Observing Patterns in Inherited Traits

Chapter 13

Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011.

Cystic Fibrosis

Most common fatal genetic disorder in the US Gene CFTR (encodes protein that moves chloride ions

out of the epithelial cells)• Water follows chloride ions osmosis• Goal thin film of water on surface of epithelial sheets that

allow mucus to slide easily over the wet sheets of cells• Mutation in CFTR gene deletion of three base pairs

•Disrupts membrane trafficking of CFTR •Protein is made by never released from the cell

•Prevents binding with bacteria at the cell surface decrease immune response

Menacing Mucus

• Outcome transport of chloride ions and water out of the epithelial cells are interrupted•Thick globs of mucus accumulate and clog passageways

• Breathing becomes difficult due to mucus obstruction

• Treatment •posture change, thumps on chest and back to clear the

lungs•Antibiotics to control infections

• Lifespan 30 yrs Genetics must have 2 copies of the inherited

mutated gene (one from each parent)• 1:3,300 births

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

Producing Hybrids

Hybrids Heterozygous individual• 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

Mendel’s Experimental Approach

Control over reproduction • Removing a flower’s pollen-bearing anthers

“Bred true” all offspring have the same form of the traits as the parent(s), generation after generation, mutations aside

“Cross-fertilize” transfer pollen among individuals that have different traits• Mendel discovered traits of the offspring from

cross-fertilized pea plants appear in predictable patterns•Outcome Hereditary information is passed from

one generation to the next in discrete units

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 Locus location of a gene on a chromosome

• Figure 13.3 and 13.4 in text 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

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

• A set of alleles carried by an individual Phenotype

• An individual’s observable traits Dominant allele may mask effect of a recessive allele

paired with it Recessive allele is masked by a dominant allele

Modern Genetic Terms

Homozygous having identical alleles of a gene Heterozygous having 2 different alleles of a gene Homozygous dominant

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

• Has two recessive alleles (aa) Heterozygote

• Has two nonidentical alleles (Aa)

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

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

• Law of Segregation 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

• Monohybrid cross•Breeding experiment Aa x Aa (heterozygote's) are

crossed• Frequency of traits among the offspring offers information

about the dominance relationship between the alleles

Punnett square predict the genetic and phenotypic outcome of a cross

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

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

Predicting F2 Offspring (3:1)

TESTCROSS

Testcross • Method of determining genotype in which an

individual of unknown genotype is crossed with one that is known to be homozygous recessive

• Ex. Offspring is homozygous recessive for a trait•One parent is homozygous dominant for that trait,

what is the genotype for the other parent???• SOLVE and WRITE here.

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

Law of segregation• Diploid cells carry pairs of genes, on pairs of

homologous chromosomes• The two genes of each pair are separated from each

other during meiosis, so they end up in different gametes

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)

Frequency of traits among the offspring provides information about the dominance relationship between the paired alleles

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

Linkage Groups

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

Crossing over between homologous chromosomes disrupts gene linkages

Crossover Distance between gene

•Genes far a parts on a chromosome high frequency of crossing over occurs between them•Assort into gametes independently, just as if

they were on different chromosomes •Genes close together on a chromosome

DO NOT assort independently and low frequency of crossing over •Tightly linked genes stay together during

meiosis more frequently

Linkage and Crossing Over

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

Beyond Simple Dominance

Other types of gene expression• Codominant alleles

• Incomplete dominance

• Epistasis

• Pleiotropy

Codominant Alleles

Both alleles are expressed at the same time in heterozygotes • Example: Multiple allele system (gene for which

three or more alleles persist in a population)• ABO blood typing

Fig. 10.10, p.160

Phenotypes(Blood type):

Genotypes:

A AB B O

or

ABAO BO OO

BBAAor

ABO Gene

O is recessive when paired with A or B Incorrect blood transfusion Dangerous

• Result: Red blood cells can clump or burst O type = universal donor AB type = universal recipient

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

Epistasis

Interacting products of one or more genes affect the same trait

Trait is influenced by the products of multiple genes

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

More Epistasis – Skin color and Fur color

Color• Dominant B = black

• Recessive b = brown

• Dominant E = melanin to deposit in fur

• Recessive e = reduces melanin deposition

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

Pleiotropy

A single gene may affect two or more traits• Example: sickle-cell anemia, cystic fibrosis, and

Marfan syndrome Marfan syndrome

• Gene encodes fibrillin give elasticity to the heart, skin, blood vessels, etc•Mutation makes vessels thin and leaky

• Inflamed, thinned, and weakened aorta can rupture during exercise

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 a

range of small differences in a shared trait in a population• Ex. Eye Color

Continuous Variation

# of individuals fall into each category. This gives the relative frequencies ofPhenotypes across a range of measurable Values. Plot data on bar chart

If bell curve trait varies continuously

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.

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rait

Environmental Effect on Phenotype

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

•Length of day affect production of melanin that affect skin and fun color

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)

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60

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Predation and Body Form

Few Predators.

Round Head

ManyPredators. Pointy head

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

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

Animation: Coat color in Labrador retrievers

Animation: Coat color in the Himalayan rabbit

Animation: Codominance: ABO blood types

Animation: Comb shape in chickens

Animation: Continuous variation in height

CLICK HERE TO PLAY

Animation: Crossing garden pea plants

Animation: Crossover review

Animation: Dihybrid cross

Animation: F2 ratios interaction

Animation: Genetic terms

Animation: Incomplete dominance

Animation: Monohybrid cross

Animation: Pleiotropic effects of Marfan syndrome

Animation: Testcross