions

Eggs

All offspring purple

or1 2 offspring purple and

offspring white1 2

Eggs

Results

Technique

If purple-floweredparent is PP

If purple-floweredparent is Pp

Sperm Sperm

or

pp p p

P

p

P

P

Pp Pp

PpPp pp

Pp Pp

pp

• Mendel identified his second law of inheritance by following two characters at the same time

• Crossing two, true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters

• A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

The Law of Independent Assortment

LE 14-8P Generation

F1 Generation

YYRR

Gametes YR yr

yyrr

YyRrHypothesis ofdependentassortment

Hypothesis of independent assortment

SpermEggs

YR

Yr

yrYR

YR

yr

Eggs

YYRR YyRr

YyRr yyrr yR

yrPhenotypic ratio 3:1

F2 Generation(predictedoffspring)

YYRR YYRr YyRR YyRr

YYRr YYrr YyRr Yyrr

YyRR YyRr yyRR yyRr

YyRr Yyrr yyRr yyrr

Phenotypic ratio 9:3:3:1

YR Yr yR yr

Sperm

1 21 4

1 4

1 4

1 4

1 43 4

1 2

1 2

1 2

1 4

9 16 3 16 3 16 3 16

1 4 1 4 1 4

• Using a dihybrid cross, Mendel developed the law of independent assortment

• The law of independent assortment states that each pair of alleles segregates independently of other pairs of alleles during gamete formation

• Strictly speaking, 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 14.2: The laws of probability govern Mendelian inheritance

• Mendel’s laws of segregation and independent assortment reflect the rules of probability

• 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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The Multiplication and Addition Rules Applied to Monohybrid Crosses

• The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities

• Probability in an F1 monohybrid cross can be determined using the multiplication rule

• Segregation in a heterozygous plant is like flipping a coin: Each gamete has a 1/2 chance of carrying the dominant allele and a 1/2 chance of carrying the recessive allele


Eggs

1 2

Sperm

RrSegregation of

alleles into eggs

RrSegregation of

alleles into sperm

1 2

1 4

1 4

1 4

1 4

1 2 1 2R

RRR

R

r

r

rrr

Rr

• The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities

• The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous


Solving Complex Genetics Problems with the Rules of Probability

• We can apply the rules of multiplication and addition 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

Binomial Expansion Equation• Represents all of the possibilities for a given set of

unordered events

2-63Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

n!x! (n – x)! px qn – x

• where – p = probability that the unordered number of events will occur

– n = total number of events– x = number of events in one category– p = individual probability of x– q = individual probability of the other category

P =

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

• Note:– p + q = 1– The symbol ! denotes a factorial

• n! is the product of all integers from n down to 1– 4! = 4 X 3 X 2 X 1 = 24– An exception is 0! = 1

2-64

• Question– Two heterozygous brown-eyed (Bb) individuals have

five children– What is the probability that only two of the couple’s five

children will have blue eyes?

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

• Applying the binomial expansion equation– Step 1: Calculate the individual probabilities

• This can be obtained via a Punnett square

1/4 P(blue eyes) = p =

3/4 P(brown eyes) = q =

2-65

– Step 2: Determine the number of events• n = total number of children = 5• x = number of blue-eyed children = 2

– Step 3: Substitute the values for p, q, x, and n in the binomial expansion equation

2-66Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

• Therefore 26% of the time, a heterozygous couple’s five children will contain two with blue eyes and three with brown eyes

n!x! (n – x)! px qn – x P =

5!2! (5 – 2)! (1/4)2 (3/4)5 – 2 P =

5 X 4 X 3 X 2 X 1(2 X 1) (3 X 2 X 1)

(1/16) (27/64) P =

P = 0.26 or 26%

2. Imagine a locus with four different alleles for fur color in an animal. The alleles are named Da, Db, Dc, and Dd. If you crossed two heterozygotes, DaDb and DcDd, what genotype proportions would you expect in the offspring?

a. 25% DaDc, 25% DaDd, 25% DbDc, 25% DbDd

b. 50% DaDb, 50% DcDd

c. 25% DaDa, 25% DbDb, 25% DcDc, 25% DdDdDcDd

d. 50% DaDc, 50% DbDd

e. 25% DaDb, 25% DcDd, 25% DcDc, 25% DdDd

2. Imagine a locus with four different alleles for fur color in an animal. The alleles are named Da, Db, Dc, and Dd. If you crossed two heterozygotes, DaDb and DcDd, what genotype proportions would you expect in the offspring?

a. 25% DaDc, 25% DaDd, 25% DbDc, 25% DbDdb. 50% DaDb, 50% DcDd

c. 25% DaDa, 25% DbDb, 25% DcDc, 25% DdDdDcDd

d. 50% DaDc, 50% DbDd

e. 25% DaDb, 25% DcDd, 25% DcDc, 25% DdDd

3. An individual with the genotype AaBbEeHH is crossed with an individual who is aaBbEehh. What is the likelihood of having offspring with the genotype AabbEEHh?

a. 1/8b. 1/16c. 1/32d. 1/64e. That genotype would be impossible.

3. An individual with the genotype AaBbEeHH is crossed with an individual who is aaBbEehh. What is the likelihood of having offspring with the genotype AabbEEHh?

a. 1/8b. 1/16c. 1/32 ( ½ x ¼ x ¼ x 1)d. 1/64e. That genotype would be impossible.

Extending Mendelian Genetics for a Single Gene

• Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:– When alleles are not completely dominant or

recessive– When a gene has more than two alleles– When a gene produces multiple phenotypes

Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics

The Spectrum of Dominance • Complete dominance occurs when phenotypes

of the heterozygote and dominant homozygote are identical

• In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

• In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

LE 14-10

RedCRCR

Gametes

P Generation

CR CW

WhiteCWCW

PinkCRCW

CRGametes CW

F1 Generation

F2 Generation EggsCR CW

CRCRCR CRCW

CRCW CWCWCW

Sperm

12

12

12

12

12

12

The 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

• 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 inthe brain– At the organismal level, the allele is recessive– At the biochemical level, the phenotype (i.e., the

enzyme activity level) is incompletely dominant– At the molecular level, the alleles are codominant

Frequency of Dominant Alleles• Dominant alleles are not necessarily more

common in populations than recessive alleles• For example, one baby out of 400 in the United

States is born with extra fingers or toes• The allele for this unusual trait is dominant to

the allele for the more common trait of five digits per appendage

• In this example, the recessive allele is far more prevalent than the dominant allele in the population

Multiple Alleles• Most genes exist in populations in more than

two allelic forms• For example, 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, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

Figure 14.11The three alleles for the ABO blood groups and theircarbohydrates

Blood group genotypes and phenotypes

Allele

Carbohydrate

Genotype

Red blood cellappearance

Phenotype(blood group) A B AB O

iiIAIBIBIB IBiororIAIA IAi

IA IB i

(a)

(b)

A B none

Pleiotropy• Most genes have multiple phenotypic effects, a

property called pleiotropy • For example, pleiotropic alleles are responsible

for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

Extending Mendelian Genetics for Two or More Genes

• Some traits may be determined by two or more genes

Epistasis• In epistasis, a gene at one locus alters the

phenotypic expression of a gene at a second locus• For example, in mice and many other mammals,

coat color depends on two genes• One gene determines the pigment color (with

alleles B for black and b for brown)• The other gene (with alleles C for pigment color

and c for no pigment color ) determines whether the pigment will be deposited in the hair

BbEe BbEe

Sperm

Eggs

9 3 4::

BBEE

BbEE

BBEe

BbEe bbEe

BbEe

bbEE

BbEE BBEe

BbEe

BBee

Bbee bbee

Bbee

bbEe

BbEe

beBe1 4 1 4

1 4

1 4

1 4

1 4

1 4 1 4bEBE

BE

bE

Be

be

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

Figure 14.13

Eggs

Sperm

AaBbCc AaBbCc

Phenotypes:Number ofdark-skin alleles: 0 1 2 3 4 5 6

1 8

1 8

1 8

1 8

1 8

1 8

1 8

1 8

1 64 6 64 15 64 20 64 15 64 6 64 1 64

1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8


Nature and Nurture: The Environmental Impact on Phenotype

• Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype

• The norm of reaction is the phenotypic range of a genotype influenced by the environment

• For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity


Figure 14.14

• Norms of reaction are generally broadest for polygenic characters

• Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype

Concept 14.4: Pedigrees for Human Traits

• Humans are not good subjects for genetic research because generation time is too long; parents produce relatively few offspring; and breeding experiments are unacceptable

• A pedigree is a family tree that describes the interrelationships of parents and children across generations

• Inheritance patterns of particular traits can be traced and described using pedigrees

Figure 14.15Key

Male Female Affectedmale

Affectedfemale

Mating Offspring, inbirth order(first-born on left)

ff FfFfFf

FF Ff ff ff Ff Ff ff

ff FF

Ffor

or

No widow’s peak

1st generation(grandparents)

2nd generation(parents, aunts,and uncles)

3rd generation(two sisters)

(a) Is a widow’s peak a dominant or recessive trait?

(b) Is an attached earlobe a dominantor recessive trait?

Attached earlobe Free earlobe

Wwww

wwWwWwWw ww ww

wwWw

Ww

WWor

ww

Widow’s peak

• Pedigrees can also be used to make predictions about future offspring

• We can use the multiplication and addition rules to predict the probability of specific phenotypes

Recessively Inherited Disorders• Many genetic disorders are inherited in a

recessive manner• 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

• Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele

Cystic Fibrosis• Cystic fibrosis is the most common lethal genetic

disease in the United States, striking one out of every 2,500 people of European descent

• The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes leading to a buildup of chloride ions outside the cell

• Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

Sickle-Cell Disease• Sickle-cell disease affects one out of 400 African-Americans• The disease is caused by the substitution of a single amino

acid in the hemoglobin protein in red blood cells• Symptoms include physical weakness, pain, organ damage,

and even paralysis• Heterozygotes are less susceptible to the malaria parasite, so

there is an advantage to being heterozygous in regions where malaria is common

Figure 14.17Sickle-cell alleles

Low O2

Sickle-cellhemoglobinproteins

Sickled redblood cells

Part of a fiber ofsickle-cell hemo-globin proteins

Sickle-cell alleleNormal allele

Very low O2

Sickle-celldisease

Sickle-celltrait

Sickled andnormal redblood cells

Part of a sickle-cellfiber and normalhemoglobin proteins

Sickle-cell andnormal hemo-globin proteins

Homozygote with sickle-cell disease: Weakness, anemia, pain and fever,organ damage

Heterozygote with sickle-cell trait: Some symptoms when blood oxygen isvery low; reduction of malaria symptoms

(a)

(b)

Dominantly Inherited Disorders• Some human disorders are due to dominant

alleles• One example is achondroplasia, a form of

dwarfism that is lethal when homozygous for the dominant allele

ParentsNormaldd

DwarfDd

Sperm

DdDwarf

DdDwarf

ddNormal

ddNormal

d

d

dD

• Huntington’s disease is a degenerative disease of the nervous system

• The disease has no obvious phenotypic effects until about 35 to 40 years of age

4. Imagine that the last step in a biochemical pathway to the red skin pigment of an apple is catalyzed by enzyme X, which changes compound C to compound D. If an effective enzyme is present, compound D is formed and the apple skin is red because there is very little substrate relative to enzyme in the cell. However, if the enzyme is not effective, only compound C is present and the skin is yellow. What can you accurately say about a heterozygote with one allele for an effective enzyme X and one allele for an ineffective enzyme X?

a. The phenotype will probably be yellow but cannot be red.

b. The phenotype will probably be red but cannot be yellow.

c. The phenotype will be a yellowish red.d. The phenotype will be either yellow or red.e. The phenotype will be either yellowish

red or red.

4. Imagine that the last step in a biochemical pathway to the red skin pigment of an apple is catalyzed by enzyme X, which changes compound C to compound D. If an effective enzyme is present, compound D is formed and the apple skin is red because there is very little substrate relative to enzyme in the cell. However, if the enzyme is not effective, only compound C is present and the skin is yellow. What can you accurately say about a heterozygote with one allele for an effective enzyme X and one allele for an ineffective enzyme X?

a. The phenotype will probably be yellow but cannot be red.

b. The phenotype will probably be red but cannot be yellow.

c. The phenotype will be a yellowish red.d. The phenotype will be either yellow or red.e. The phenotype will be either yellowish

red or red. This could be the answer if the enzyme amount was limited.

Genetic Testing and Counseling

Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease

Fetal Testing• In amniocentesis, the liquid that bathes the fetus is

removed and tested• In chorionic villus sampling (CVS), a sample of the

placenta is removed and tested• Other techniques, ultrasound and fetoscopy, allow

fetal health to be assessed visually in utero

Figure 14.19(a) Amniocentesis (b) Chorionic villus sampling (CVS)Ultrasoundmonitor

FetusPlacentaUterus Cervix

Amnioticfluidwithdrawn

Karyotyping

Severalhours

Severalhours

Fetal cells

Cervix

Suctiontubeinsertedthroughcervix

Ultrasoundmonitor

FetusPlacentaChorionicvilli

Uterus

CentrifugationSeveralhours

Severalweeks

Severalweeks

Biochemicaland genetic

tests

Fluid

Fetalcells

1

2

3

2

1

chapter 14: mendel and genes

Science