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Mendel and the Gene Idea

Chapter 14

LECTURE PRESENTATIONS

For CAMPBELL BIOLOGY, NINTH EDITION

Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson

© 2011 Pearson Education, Inc.

Lectures by

Erin Barley

Kathleen Fitzpatrick

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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)

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  • The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes)
  • This hypothesis can explain the reappearance of traits after several generations
  • Mendel documented a particulate mechanism through his experiments with garden peas

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Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance

  • Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments

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  • 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 or cross- pollinate with other F1 hybrids, the F2 generation is produced

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The Testcross

  • How can we tell the genotype of an individual with the dominant phenotype?
  • Such an 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 (“true-breeding”)
  • If any offspring display the recessive phenotype, the mystery parent must be heterozygous

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Table 14.1

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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: alleles, inheritance, dominance, & segregation
  • These concepts can be related to what we now know about genes and chromosomes

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Figure 14.4

Allele for purple flowers

Locus for flower-color gene

Allele for white flowers

Pair of�homologous�chromosomes

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  • 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

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Figure 14.6

Phenotype

Purple

Purple

Purple

White

3

1

1

1

2

Ratio 3:1

Ratio 1:2:1

Genotype

PP�(homozygous)

Pp�(heterozygous)

Pp�(heterozygous)

pp�(homozygous)

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  • Mendel derived the law of segregation by following a single character
  • 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

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  • 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

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Figure 14.8

P Generation

F1 Generation

Predictions

Gametes

EXPERIMENT

RESULTS

YYRR

yyrr

yr

YR

YyRr

Hypothesis of�dependent assortment

Hypothesis of�independent assortment

Predicted�offspring of�F2 generation

Sperm

Sperm

or

Eggs

Eggs

Phenotypic ratio 3:1

Phenotypic ratio 9:3:3:1

Phenotypic ratio approximately 9:3:3:1

315

108

101

32

1/2

1/2

1/2

1/2

1/4

1/4

1/4

1/4

1/4

1/4

1/4

1/4

9/16

3/16

3/16

1/16

YR

YR

YR

YR

yr

yr

yr

yr

1/4

3/4

Yr

Yr

yR

yR

YYRR

YyRr

YyRr

yyrr

YYRR

YYRr

YyRR

YyRr

YYRr

YYrr

YyRr

Yyrr

YyRR

YyRr

yyRR

yyRr

YyRr

Yyrr

yyRr

yyrr

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Mendel’s Inheritance Laws

Law of Segregation - alleles from a parent split during anaphase I of meiosis when making gametes

​

​

​

​

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Mendel’s Inheritance Laws

Law of Independent Assortment - each trait is inherited independently of other traits (Prophase I, homologous pairing)

​

​

​

​

​

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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

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Mathematical Rules Used to Calculate Probabilities

  • The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities
  • This law is used for calculating the probability of ONE gamete or ONE offspring
  • Probability in an F1 monohybrid cross can be determined using the multiplication rule

​

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Figure 14.9

Segregation of�alleles into eggs

Segregation of�alleles into sperm

Sperm

Eggs

1/2

1/2

1/2

1/2

1/4

1/4

1/4

1/4

Rr

Rr

R

R

R

R

R

R

r

r

r

r

r

×

r

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Mathematical Rules Used to Calculate Probabilities

  • The addition rule states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities
  • This law is used for calculating the probability of MORE THAN ONE offspring
  • 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

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Solving Complex Genetics Problems with the Rules of Probability

  • We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters
  • A dihybrid or other multi-character 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

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Figure 14.UN01

Probability of YYRR

Probability of YyRR

1/4 (probability of YY)

1/2 (Yy)

1/4 (RR)

1/4 (RR)

1/16

1/8

=

×

×

=

=

=

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Figure 14.UN02

Chance of at least two recessive traits

ppyyRr

ppYyrr

Ppyyrr

PPyyrr

ppyyrr

1/4 (probability of pp) × 1/2 (yy) × 1/2 (Rr)

1/4 × 1/2 × 1/2

1/2 × 1/2 × 1/2

1/4 × 1/2 × 1/2

1/4 × 1/2 × 1/2

= 1/16

= 1/16

= 2/16

= 1/16

= 1/16

= 6/16 or 3/8

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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

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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

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Degrees of Dominance

  • Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical

​

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Degrees of Dominance

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

​

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Degrees of Dominance

  • Incomplete dominance yields a phenotypic ratio of 1:2:1 in monohybrid crosses

​

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Degrees of Dominance

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

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Multiple Alleles

  • Most genes exist in populations in more than two allelic forms
  • In this case, a trait is controlled by a single gene with more than two alleles
  • 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.

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Figure 14.11

Carbohydrate

Allele

(a) The three alleles for the ABO blood groups and their� carbohydrates

(b) Blood group genotypes and phenotypes

Genotype

Red blood cell�appearance

Phenotype�(blood group)

A

A

B

B

AB

none

O

IA

IB

i

ii

IAIB

IAIA or IAi

IBIB or IBi

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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

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Epistasis

  • In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus
  • For example, in Labrador retrievers 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 color and c for no color) determines whether the pigment will be deposited in the hair

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Figure 14.12

Sperm

Eggs

9

: 3

: 4

1/4

1/4

1/4

1/4

1/4

1/4

1/4

1/4

BbEe

BbEe

BE

BE

bE

bE

Be

Be

be

be

BBEE

BbEE

BBEe

BbEe

BbEE

bbEE

BbEe

bbEe

BBEe

BbEe

BBee

Bbee

BbEe

bbEe

Bbee

bbee

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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
  • Article on the evolution of skin color

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Figure 14.13

Eggs

Sperm

Phenotypes:

Number of�dark-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

1/8

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

AaBbCc

AaBbCc

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

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Figure 14.14

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Concept 14.4: Many human traits follow Mendelian patterns of inheritance

  • Humans are not good subjects for genetic research
    • Generation time is too long
    • Parents produce relatively few offspring
    • Breeding experiments are unacceptable
  • However, basic Mendelian genetics endures as the foundation of human genetics

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Pedigree Analysis

  • 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
  • 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

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Figure 14.15

Key

Male

Female

Affected�male

Affected �female

Mating

Offspring

1st�generation

2nd�generation

3rd�generation

1st�generation

2nd�generation

3rd�generation

Is a widow’s peak a dominant or�recessive trait?

(a)

Is an attached earlobe a dominant�or recessive trait?

b)

Widow’s�peak

No widow’s�peak

Attached�earlobe

Free�earlobe

FF�or�Ff

WW�or�Ww

Ww

ww

ww

Ww

Ww

Ww

Ww

ww

ww

ww

ww

Ff

Ff

Ff

Ff

Ff

ff

ff

ff

ff

FF or Ff

ff

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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 individuals with recessive disorders are born to carrier parents
  • Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair

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Figure 14.16

Parents

Normal�Aa

Sperm

Eggs

Normal�Aa

AA

Normal�

Aa

Normal�(carrier)�

Aa

Normal�(carrier)�

aa

Albino�

A

A

a

a

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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

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Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications

  • 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
  • In homozygous individuals, all hemoglobin is abnormal (sickle-cell)
  • Symptoms include physical weakness, pain, organ damage, and even paralysis

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  • Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms
  • About one out of ten African Americans has sickle cell trait, an unusually high frequency of an allele with detrimental effects in homozygotes
  • Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous

Fig. 14-UN1

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Huntington’s Disease: A Late-Onset Lethal Disease

  • The timing of onset of a disease significantly affects its inheritance
  • Huntington’s disease is a degenerative disease of the nervous system
  • The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age
  • Once the deterioration of the nervous system begins the condition is irreversible and fatal

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Counseling Based on Mendelian Genetics and Probability Rules

  • Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders
  • Probabilities are predicted on the most accurate information at the time; predicted probabilities may change as new information is available

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Tests for Identifying Carriers

  • For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately

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Fetal Testing

  • In amniocentesis, the liquid that bathes the fetus is removed and tested
  • In chorionic villus sampling (CVS), a sample of the placenta is removed and tested
  • Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero

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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

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