CHAPTER 14

MENDEL AND THE GENE IDEA

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I. OVERVIEWI. OVERVIEW

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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) offspring

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 which began in 1860’s

I. INTRODUCTIONI. INTRODUCTIONA. Blending Theory of Heredity

• Pre-Mendel• Proposed that hereditary material from each parent mixes or blends in the offspring

B. Particulate Theory of Heredity• Gregor Mendel’s theory• Parents transmit to their offspring inheritable factors (genes)

• Began in 1860’s

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II. GREGOR MENDELA. Used quantitative approachB. Austrian monkC. Father of GeneticsD. Two reasons he used peas in

his experiments:1. Many easily recognized

characteristics (traits)2. Easy to cross-pollinate

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III. VOCABULARY1. gene—segment of DNA

--responsible for production of 1 protein--Mendel’s factor

2. locus—specific position of a gene on a chromosome

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3. characteristic—inherited feature (Ex: flower color)4. trait—variant of a characteristic (Ex: white or red flowers)5. diploid—refers to cells with 2 sets of chromosomes (2n)6. haploid—refers to cells with 1 set of chromosomes (n)7. allele—alternative forms of a gene8. homozygous—two alleles for a particular characteristic are

identical--TT or tt

9. heterozygous—two alleles for a particular characteristics are different

--Tt

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10. genotype—actual genetic makeup--symbols--Tt, TT, or tt

11. phenotype—effects of the genes--description of appearance--tall or short

12. dominant—refers to trait that always appear in offspring of parents with contrasting traits

tall x short | tall

13. recessive—refers to trait that does not appear in offspring of parents with contrasting traits

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14. monohybrid cross—cross in which only one character is considered

TT x tt | Tt (monohybrid)

15. dihybrid cross—cross in which two characters are considered

TTyy x ttYY | TtYy (dihybrid)

16. P—parental generation --true breeding

17. F1—first filial generation--offspring of P

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18. F2—second filial generation--offspring of F1

19. intercross—cross in which both parents are heterozygous for a characteristic

--1 Factor Intercross (1FIC)•Tt x Tt•Phenotypic ratio 3:1•Genotypic ratio 1:2:1

--2 Factor Intercross (2FIC)•TtYy x TtYy•Phenotypic ratio 9:3:3:1

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20. backcross—cross with a parent or parent-type--Tt x TT or tt

21. testcross—cross unknown genotype with a homozygous recessive

--used to attempt to determine if a dominant trait is homozygous or heterozygous

--T_ x tt22. true breeding—offspring have same traits as parents when

parents self-fertilize--pure breeding--homozygous

23. hybrid—offspring from a cross of parents differing in one or more characteristics

--heterozygous

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IV. MENDEL’S EXPERIMENTSP purple x white

F1 purple (self-pollinate)

F2 705 purple 224 white

Ratio 3.15 :1 ~ 3:113

Mendel’s Characters

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A. Mendel’s Conclusions

1. For each character, an organism inherits two genes (factors), one from each parent.

2. Alternative forms of genes are responsible for variations in inherited characters.

If the two alleles (factors) differ, one is fully expressed (dominant allele); the other is completely masked (recessive allele).

•Symbol for dominant allele—capital letter•Symbol for recessive allele—lowercase letter

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Law of Segregation3. Law of Segregation

• The two genes (factors) for each character segregate during gamete formation

• Occurs during meiosis

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4. Mendel’s Law of Independent Assortment• States: Each allele pair segregates independently of

other gene pairs during gamete formation.• Developed from dihybrid crosses• Refers to behavior of genes during gamete formation• Refers to genes on different chromosomes.• Ex: RrTt x RrTt

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Law of Independent Assortment

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B. Punnett Square• Named for R. C. Punnett• Special chart used to

predict outcome of genetic crosses

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C. Inheritance as a Game of Chance

1. Laws of Probability• Probability of 1 means event certain to occur• Probability of 0 means event certain not to occur• Probability of all possible outcomes for an event add up

to 1• Two basic rules of probability:

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a. Rule of Multiplication

• the probability that independent events will occur simultaneously is the product of their individual probabilities

• What is the probability that offspring will be pp if the parents are Pp x Pp?

-First determine the probability that an egg will receive a “p” allele (½)

-Then determine the probability that a sperm will receive a “p” allele (½)-Solution:

½ x ½ = ¼

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b. Rule of Addition• The probability of an event that can occur in two or more

independent ways is the sum of the separate probabilities of the different ways.

• In the cross Pp x Pp, what is the probability of the offspring being heterozygous (Pp)?

-2 ways:egg sperm probabilityP (½) p (½) = Pp ¼p (½) P (½) = Pp ¼

-Solution¼ + ¼ = ½

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IV. Concept 14.3: Extending IV. Concept 14.3: Extending Mendelian GeneticsMendelian Genetics

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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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A. Extending Mendelian Genetics for a Single Gene

1. 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

V. EXTENDING MENDELIAN GENETICS

Complete Dominance• The phenotypes of the heterozygote and the dominant homozygote are indistinguishable.

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V. EXTENDING MENDELIAN GENETICS

A. Incomplete Dominance• Dominant phenotype is not fully expressed in

heterozygote, resulting in a third phenotype that is intermediate between homozygous dominant and homozygous recessive.

• Example:P red flowers x white flowers

F1 pink flowers (self-pollinate)

F2 ¼ red ½ pink ¼ white

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Degrees of Dominance

• Complete Dominance- the phenotypes of the heterozygote and the dominant homozygote are indistinguishable.

• Incomplete Dominance- neither allele is completely dominant and the F1 hybrids have a phenotype somewhere between those of the two parental varieties.

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

• Symbols: R—red W—whiteRR x WW

RW x RW

¼ RR ½ RW ¼ WW

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R W

R RR RW

W RW WW

B. What is a dominant allele?Complete Incomplete CodominanceDominance Dominance (A is dominant) (A is incompletely (no dominance) dominant)

AA and Aa Aa Aa Same Intermediate Both allelesphenotype phenotype expressed between

AA and aa

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The Relation Between Dominance and Phenotype

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1. A dominant allele does not subdue a recessive allele; alleles don’t interact

2. Alleles are simply variations in a gene’s nucleotide sequence

3. 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 in the brain

B. Codominance• Defined as the full expression of both alleles in a heterozygote• Ex: MN blood type

• Tay Sachs• 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

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Blood Type Genotype

M MM

N NN

MN MN

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Frequency of Dominant Alleles1. Dominant alleles are not necessarily more common in

populations than recessive alleles

2. For example, one baby out of 400 in the United States is born with extra fingers or toes

3. The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage

D. Multiple Alleles• More than two alternative forms (alleles) of a gene• Ex: ABO blood group• The four phenotypes of the ABO blood group (A,

B, AB, O) 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.

• 3 alleles produce 4 possible phenotypes—A, B, O, AB

• IA and IB are dominant to i (recessive).• IA and IB are codominant to each other.• Each person carries only 2 alleles.

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ABO Blood Group

Blood Type Genotypes Antigen on rbc

Antibodies In serum

A IA IA

IA i

A Anti-B

B IB IB

IB i

B Anti-A

AB IA IBB A and B None

O ii None Anti-A and Anti-B

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Blood Problem• Identification bracelets were accidentally removed

from three newborn babies. Blood typings were taken to help in the identification procedures. The blood types for the babies and their parents were: Baby 1- type A, Baby 2- type O, Baby 3- type AB• Mr. Black = type A Mr. Black = type B• Mr. Green = type AB Mrs. Green = type O• Mr. White = type O Mrs. White = type O

• Which baby could belong to Mr. and Mrs. Black?• Which baby could belong to Mr. and Mrs. Green?• Which baby could belong to Mr. and Mrs. White?

E. Pleiotrophy• Ability of a single gene to have multiple phenotypic

effects• For example, pleiotropic alleles are responsible for

the multiple symptoms of certain hereditary diseases

• Ex: sickle cell anemia, cystic fibrosis

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F. Epistasis• Interaction between two nonallelic genes in which one

alters the phenotypic expression of the other (a gene at one locus alters the phenotypic expression of a gene at a second locus)

• Dihybrid cross results will deviate from expected ratio of 9:3:3:1

• 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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F. Epistasis• Ex: albinism (no pigmentation)

BB, Bb-black coat colorbb-brown coat colorCC, Cc-can make pigment (melanin)cc-albino (can not make pigment)

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BbCc x BbCc

BC Bc bC bc

BC BBCC BBCc BbCC BbCc

Bc BBCc BBcc BbCc Bbcc

bC BbCC BbCc bbCC bbCc

bc BbCc Bbcc bbCc bbcc

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9 Black B_ C_

10Brown bb C_

11Albino __ cc

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•Quantitative characters are those that vary in the population along a continuum•Quantitative variation usually indicates polygenic inheritance

G. Polygenic Inheritance

• Additive effect of two or more genes determines a single phenotypic character

• Ex: Skin pigmentation in humans is controlled by at least 3 separately inherited genes.

--incomplete dominance--AABBCC- very dark--aabbcc-very light--AaBbCc-intermediate shade--phenotype can be affected by

environmental factors (sun exposure)

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H. Environmental Effects on Phenotype• Coat color can be influenced by temperature.

-Ex: Siamese cats• Curly wing mutant of Drosophila loses the ability to fly

at less than 16° C• Color of flowers of Hydrangeas because of soil pH

I. Phenocopy• Environmentally produced phenotype that simulates

the effects of a particular gene-Ex: The drug thalidomide mimics the birth

defect phocomelia.

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Environmental Genetic Interaction

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Environmental Effects on Phenotype (Nature vs. Nuture)•Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype

•Coat color can be influenced by temperature.-Ex: Siamese cats

•Curly wing mutant of Drosophila loses the ability to fly at less than 16° C

•Color of flowers of Hydrangeas because of soil pH

Siamese Cat--Coco

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J. Norm of Reaction• Refers to range of phenotypic possibilities due to

environmental influences• The norm of reaction is the phenotypic range

of a genotype influenced by the environment• Nature vs. Nurture (genetics vs. environment)• Norms of reaction are broadest for polygenic

characters such as skin color which are usually referred to as multifactorial (both genetic and environmental factors influence phenotype).

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

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Environmental Genetic Interaction

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VI. MENDELIAN INHERITANCE IN HUMANS

A. Difficult to study because:• generation time• number of offspring• can not control breeding experiments• different genetic backgrounds

B. Human Pedigrees (Family Tree)• Shows inheritance pattern of a particular phenotypic

character• Used to show Mendelian inheritance in humans• Helps to understand the past and predict the future• Important in genetic counseling for disabling or lethal

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JamesPotter

Thomas

Anna LoraineBrackettThomas

Marjorie LoraineThomas Rickard

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Jean EvonThomasCoffman

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BillyJohn

Thomas

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CharlieLouis

Thomas

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RDRickard

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WayneLeon

Austin

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FranklinDale

Young

BrandonShaneYoung

TonyaPattyRickard

Jackie KayeKnight YoungUnderw ood

KimberlySutherlinAustin

AndreaLynn Jones

Austin

LauraBeth

Austin

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KellyJan

Austin

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StephenMichaelAustin

8

LukeAndrewAustin

6

BenjaminThomasAustin

BryanAdamAustin

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AshleyMischelle

Austin

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AlyssaKendallAustin

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HadleyMischelle

Hardin

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AustinMichaelMoore

Janet LouiseRickardAustin

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Susan KayRickardYoung

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DavidRickard

ElyBrandonYoung

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GavinLee

Young

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RogerWayneAustin

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BryanKeith

Austin

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MichaelShaneAustin

Grace LouiseThomasTotton

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LynnRickard

SallyRickard

RobertCarn

Woosley

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Callie BeeMcElw ainWoosley

83

GrundyRickard

JoeRickard

JessicaMarieHale

Martha LeonaWoosleyAustin

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Guy OrtherWoosley

AubreyWoosley

ClintAustin

Eff ie ElizabethLindseyAustin

DarrellAustin

HarrellAustin

FriedaAustin

GladysWoosley

MableCrabtree

CharlesAustin

JohnBrackett

Archie CastleHuckleberry

Brackett

RoyBrackett

PearlBrackett

AdamHardin

MichaelMoore

ChristinaMichelleRickard

DavidAndrewRickard

TomGrundy

WaltRickard

CharlesRickard

FrankRickard

LeeRickardCarrico

WilliamAustin

Eff ie JaneHayesAustin

BerniceAustin

Grimmizon

UraAustin

Brackett

EdSmith

EmuelSmith

PotterDame

DeloresLee Dame

Ferrell

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AlvinLindsey

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Ann ArpisineWoosleyLindsey

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John Jim SeaberryEvaMittie

JohnJack

Woosley

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RosemanJones

Woosley

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JamesS.

Jones

Anna ArpaDuvallJones

JosephL.

Woosley

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Marth E.PressleyWoosley

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VerdaMorehead

My Family Pedigree

Pedigree of Queen Victoria

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C. Autosomal Recessive Disorders

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1. Recessively inherited disorders show up only in individuals homozygous for the allele

2. Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented)

3. Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair

4. If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low

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

Autosomal Recessive Disorders• Usually involves a malfunctional protein or no protein

production at all• Consanguinity

-genetic relationship resulting from matings between closely related people-often produces homozygous recessive offspring-not always harmful

• Heterozygotes can be normal• Examples:

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1. Cystic Fibrosis• Most common lethal genetic disease in US• Common in Caucasians• Caused by lack of or defective membrane protein that pumps Cl- out of

cells• striking one out of every 2,500 people of European descent• Increased secretions of mucus from pancreas and lungs• No cure• Symptoms include mucus buildup in some internal organs and

abnormal absorption of nutrients in the small intestine• Treat with diet and antibiotics

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2. Tay-Sachs• Most common in individuals of Central European Jewish descent• Enzyme does not function properly; can not metabolize gangliosides

(lipid)• Lipids accumulate in brain• Lethal gene

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3. Sickle-cell Anemia• Most common in African-Americans• Caused by single amino acid substitution in hemoglobin

(valine for glutamic acid)• Causes abnormally shaped rbc• Heterozygous individuals are said to have sickle-cell trait• Heterozygous condition is malaria resistant

4. Albinism• Decreased or lack of production of melanin

5. Glactosemia• Lack an enzyme to break down galactose

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ALBINISM

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6. PKU• Phenylketanuria• Can not metabolize amino acid phenylalanine• Can result in mental retardation

7. Sickle- Cell Anemia• Most common in African-Americans• Caused by single amino acid substitution in hemoglobin (valine for glutamic acid)

• Causes abnormally shaped rbc• Heterozygous individuals are said to have sickle-cell trait

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Autosomal Dominant Disorders 1. Achondroplasia

• Type of dwarfism• Lethal in homozygous state

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AchondroplasiaAchondroplasia

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2. Huntington’s Disease•Degeneration of nervous system•Late-acting lethal dominant•Onset about age 40

3. Manic-Depressive Disease4. Polydactyly

•More than 5 digits on hand and/or feet

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E. Genetic Screening and Counseling1. Carrier Recognition

•Tests are available to detect carriers of Tay-Sachs, cystic fibrosis, and sickle-cell trait.

2. Fetal Testinga. Amniocentesis

•involves removal of amniotic fluid•can analyze fluid for chemical content•culture cells for karyotyping•done between 14-16 weeks

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Amniocentesis Chorionic Villus Sampling

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b. Chorionic Villi Sampling• fetal tissue suctioned from chorionic villi of placenta• results within 24 hours• done at 8-10 weeks

c. Ultrasound• sound waves used to study fetal structure

d. Fetoscopy• involves insertion of fiber-optic scope into uterus

e. Newborn Screening• PKU screening

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F. Multifactorial Disorders• Disease that have both genetic and environmental influences

• Ex: heart disease, diabetes, cancer, alcoholism, some forms of mental illness

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You should now be able to:

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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

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4. Define and give examples of pleiotropy and epistasis5. Explain why lethal dominant genes are much rarer than

lethal recessive genes6. Explain how carrier recognition, fetal testing, and

newborn screening can be used in genetic screening and counseling

Monohybrid Cross• A brown dog is homozygous for the gene

that controls coat color. The brown dog is mated with an albino (all white) dog. The dogs have many puppies. All of the puppies have brown coat color.• Draw a punnett square for this cross and give

the expected genotypic and phenotypic outcomes.

• What are the dominant and recessive alleles? Provide symbols for both alleles.

• What would be the results if these offspring mated with an albino dog? A homozygous brown dog? A heterozygote?

Dihybrid Cross• About 70% of Americans perceive a bitter taste from the

chemical phenylthiocarbamide (PTC). The ability to taste this chemical results from a dominant allele (T) and not being able to taste PTC is the result of having two recessive alleles (t). Albinism is also a single locus trait with normal pigment being dominant (A) and the lack of pigment being recessive (a). A normally pigmented woman who is heterozygous for PTC tasting, has a father who is homozygous for both albinism and PTC tasting. She marries a heterozygous, normally pigmented man who is a taster but who has a mother that does not taste PTC• Give the phenotypic and genotypic ratios of the offspring.

• A blue-eyed, left-handed woman marries a brown-eyed, right handed man who is heterozygous for both traits. Blue eyes and left-handedness are recessive.• Give the phenotypic and genotypic ratios of the offspring.

Exit Slip• If a man with type AB blood marries

a woman with type O blood, what blood types would you expect in their children. (Popcorn)

Homework:

• Read chapter 14, sections 3