Observing Patterns in Inherited Traits

Chapter 10

10.1 Mendel, Pea Plants, and Inheritance Patterns

By experimenting with pea plants, Mendel was the first to gather evidence of patterns by which parents transmit genes to offspring

Mendel’s Experiments

a Garden pea flower, cut in half. Sperm form in pollen grains, which originate in male floral parts (stamens). Eggs develop, fertilization takes place, and seeds mature in female floral parts (carpels).

b Pollen from a plant that breeds true for purple flowers is brushed onto a floral bud of a plant that breeds true for white flowers. The white flower had its stamens snipped off. This is one way to assure cross-fertilization of plants.

c Later, seeds develop inside pods of the cross-fertilized plant. An embryo within each seed develops into a mature pea plant.

d Each new plant’s flower color is indirect but observable evidence that hereditary material has been transmitted from the parent plants.

Fig. 10.3, p.154

carpel stamen

Animation: Crossing garden pea plants

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Producing Hybrids

Hybrids • Offspring of a cross between two individuals that

breed true for different forms of a trait

Each inherits nonidentical alleles for a trait being studied

Producing Hybrid Offspring

fertilization produces heterozygous offspring

meiosis II

meiosis I

(chromosomes duplicated

before meiosis)

Homozygous dominant parent

Homozygous recessive parent

(gametes) (gametes)

Fig. 10.5, p.156

Heritable Units of Information

Genes • Heritable units of information about traits• Each has its own locus on the chromosome

Alleles• Different molecular forms of the same gene

Mutation• Permanent change in a gene’s information

Heritable Units of Information

b A gene locus (plural, loci), the location for a specific gene on a chromosome. Alleles are at corresponding loci on a pair of homologous chromosomes

d Three pairs of genes (at three loci on this pair of homologous chromosomes); same thing as three pairs of alleles.

Fig. 10.4, p.155

a A pair of homologous chromosomes,both unduplicated. In most species, oneis inherited from a female parent and itspartner from a male parent.

c A pair of alleles may be identicalor not. Alleles are represented in the text by letters such as D or d.

Modern Genetic Terms

Homozygous dominant • Has two dominant alleles for a trait (AA)

Homozygous recessive • Has two recessive alleles (aa)

Heterozygote • Has two nonidentical alleles (Aa)

Modern Genetic Terms

Dominant allele may mask effect of recessive allele on the homologous chromosome

Genotype • An individual’s alleles at any or all gene loci

Phenotype• An individual’s observable traits

Animation: Genetic terms

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Key Concepts: MODERN GENETICS

Gregor Mendel gathered the first indirect, experimental evidence of the genetic basis of inheritance

His meticulous work tracking traits in many generations of pea plants gave him clues that heritable traits are specified in units

The units, distributed into gametes in predictable patterns, were later identified as genes

10.2 Mendel’s Theory of Segregation

Mendel’s Theory of Segregation: • Diploid organisms have pairs of genes, on pairs

of homologous chromosomes • Based on monohybrid experiments

During meiosis• Genes of each pair separate• Each gamete gets one or the other gene

Producing Hybrid Offspring

Crossing two true-breeding parents of different genotypes yields hybrid offspring

All F1 offspring are heterozygous for a gene, and can be used in monohybrid experiments• All F1 offspring of parental cross AA x aa are Aa

A Monohybrid Cross

Crosses between F1 monohybrids resulted in these allelic combinations among F2 offspring

• Phenotype ratio 3:1• Evidence of dominant and recessive traits

F2 Offspring: Dominant and Recessive Traits

Trait Studied

Dominant Form

Recessive Form

F2 Dominant-to-Recessive

Ratio

Seed shape

Seed color

Pod shape

Pod color

Flower color

Flower position

Stem length

2.98:1

3.01:1

2.95:1

2.82:1

3.15:1

3.14:1

2.84:1787 tall 277 dwarf

651 long stem 207 at tip

705 purple 224 white

152 yellow428 green

299 wrinkled882 inflated

6,022 yellow 2,001 green

5,474 round 1,850 wrinkled

Fig. 10.6, p.156

Predicting Probability: Punnett Squares

female gametes

mal

e g

amet

es

Fig. 10.7a, p.157

a From left to right, step-by-step construction of a Punnett square. Circlessignify gametes. A stands for a dominant allele and a for a recessive allele atthe same gene locus. Offspring genotypes are indicated inside the squares.

A

A A A A

AAAAA

Aa

AaAa

AaAaaa aa aa aa

a

a

a

a

a

a

a

a

aA

aaAa

A

a

aA

aaAa

AaA

a

aA

aaAa

AaAAA

a

aA

a aa

A

female gametes

mal

e g

amet

es

Stepped Art

Fig. 10-7a, p.157

Predicting F1 Offspring

Fig. 10.7b, p.157

A

AA

Aaa Aa

AaAa

A

a

aa

Aa Aa

AaAa

True-breeding homozygousrecessive parent plant

F1 offspring

b Cross between two plants that breed true for different forms of a trait.

True-breeding homozygousdominant parent plant

Predicting F2 Offspring

Fig. 10.7c, p.157

AAa

AAa

Aa

a

a

Aa

AA Aa

aaAa

F2 offspring

HeterozygousF1 offspring

HeterozygousF1 offspring

c Cross between heterozygous F1 offspring.

aa

AA

Key Concepts: MONOHYBRID EXPERIMENTS

Some experiments yielded evidence of gene segregation

When one chromosome separates from its homologous partner during meiosis, the pairs of alleles on those chromosomes also separate and end up in different gametes

Animation: Monohybrid cross

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Animation: F2 ratios interaction

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Animation: Testcross

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10.3 Mendel’s Theory of Independent Assortment

Mendel’s Theory of Independent Assortment:• Meiosis assorts gene pairs of homologous

chromosomes independently of gene pairs on all other chromosomes

• Based on dihybrid experiments

Pairs of homologous chromosomes align randomly at metaphase I

Independent Assortment in Meiosis I

Fig. 10.8, p.158

One of two possible alignments The only other possible alignment

c Possiblecombinationsof alleles ingametes:

b The resultingalignments atmetaphase II:

a Chromosomealignments atmetaphase I:

A

a

AB Abab aB

a

aa

a abb

b b

b

b

A A

AA

b b

A A B B

BB

B B

aa

aa

aa

b

b

bbB

BB

B B

B

AA

AA

Dihybrid Experiments

Start with a cross between true-breeding heterozygous parents that differ for alleles of two genes (AABB x aabb)

All F1 offspring are heterozygous for both genes (AaBb)

Mendel’s Dihybrid Experiments

AaBb x AaBb

Phenotypes of the F2 offspring of F1 hybrids were close to a 9:3:3:1 ratio• 9 dominant for both traits• 3 dominant for A, recessive for b• 3 dominant for B, recessive for a• 1 recessive for both traits

Results of Mendel’s Dihybrid Experiments

Fig. 10.9, p.159

parent homozygous recessivefor white flowers, short stemsGametes at fertilization

parent homozygous dominantfor purple flowers, tall stems

Meiosis, gamete formationin true-breeding parent

plants

Possible genotypes resulting from a cross between two F1 plants:meiosis,gameteformation

All F1 plants are AaBbheterozygotes with purpleflowers and tall stems.

meiosis,gameteformation

Key Concepts: DIHYBRID EXPERIMENTS

Some experiments yielded evidence of independent assortment

During meiosis, the members of a pair of homologous chromosomes are distributed into gametes independently of all other pairs

Animation: Dihybrid cross

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10.4 Beyond Simple Dominance

Other types of gene expression• Codominant alleles • Incomplete dominance • Epistasis• Pleiotropy

Codominant Alleles

Both expressed at the same time in heterozygotes • Example: Multiple alleles in ABO blood typing

Fig. 10.10, p.160

Phenotypes(Blood type):

Genotypes:

A AB B O

or

ABAO BO OO

BBAAor

Animation: Codominance: ABO blood types

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Incomplete Dominance

An allele is not fully dominant over its partner on a homologous chromosome• Both are expressed • Produces a phenotype between the two

homozygous conditions

Incomplete Dominance

Fig. 10.11, p.160

Cross two of theF1 plants, andthe F2 offspringwill show threephenotypes ina 1:2:1 ratio:

homozygousparent (RR)

homozygousparent (rr)

heterozygousF1 offspring (Rr)x

RR Rr Rr rr

Animation: Incomplete dominance

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Epistasis

Interacting products of one or more genes affect the same trait

Fig. 10.12, p.161

9/16 walnut 3/16 rose 3/16 pea 1/16 single comb

F2 offspring:

RRpp (rose comb) X rrPP (pea comb)

F1 of spring: RrPp (all walnut comb)

RrPp RrPp

RRPP, RRPp,RrPP, or RrPp

RRpp or Rrpp rrPP or rrPp rrpp

X

Animation: Comb shape in chickens

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More Epistasis

Fig. 10.13, p.161

EB Eb eB eb

EB

Eb

eB

eb EeBbblack

EeBBblack

EEBbblack

EEBBblack

EEBbblack

EeBBblack

EeBbblack

Eebbchocolate

EeBbblack

EEbbchocolate

EeBbblack

Eebbchocolate

eeBByellow

eeBbyellow

eebbyellow

eeBbyellow

Animation: Coat color in Labrador retrievers

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Pleiotropy

A single gene may affects two or more traits• Example: Marfan syndrome

Animation: Pleiotropic effects of Marfan syndrome

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10.5 Linkage Groups

All genes on the same chromosome are part of one linkage group

Crossing over between homologous chromosomes disrupts gene linkages

Linkage Groups and Meiosis

During meiosis, genes relatively close together on a chromosome tend to stay together• Few crossover events occur between them

Genes that are relatively far apart tend to assort independently into gametes • Greater frequency of crossing over between them

Linkage and Crossing Over

Fig. 10.15, p.162

Parentalgeneration

F1 offspring

Gametes

Most gametes haveparental genotypes

A smaller number haverecombinant genotypes

meiosis, gamete formation

All AaCc

acAC

×

A

A

A A aa

a

a

c

c

c c CC

C

C

Animation: Crossover review

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10.6 Genes and Environment

Environmental factors may affect gene expression in individuals• Example: Temperature and fur color

Animation: Coat color in the Himalayan rabbit

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Elevation and Plant Height

Fig. 10.17, p.163

c Mature cuttingat low elevation(30 meters abovesea level)

b Mature cuttingat mid-elevation(1,400 metersabove sea level)

a Mature cuttingat high elevation(3,060 metersabove sea level)

He

igh

t (c

en

tim

ete

rs)

He

igh

t (c

en

tim

ete

rs)

He

igh

t (c

en

tim

ete

rs) 60

0

60

0

60

0

Predation and Body Form

10.7 Complex Variations in Traits

Polygenic Inheritance • When products of

many genes influence a trait, individuals of a population show a range of continuous variation for the trait

Continuous Variation

Fig. 10.19a, p.164

Range of values for the trait

This red graph line of therange of variation for a traitin a population plots out asa bell-shaped curve. Suchcurves indicate continuousvariation in a population.

Nu

mb

er

of

ind

ivid

ual

sw

ith

a m

eas

ura

ble

val

ue

fo

r th

e t

rait

Animation: Continuous variation in height

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Variations in Gene Expression

Gene interactions and environmental factors affect most phenotypes • Gene products control metabolic pathways• Mutations may alter or block pathways

Key Concepts: VARIATIONS ON MENDEL’S THEME

Not all traits have clearly dominant or recessive forms

One allele of a pair may be fully or partially dominant over its nonidentical partner, or codominant with it

VARIATIONS ON MENDEL’S THEME (cont.)

Two or more gene pairs often influence the same trait, and some single genes influence many traits

The environment also influences variation in gene expression

Video: One Bad Transporter and Cystic Fibrosis

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