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Patterns of Inheritance/Mendelian

GeneticsChapter 9, 12

Student Learning Goals & Achievement Scale – Biology

Mendel’s Laws, Genetics, and Patterns of Inheritance

SC.912.L.16.1

• Goals: Use Mendel’s Laws of Segregation and Independent

Assortment to analyze patterns of inheritance.

• 4 - Explore Mendel’s Laws of Segregation and Independent

Assortment to analyze patterns of inheritance.

• 3 - Use Mendel’s Laws of Segregation and Independent

Assortment to analyze patterns of inheritance.

• 2 - Summarize Mendel’s Laws of Segregation and Independent

Assortment to analyze patterns of inheritance.

• 1 – Define Mendel’s Laws of Segregation and Independent

Assortment to analyze patterns of inheritance.

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Learning Objectives1. Describe how Mendel was able to control how

his pea plants were pollinated.

2. Describe the steps in Mendel’s experiments on

true-breeding garden peas.

3. Distinguish between dominant and recessive

traits.

4. State two laws of heredity that were developed

from Mendel’s work

5. Describe how Mendel’s results can be

explained by scientific knowledge of genes and

chromosomes3

Learning Objectives

6. Differentiate between the genotype and phenotype of an organism.

7. Explain how probability is used to predict the results of genetic crosses.

8. Use a Punnett square to predict the results of a monohybrid and dihybrid

genetic crosses.

9. Explain how a testcross is used to show the genotype of an individual

whose phenotype expresses the dominant trait.

10. Differentiate a monohybrid cross from a dihybrid cross.

11. Distinguish between sex chromosomes and autosomes.

12. Explain the role of sex chromosomes in sex determination.

13. Describe how an X or Y-linked gene affects the inheritance of traits.

14. Explain the effect of crossing-over on the inheritance of genes in linkage

groups.

15. Distinguish between chromosome mutations and gene mutations.

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Learning Objectives

16. Analyze pedigrees to determine how

genetic traits and genetic disorders are

inherited.

17.Explain the inheritance of the ABO blood

type

18.Explain how geneticists can detect and

treat genetic disorders.

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Schedule and Announcements

• Quiz Thursday December 3

• Exam 3- Tuesday December 8 over 9, 12

• Semester Exam Tuesday December 15 @

2:15 (cumulative)

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Early Ideas of Heredity

• Before the 20th century, 2 concepts were

the basis for ideas about heredity:

1.) heredity occurs within species

2.) traits are transmitted directly from parent to

offspring (The homunculus myth)

• This led to the belief that inheritance is a

matter of blending traits from the parents.

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Gregor Mendel

• Mendel song

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

2xpTz7SUbnc)

• Born in 1822

• Education: University of Vienna

– Failed exit examinations

• Returned to monastery

• Mendel published his work in 1865.

• That work was lost until ca. 1900.

• With the “rediscovery” of Mendel’s

conceptual work the hunt was on for

the physical nature of the gene.

• What was it and how did it function?

• These questions were largely

answered from 1940’s through the

1960’s and lead to the biotech

revolution beginning of the 1970’s.

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The Garden Pea

• Pisum sativum

• Easy to grow

• Produces many varieties

• Male and female organs

in the same flower

– Self-fertilization

– Cross-fertilization

• What if Mendel choose to

work with sheep instead?

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Early Ideas of Heredity

• Mendel’s experimental method:

1. produce true-breeding strains for each trait he was studying

2. cross-fertilize true-breeding strains having alternate forms of a trait

-perform reciprocal crosses as well

3. allow the hybrid offspring to self-fertilize and count the number of offspring showing each form of the trait

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Monohybrid Crosses

• Monohybrid cross: a cross to study only

2 variations of a single trait

• Mendel produced true-breeding pea

strains for 7 different traits

– each trait had 2 alternate forms (variations)

– Mendel cross-fertilized the 2 true-breeding

strains for each trait

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Monohybrid Crosses

• F1 generation (1st filial generation):

offspring produced by crossing 2 true-

breeding strains

• For every trait Mendel studied, all F1

plants resembled only 1 parent

– no plants with characteristics intermediate

between the 2 parents were produced

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Monohybrid Crosses

• F1 generation: offspring resulting from a cross of true-breeding parents

• F2 generation: offspring resulting from the self-fertilization of F1 plants

• dominant: the form of each trait expressed in the F1 plants

• recessive: the form of the trait not seen in the F1 plants

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Monohybrid Crosses

• F2 plants exhibited both forms of the trait in a very specific pattern:

• ¾ plants with the dominant form

• ¼ plant with the recessive form

• The dominant to recessive ratio was 3 : 1.

• Mendel discovered the ratio is actually:• 1 true-breeding dominant plant

• 2 not-true-breeding dominant plants

• 1 true-breeding recessive plant

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Monohybrid Crosses

• gene: information for a trait passed from

parent to offspring

• alleles: alternate forms of a gene

• homozygous: having 2 of the same allele

• heterozygous: having 2 different alleles

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Monohybrid Crosses

• genotype: total set of alleles of an

individual

– PP = homozygous dominant

– Pp = heterozygous

– pp = homozygous recessive

• phenotype: outward appearance of an

individual

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Monohybrid Crosses

• Principle of Segregation

Two alleles for a gene segregate during

gamete formation and are rejoined at

random, one from each parent, during

fertilization.

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Dihybrid Crosses

• Dihybrid cross: examination of 2

separate traits in a single cross

– for example: RR YY x rryy

• The F1 generation of a dihybrid cross

(RrYy) shows only the dominant

phenotypes for each trait.

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Dihybrid Crosses

• The F2 generation is produced by crossing

members of the F1 generation with each

other or allowing self-fertilization of the F1.

– for example RrYy x RrYy

• The F2 generation shows all four possible

phenotypes in a set ratio:

– 9 : 3 : 3 : 1

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Dihybrid Crosses

• Principle of Independent Assortment

In a dihybrid cross, the alleles of each gene

assort independently.

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Probability – Predicting Results

• Rule of addition: the probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities.

• When crossing Pp x Pp, the probability of producing Pp offspring is

– probability of obtaining Pp (1/4), PLUS probability of obtaining pP (1/4)

– ¼ + ¼ = ½

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Probability – Predicting Results

• Rule of multiplication: the probability of 2 independent events occurring simultaneously is the PRODUCT of their individual probabilities.

• When crossing Rr Yy x RrYy, the probability of obtaining rr yy offspring is:

– probability of obtaiing rr = ¼

– probability of obtaining yy = ¼

– probability of rr yy = ¼ x ¼ = 1/16

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Testcross

• Testcross: a cross used to determine the

genotype of an individual with dominant

phenotype

– cross the individual with unknown genotype

(e.g. P_) with a homozygous recessive (pp)

• the phenotypic ratios among offspring are

different, depending on the genotype of the

unknown parent

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Extensions to Mendel

• Mendel’s model of inheritance assumes

that:

– each trait is controlled by a single gene

– each gene has only 2 alleles

– there is a clear dominant-recessive

relationship between the alleles

• Most genes do not meet these criteria.

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

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Extensions to Mendel

• Incomplete dominance: the heterozygote

is intermediate in phenotype between the

2 homozygotes.

• Codominance: the heterozygote shows

some aspect of the phenotypes of both

homozygotes.

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• Tay-Sachs disease is fatal; a dysfunctional

enzyme causes an accumulation of lipids in

the brain

– 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

Multiple Alleles

• Most genes exist in populations in more than

two allelic forms

• For example, the four phenotypes of the ABO

blood group in humans are determined by

three alleles of the gene: IA, IB, and i.

• The enzyme (I) adds specific carbohydrates to

the surface of blood cells

• The enzyme encoded by IA adds the A

carbohydrate, and the enzyme encoded by IB

adds the B carbohydrate; the enzyme encoded

by the i allele adds neither

Figure 11.11

Carbohydrate

(b) Blood group genotypes and phenotypes

Allele

Red blood cellappearance

Genotype

noneBA

IB

Phenotype(blood group)

iIA

IAIB iiIAIA or IAi IBIB or IBi

BA OAB

(a) The three alleles for the ABO blood groups and theircarbohydrates

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Extensions to Mendel

• Polygenic inheritance occurs when multiple genes are involved in controlling the phenotype of a trait.

• The phenotype is an accumulation of contributions by multiple genes.

• These traits show continuous variationand are referred to as quantitative traits.

• For example – human height

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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

Figure 11.12

¼

¼

¼

¼

¼ ¼¼¼ BE Be

BE

be

BBEE

bbee

BbEE BbEe

bE be

bE

Be

BBEe

BbEE bbEE bbEeBbEe

BBEe BbEe BbeeBBee

BbEe bbEe Bbee

9 : 4: 3

Eggs

Sperm

BbEe BbEe

Polygenic Traits

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Extensions to Mendel

• Pleiotropy refers to an allele which has

more than one effect on the phenotype.

• This can be seen in human diseases such

as cystic fibrosis or sickle cell anemia.

• In these diseases, multiple symptoms can

be traced back to one defective allele.

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Extensions to Mendel

• The expression of some genes can be

influenced by the environment.

• for example: coat color in Himalayan

rabbits and Siamese cats

– an allele produces an enzyme that allows

pigment production only at temperatures

below 30oC

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Extensions to Mendel

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Figure 11.14

WW

or

Ww

ww

ww ww

ww ww

WwWw

Ww Ww

No widow’s peakWidow’s peak

wwWw

1st generation(grandparents)

3rd generation(two sisters)

2nd generation(parents,aunts, anduncles)

Affected male

Affected female

Male Female

Key

Mating

Attachedearlobe

Freeearlobe

Offspring, inbirth order(first-born on left)

FF

or

Ff

ff

ff ff

Ff ff

FfFF or

Ff

Ff Ff

ffFf

(a) Is a widow’s peak a dominant or recessive trait? (b) Is an attached earlobe a dominant

or recessive trait?

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 people who have recessive disorders

are born to parents who are carriers of the

disorder

Figure 11.15

Parents

Sperm

NormalAa

NormalAa

EggsAA

Normal

AaNormal(carrier)

AaNormal(carrier)

aaAlbino

A

a

A a

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

• 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

Figure 11.UN05

In the whole population,some genes have morethan two alleles

Pleiotropy

Relationship amongalleles of a single gene Description Example

Codominance

Multiple alleles

Incomplete dominance

of either allele

Complete dominance

of one allele

One gene is able to affectmultiple phenotypiccharacters

Both phenotypesexpressed inheterozygotes

Heterozygous phenotypeintermediate betweenthe two homozygousphenotypes

Heterozygous phenotypesame as that of homo-zygous dominant

ABO blood group alleles

Sickle-cell disease

IAIB

IA, IB, i

CRCR CRCW CWCW

PP Pp

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Figure 11.UN06

Polygenic inheritance A single phenotypiccharacter is affected bytwo or more genes

The phenotypicexpression of one geneaffects the expressionof another gene

Epistasis

Relationship amongtwo or more genes Description Example

AaBbCc AaBbCc

BbEe BbEe

BE

BE bE

bE

be

be

Be

Be

9 : 3 : 4

Chromosomes, Mapping, and the

Meiosis-Inheritance Connection

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Chromosome Theory

• Chromosomal theory of inheritance

– developed in 1902 by Walter Sutton

– proposed that genes are present on chromosomes

– based on observations that homologous chromosomes pair with each other during meiosis

– supporting evidence was provided by work with fruit flies

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Chromosome Theory

• T.H. Morgan isolated a mutant white-eyed

Drosophila

• red-eyed female X white-eyed male gave

a F1 generation of all red eyes

• Morgan concluded that red eyes are

dominant

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Chromosome Theory

• Morgan crossed F1 females X F1 males

• F2 generation contained red and white-

eyed flies but all white-eyed flies were

male

• testcross of a F1 female with a white-eyed

male showed the viability of white-eyed

females

• Morgan concluded that the eye color gene

is linked to the X chromosome

• Chromosomal basis

of sex linkage

– White-eyed male flies

X red-eyed females

– F1 flies all have red

eyes

– F2 flies, all of the

white-eyed flies are

males because the Y

chromosome lacks

the white gene

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Sex Chromosomes

• Sex determination in Drosophila is based on the

number of X chromosomes

• 2 X chromosomes = female

• 1 X and 1 Y chromosome = male

• Sex determination in humans is based on the

presence of a Y chromosome

• 2 X chromosomes = female

• having a Y chromosome (XY) = male

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Sex Chromosomes• In many organisms, the Y chromosome is

greatly reduced or inactive.

• genes on the X chromosome are present in only 1 copy in males

• sex-linked traits: controlled by genes present on the X chromosome

• Human X-linked disorders

– Color blindness, Muscular dystrophy, Hemophilia, Fragile X syndrome

• Sex-linked traits show inheritance patterns different than those of genes on autosomes.

Royal Hemophilia Pedigree

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Sex Chromosomes

• Dosage compensation ensures an equal expression of genes from the sex chromosomes even though females have 2 X chromosomes and males have only 1.

• In each female cell, 1 X chromosome is inactivated and is highly condensed into a Barr body.

• Females heterozygous for genes on the X chromosome are genetic mosaics.

Genetic basis behind a calico

cat

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Chromosome Theory Exceptions

• Mitochondria and chloroplasts contain

genes.

• traits controlled by these genes do not

follow the chromosomal theory of

inheritance

• genes from mitochondria and chloroplasts

are often passed to the offspring by only

one parent

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Chromosome Theory Exceptions

• Maternal inheritance: uniparental (one-

parent) inheritance from the mother

• the mitochondria in a zygote are from the

egg cell; no mitochondria come from the

sperm during fertilization

• in plants, the chloroplasts are often

inherited from the mother, although this is

species dependent

Human X Chromosome Gene

Map

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Human Genetic Disorders

• Some human genetic disorders are caused by altered proteins.

• the altered protein is encoded by a mutated DNA sequence

• the altered protein does not function correctly, causing a change to the phenotype

• the protein can be altered at only a single amino acid (e.g. sickle cell anemia)

Sickle-Cell Anemia

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Human Genetic Disorders

• Some genetic disorders are caused by a

change in the number of chromosomes.

• nondisjunction during meiosis can create

gametes having one too many or one too

few chromosomes

• fertilization of these gametes creates

trisomic or monosomic individuals

• Down syndrome is trisomy of chromosome

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Down Syndrome

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Human Genetic Disorders• Nondisjunction of sex chromosomes can

result in:Syndrome Sex Disorder Chromosome

#

Spontaneous

abortions

Live

births

Turner F XO 45 1/18 1/ 2,500

Klinefelter M XXY OR

XXXY

47 or 48 1/300 1/800

Poly-X F XXX OR

XXXX

47 or 48 0 1/ 1,500

Jacobs M XYY 47 ? 1/1,000

Down M or F Trisomy 21 47 1/40 1/800

Abnormalities in the # of sex

chromosomes

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Human Genetic Disorders

• Genetic counseling can use pedigree analysis to determine the probability of genetic disorders in the offspring.

• Some genetic disorders can be diagnosed during pregnancy.

• amniocentesis collects fetal cells from the amniotic fluid for examination

• chorionic villi sampling collects cells from the placenta for examination

Amniocentesis

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Chorionic villi sampling

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