Chapter 14 Mendel and the Gene Idea. Overview: Drawing from the Deck of Genes What genetic...

Chapter 14Chapter 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)

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

• 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


Beliefs about Heredity

Fig. 1. De la propagation du genre humain, ou manuel indispensable pour ceux qui veulent avoir de beaux enfants de l’un ou l’autre sexe (Paris, Year VII). Image courtesy of the Bibliothèque Interuniversitaire de Médecine, Paris.

HomunculusHow is “heredity passed on:

Spermist vs Ovists

Spermist conception of a

human sperm

Homunculus

Leeuwenhoek’s black male and white female rabbit experiments: spermist “proof”

Darwin

What he got right. What he got wrong.

• Acquired characteristics

Ex. Blind cave animals

• Sex. Repro.- gemmules from all over body are packed in sperm and egg

• Blended inheritance

• Likes produce likes

• Change can be permanent

• There is no limit to cumulative change

Gregor MendelMendelian GeneticsMendelian Genetics

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

PPP Pp

p

pPp pp

3 1

Mendel’s Three Principles

• Dominance

• Segregation

• Independent Assortment

The foundation of “classical” scienceThe foundation of “classical” science

(1822-1884)

Dominance

• Traits of both parents inherited, but one shows over the other

• Traits are not blended

Dominance Mechanism

• Two alleles are carried for each trait

• In true-breeding individuals, both alleles are the same.

• Hybrids, on the other hand, have one of each kind of allele.

• One trait is dominant, the other trait is recessive

Segregation

• Half the gametes (egg or sperm) will carry the traits of one parent and half the traits for the other parent

Pairs of alleles are separated (=segregated) during meiosisPairs of alleles are separated (=segregated) during meiosis

Two different parental characteristics will be inherited independently of one another during gamete formation.

Independent Assortment

Example: flower color and leaf shape

• Genes- genetic material on a chromosome that codes for a specific trait

• Genotype- the genetic makeup of the organism

• Phenotype- the expressed trait

• Allel- an alternative form of a gene

Dominance Mechanism

• Two alleles are carried for each trait

• In true-breeding individuals, both alleles are the same (homozygous).

• Hybrids, on the other hand, have one of each kind of allele (heterozygous).

• One trait is dominant, the other trait is recessive

Genetic Information

Genes are traits

“Eye color”

Ear lobe connectedness

Genes produce proteins

Enzymes are proteins

Homologous Chromosomes

gene: location

allele: specific trait

Allele Example

Gene = “eye color”

Alleles

brownbluegreenlavender

Allele Examples

appearance

eye color:homozygous

Allele Examples

appearance

eye color:heterozygous,

brown dominant over blue

Genotype vs Phenotype

homozygous(dominant)

heterozygous

homozygous

(recessive)

genotype phenotype

appearanceappearance

Phaner = visible

Punnett Square

If male & female are heterozygous for eye color

X

brown: 3/4 offspring

blue: 1/4 offspring

male

female

PKU

Each parent carries one gene for PKU.

P p

P pX

P

p

pP

P

P

P

p

p

P

p

p

Possible genotypes: 1PP 2Pp 1pp

Possible phenotypes:no PKU PKU

Compare this to what would have happened if one parent was homozygous for sickle cell.

HbA HbA

HbS HbS

X

HbS

HbS

HbAHbA

HbA

HbS

HbA

HbS

HbS

HbA

HbS

HbA

all offspring are carriers of sickle cell trait

Color blind- sex linked

XS Xs

Xs Y

X

Xs

Y

XsXS

XS

Xs

Xs

Xs

Y

XS

Y

XsS= color blind

s= normal

Phenotype

HbA Hb_

HbA HbB

X

HbA

HbB

Hb_HbA

HbA

HbA

HbA

Hb_

HbB

HbA

Hb_

HbB

A

B

AB

O

GenotypeBlood Type

AA, A_

BB, B_

AB

_ _

Outcome: AA, A_, AB, B_

Where Does Genetic Diversity Come Where Does Genetic Diversity Come From?From?

• Mutation

• Chromosomal Aberrations

• Genetic Recombination (e.g., from sexual reproduction)

MutationMutation

Mutations are changes to the sequence of nucleotide bases within a gene.

A possible consequence is the change in the amino acid sequence of a protein.

Mutations are rare, but occur often enough to be important (each of us will probably pass at least one mutation on to our offspring).

mutationmutation

Sickle Cell Mutation

CTG ACT CCT GAG GAG AAG TCT

Leu Thr Pro Glu Glu Lys Ser

CTG ACT CCT GAG GTG AAG TCT

Leu Thr Pro Glu Val Lys Ser

NORMAL Hb

SICKLE CELL

MutationMutation• Some mutations may be deleterious, while

others are may enhance chances for survival.

• Probably most mutations are neutral, having little or no effect on the fitness of the organism.

• Mutations may accumulate generation after generation, eventually leading to substantial differences in the characteristics of a species over time.

• Some mutations may be deleterious, while others are may enhance chances for survival.

• Probably most mutations are neutral, having little or no effect on the fitness of the organism.

• Mutations may accumulate generation after generation, eventually leading to substantial differences in the characteristics of a species over time.

Autosomes and Sex Chromosomes

Red-Green Color Blindness

Sex-linked trait

XC Y

XC Xc

X

XC

Xc

YXC

XC

XC

XC

Y

Xc

XC

Y

Xc

Normal male

Normal female recessive gene

Possible outcomes: XCXC XCXc XCY XcYNormal

femaleNormal

Female

(carrier)

Normal

male

Color-blind male

Dominance

Most traits show complete dominance

Blending unexpected

E unconnected earlobe

e connected earlobe

E unconnected earlobe

e connected earlobe

alleleallele genegene

P EE x ee

gametes E e

F1 Ee

unconnected

connected

F1 Ee x Ee

gametes1/2 E 1/2 e1/2 E 1/2 e

E

e

E e

EE Ee

Ee ee

F2 1 EE 2 Ee 1 ee

PunnettSquare

P

F1

F2

EE, ee

Ee

EE, 2 Ee, ee

50%

100%

75%

1:1

1:1

1:1

generation genotypes unconnected E:e

Basis of the Castle-Hardy-Weinberg Law

phenotypesphenotypes

ratio of alleles in the populationratio of alleles in the population

Genotypes Phenotypes

Experiment to determinedominant vs. recessiveExperiment to determinedominant vs. recessive

Genetic Sleuthing

My eye color phenotype is brown.

What is my genotype?

Pedigree phenotypesphenotypes

infer genotypes

Alternative:

look directly at the DNA

Complexities

Multiple genes for one trait

Example: eye color

Blended traits (“incomplete dominance”)

Influence of the environment

Degrees of Dominance

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

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

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


Fig. 14-10-1

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

Fig. 14-10-2

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes 1/21/2

Fig. 14-10-3

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes 1/21/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR CRCW

CRCW CWCW

1/21/2

1/2

1/2

Fig. 14-UN2

Degree of dominance

Complete dominanceof one allele

Incomplete dominanceof either allele

Codominance

Description

Heterozygous phenotypesame as that of homo-zygous dominant

Heterozygous phenotypeintermediate betweenthe two homozygousphenotypes

Heterozygotes: Bothphenotypes expressed

Multiple alleles

Pleiotropy

In the whole population,some genes have morethan two alleles

One gene is able toaffect multiplephenotypic characters

CRCR CRCW CWCW

IAIB

IA , IB , i

ABO blood group alleles

Sickle-cell disease

PP Pp

Example

Disorders

Down’s Syndrome (chrom 21)

Huntington’s (chrom 4)

Alzheimer’s (chrom 1, 10, 14, 19, 21)

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


Fig. 14-14

Tongue Roller

R = Tongue Rollerr = Unable to Roll Tongue

Widow’s Peak

W = Widows Peakw = Lack of Widow’s Peak

Free Ear Lobe Attached Ear Lobe

E = Free Ear Lobee = Attached Ear Lobe

Hitchhiker’s Thumb

Hi = Straight Thumbhi = Hitchhiker’s Thumb

Bent Little Finger

Bf = Bent Little Fingerbf = Straight Little Finger

Mid-digital Hair

M = Mid-Digital Hairm = Absence of Mid-Digital Hair

Dimples

D = Dimplesd = Absence of Dimples

Short Hallux

Ha = Short Halluxha = Long Hallux

Short Index Finger

Ss = Short Index FingerS1 = Long Index Finger

*Sex-Influenced Trait*Sex-Influenced Trait

http://www.youtube.com/watch?v=gCPuHzbb5hA

Human Genome Project

U.S. govt. project coordinated by the Department of Energy and the National Institutes of Health, launched in 1986 by Charles DeLisi.

Definition: GENOME – the whole hereditary information of an organism that is encoded in the DNA.

Project Goal: to identify the approximate 100,000 genes in the human DNA.

- determine the sequences of the 3 billion bases that make up human DNA.

- store this information in databases.

- develop tools for data analysis.

- address the ethical, legal, and social issues that arise from genome research.

• Bacteria (E. coli, influenza, several others)

• Yeast (Saccharomyces cerevisiae)

• Plant (Arabidopsis thaliana)

• Fruit fly (Drosophila melanogaster)

• Mouse (Mus musculus)

Modeled Organisms

DNA from 5 humans:

• 2 males, 3 females

• 2 caucasians, one each of asian, african, hispanic

• Cut up DNA with restriction enzymes

• Ligated into BACs & YACs, then grew them up

• Sequenced the BACs

• Let a supercomputer put the pieces together

Craig VenterCelera Genomics

Importance of geneticsImportance of genetics• Understanding hereditary diseases and to Understanding hereditary diseases and to

develop new treatmentsdevelop new treatments

• Donor matchesDonor matches

• PaternityPaternity

• ForensicsForensics

• EvolutionEvolution

• MigrationMigration

Polynesian OriginsPolynesian Origins

Bismarck Archipelago 3.5ka

http://www.sciencedaily.com/releases/2011/02/110203124726.htm

mtDNA

Polynesian OriginsPolynesian Origins

Genetic Testing

Would you want to know?

• Ethical concerns

• Cost

• Insurance companies

see

Genetic Testing

Gel electrophoresis

PCR is a rapid, inexpensive and simple way of copying specific DNA fragments from minute quantities of source DNA material

Three steps are involved in PCR: • Denaturation• Annealing• Extension

Polymerase Chain Reaction

Genetic Testing

Paternity Test $99

$99, looks at specific diseases

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


Tests for Identifying Carriers

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


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


Video: Ultrasound of Human Fetus IVideo: Ultrasound of Human Fetus I

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 tubeinsertedthroughcervix

About 1-4% of DNA in Modern Europeans and Asians was inherited from Neanderthals

Ozzy Osbourne's Genome Reveals Some Neanderthal Lineage

http://www.scientificamerican.com/article.cfm?id=ozzy-osbourne-genome

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


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


Chapter 14 Mendel and the Gene Idea. Overview: Drawing from the Deck of Genes What genetic...

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