Chapter 9 Patterns of Inheritance

Chapter 9Chapter 9

Patterns of InheritancePatterns of Inheritance

HeredityHeredity: The transmission of traits from : The transmission of traits from one generation to another.one generation to another.

VariationVariation: Offspring are different from their : Offspring are different from their parents and siblings.parents and siblings.

GeneticsGenetics: The scientific study of heredity : The scientific study of heredity and hereditary variation. and hereditary variation.

Involves study of cells, individuals, their Involves study of cells, individuals, their offspring, and populations.offspring, and populations.

I. History of GeneticsI. History of Genetics

Prehistoric Times:Prehistoric Times: Little is known about when Little is known about when

humans first recognized the importance of humans first recognized the importance of

heredity. heredity.

Domestication and breeding of horses, cattle, and Domestication and breeding of horses, cattle, and

various breeds of dogs around 8000 and 1000 B.C. various breeds of dogs around 8000 and 1000 B.C.

Cultivation of many plants (corn, wheat, and rice) Cultivation of many plants (corn, wheat, and rice)

around 5000 B.C. in Mexico and other regions.around 5000 B.C. in Mexico and other regions.

Artificial pollinationArtificial pollination of date palms by Assyrians of date palms by Assyrians

around 850 B.C.around 850 B.C.

I. History of GeneticsI. History of Genetics Greek Influence:Greek Influence:

Pythagoras:Pythagoras: Greek philosopher speculated around Greek philosopher speculated around

500 B.C. that human life begins with male and 500 B.C. that human life begins with male and

female fluids, or female fluids, or semenssemens, originating in body parts., originating in body parts.

Hippocrates:Hippocrates: Around 500-400 B.C., Theory of Around 500-400 B.C., Theory of

pangenesispangenesis. “Humors” from an individual’s body . “Humors” from an individual’s body

collect in their semen, and are passed on to next collect in their semen, and are passed on to next

generation. Humors could be generation. Humors could be healthyhealthy or or diseaseddiseased. .

Acquired characteristics could be inherited.Acquired characteristics could be inherited.

Aristotle:Aristotle: 384-322 B.C. Postulated that semens were 384-322 B.C. Postulated that semens were

purified blood and that purified blood and that bloodblood was the element of was the element of

heredity. The heredity. The potentialpotential to produce body features to produce body features

was inherited, not the features themselves.was inherited, not the features themselves.

I. History of GeneticsI. History of Genetics Blending Hypothesis:Blending Hypothesis: In 1800s biologists and plant In 1800s biologists and plant

breeders suggested that traits of parents mix to form breeders suggested that traits of parents mix to form

intermediateintermediate traits in offspring. traits in offspring.

ParentsParents OffspringOffspring

Red flower x White flowerRed flower x White flower Pink flowerPink flower

Tall height x Short heightTall height x Short height Medium heightMedium height

Blue bird x Yellow birdBlue bird x Yellow bird Green birdsGreen birds

Fair skin x dark skinFair skin x dark skin Medium skin colorMedium skin color

If If blending blending always occurred, eventually all always occurred, eventually all extremeextreme

characteristicscharacteristics would would disappeardisappear from the population. from the population.

Gregor Mendel:Gregor Mendel: Established genetics as a science in Established genetics as a science in

1860s. Considered the founder of 1860s. Considered the founder of modern geneticsmodern genetics..

II. Modern GeneticsII. Modern Genetics

Began as a science in 1860s. Began as a science in 1860s.

Gregor Mendel:Gregor Mendel: An Austrian monk, who was a An Austrian monk, who was a

farmer’s son. He was trained in mathematics, farmer’s son. He was trained in mathematics,

chemistry, and physics.chemistry, and physics. Studied the breeding patterns of plants for over 10 years.Studied the breeding patterns of plants for over 10 years.

Artificially crossed Artificially crossed peaspeas, watermelons, and other plants., watermelons, and other plants.

Kept Kept meticulous recordsmeticulous records of thousands of breedings and of thousands of breedings and

resulting offspring.resulting offspring.

Rejected blending hypothesisRejected blending hypothesis, and stressed that heritable , and stressed that heritable

factors (factors (genesgenes) retain their ) retain their individualityindividuality generation after generation after

generation.generation.

II. Modern GeneticsII. Modern Genetics

Gregor Mendel:Gregor Mendel: Calculated the Calculated the mathematical probabilitiesmathematical probabilities of inheriting of inheriting

many genetic traits.many genetic traits.

Published results in 1866. They were Published results in 1866. They were largely ignoredlargely ignored due due

to fervor surrounding Darwin’s publications on to fervor surrounding Darwin’s publications on

evolution.evolution.

Discouraged by the lack of attention from the scientific Discouraged by the lack of attention from the scientific

community, he quit his work and died a few years later.community, he quit his work and died a few years later.

Importance of Mendel’s work was not appreciated until Importance of Mendel’s work was not appreciated until

early 1900searly 1900s when his paper was rediscovered. when his paper was rediscovered.

III. Mendel’s ExperimentsIII. Mendel’s Experiments Used “Used “true-breedingtrue-breeding” or ” or purebredpurebred plant varieties for seven plant varieties for seven

pea characteristics. Self-pollination produces pea characteristics. Self-pollination produces all identical all identical

offspring.offspring.

Using Using artificial pollinationartificial pollination, he crossed true-bred varieties., he crossed true-bred varieties.

Trait Trait VarietiesVarieties

Flower colorFlower color Purple or whitePurple or white

Seed colorSeed color Yellow Yellow or or greengreen

Seed shapeSeed shape Round or wrinkledRound or wrinkled

Pod colorPod color GreenGreen or or YellowYellow

Pod shapePod shape Smooth or constrictedSmooth or constricted

Flower positionFlower position Axial or terminalAxial or terminal

Plant heightPlant height Tall or shortTall or short

Seven Pea Characteristics Studied by Mendel

The Pea Flower Has Both Male and Female Parts

Mendel Used Artificial Fertilization to Cross DifferentVarieties of Peas

III. Mendel’s ExperimentsIII. Mendel’s Experiments

Question:Question: What will we obtain when we cross a What will we obtain when we cross a

pea plant with purple flowers with one with white pea plant with purple flowers with one with white

flowers?flowers?

Possible outcomesPossible outcomes::

1.1. If blending hypothesis is true, then plants would If blending hypothesis is true, then plants would

be an be an intermediate colorintermediate color, e.g.: light purple., e.g.: light purple.

2.2. Some Some plantsplants will be purple, others will be white. will be purple, others will be white.

3.3. All plantsAll plants will be purple will be purple oror all plants will be all plants will be

white.white.

When Mendel Crossed Purple with White Flower Plants All Plants in the First Generation Had Purple Flowers

Purple is Dominant Over White Flower Color

III. Summary of Mendel’s ResultsIII. Summary of Mendel’s Results

All plants displayed All plants displayed one trait only.one trait only.

Trait Trait VarietiesVarieties OffspringOffspringFlower colorFlower color Purple or whitePurple or white 100% Purple100% Purple

Seed colorSeed color Yellow Yellow or or greengreen 100% Yellow100% Yellow

Seed shapeSeed shape Round or wrinkledRound or wrinkled 100% Round100% Round

Pod colorPod color GreenGreen or or YellowYellow 100% Green100% Green

Pod shapePod shape Smooth or constrictedSmooth or constricted 100% Smooth100% Smooth

Flower positionFlower position Axial or terminalAxial or terminal 100% Axial100% Axial

Plant heightPlant height TallTall or short or short 100% 100% TallTall

The trait that prevailed was The trait that prevailed was dominantdominant, the other , the other recessiverecessive..

IV. Mendel’s ConclusionsIV. Mendel’s Conclusions

1.1. Results indicate that blending hypothesis is not Results indicate that blending hypothesis is not

true.true.

2. 2. Only one of the two traits appeared in the first Only one of the two traits appeared in the first

generation. He called this the generation. He called this the dominantdominant trait. trait.

He called the trait that disappeared the He called the trait that disappeared the recessiverecessive

trait.trait.

Mendel then asked the following questions:Mendel then asked the following questions: What has happened to the What has happened to the recessiverecessive (white) trait? (white) trait?

Has it been Has it been lostlost? ?

Has it been Has it been alteredaltered??

Do the crossbred plants carry genetic Do the crossbred plants carry genetic

information for the recessive trait?information for the recessive trait?

Recessive Traits Reappear in Second Generation


1.1. Results indicate that the Results indicate that the recessive trait is intactrecessive trait is intact. .

2. 2. The crossbred plants with purple flowers must The crossbred plants with purple flowers must

be be carrying the genetic informationcarrying the genetic information to produce to produce

white flowers. white flowers.

3.3. The crossbred plants with purple flowers are The crossbred plants with purple flowers are

genetically differentgenetically different from the purebred plants, from the purebred plants,

even though they look the same.even though they look the same.


4.4. Must distinguish between: Must distinguish between:

PhenotypePhenotype:: Physical appearancePhysical appearance of individual. of individual.

Example:Example: Two phenotypesTwo phenotypes for flower color. for flower color. Purple flowersPurple flowers

White flowers.White flowers.

Genotype:Genotype: Genetic makeupGenetic makeup of an individual. of an individual.

Not all purple flowers are genetically identical.Not all purple flowers are genetically identical.


5.5. Each individual carries Each individual carries two genestwo genes for a given for a given

genetic trait. One gene comes from the genetic trait. One gene comes from the

individual’s mother, the other from the father.individual’s mother, the other from the father.

There are two There are two alternative forms of genesalternative forms of genes or or

hereditary units. hereditary units.

The alternative forms of these hereditary units The alternative forms of these hereditary units

are called are called allelesalleles. .

P: Allele for purple flowersP: Allele for purple flowers

p: Allele for white flowersp: Allele for white flowers


6. 6. In a given individual, the two genes for a given In a given individual, the two genes for a given

trait may be the same trait may be the same alleleallele (form of a gene) or (form of a gene) or

different.different.

PhenotypePhenotype Genotype:Genotype:

PurplePurple PP (Homozygous dominant)PP (Homozygous dominant)

PurplePurple Pp (Heterozygous dominant)Pp (Heterozygous dominant)

WhiteWhite pp (Homozygous recessive)pp (Homozygous recessive)

Homologous Chromosomes Bear the Two Alleles for Each Characteristic

Phenotype and Genotype of Mendel’s Pea Plants


7.7. How can we explain the consistent 3:1 How can we explain the consistent 3:1

phenotypic ratio in the F2 generation?phenotypic ratio in the F2 generation?

During gamete formation, the two alleles for a During gamete formation, the two alleles for a

given trait given trait separateseparate ( (Principle of segregationPrinciple of segregation). ).

Egg or sperm cells only contain one allele for a Egg or sperm cells only contain one allele for a

given trait.given trait.

When a sperm and egg come together during When a sperm and egg come together during

fertilization, each one contributes fertilization, each one contributes one alleleone allele to the to the

offspring, which restores the pair of alleles.offspring, which restores the pair of alleles.

Principle of Segregation: Each Parent or Gamete Contributes One Allele to Offspring

Punnet SquarePunnet Square::

Used to determine the outcome of a cross between Used to determine the outcome of a cross between two individuals.two individuals.

Heterozygotes make 1/2 P and 1/2 p gametes.Heterozygotes make 1/2 P and 1/2 p gametes.

PP p p

PP PP PP Pp Pp

pp PpPp pp pp

Offspring:Genotype: 1/4 PP, 1/2 Pp, and 1/4 pp Phenotype: 3/4 Purple and 1/4 white

Genotypic and Phenotypic Ratios of F2 Generation

V. Mendel’s Dihybrid Cross: Tracking Two TraitsV. Mendel’s Dihybrid Cross: Tracking Two Traits

Question:Question: What will we obtain in F2 generation, when What will we obtain in F2 generation, when

we cross a pea plant with round we cross a pea plant with round yellowyellow peas (RRYY) peas (RRYY)

with one with with one with wrinkledwrinkled greengreen peas (rryy)? peas (rryy)?

F1 Generation will all be round F1 Generation will all be round yellowyellow (RrYy). (RrYy).

Possible outcomes of F2 GenerationPossible outcomes of F2 Generation::

1.1. If the two traits are If the two traits are inherited as a packageinherited as a package (RY and ry), (RY and ry),

then will only get then will only get yellowyellow round and round and greengreen wrinkledwrinkled peas. peas.

2.2. If two traits are If two traits are inherited independentlyinherited independently, will get:, will get:

Not only Not only yellowyellow round and round and greengreen wrinkledwrinkled peas. peas.

But also But also yellowyellow wrinkledwrinkled and and greengreen round peas round peas

Principle of Independent Assortment is Revealed by Tracking Two Characteristics

V. Dihybrid Cross ConclusionsV. Dihybrid Cross Conclusions

1.1. Principle of Independent AssortmentPrinciple of Independent Assortment: Genetic : Genetic

traits are inherited traits are inherited independentlyindependently of one another. of one another.

One trait does not affect the inheritance of the One trait does not affect the inheritance of the

other.other.

2.2. Heterozygous individuals with Heterozygous individuals with yellowyellow round peas round peas

((RrYyRrYy) from the F1 generation, will produce ) from the F1 generation, will produce fourfour

types of gametes:types of gametes:

1/4 RY1/4 RY 1/4 rY1/4 rY 1/4 Ry1/4 Ry 1/4 ry1/4 ry

instead of only two:instead of only two:

1/2 RY1/2 RY 1/2 ry1/2 ry

V. Dihybrid Cross ConclusionsV. Dihybrid Cross Conclusions

3.3. The offspring of a dihybrid cross displays a The offspring of a dihybrid cross displays a

9:3:3:19:3:3:1 phenotypic ratio: phenotypic ratio:

9/16 9/16 YellowYellow Round (Y-R-) Round (Y-R-)

3/16 3/16 GreenGreen Round (yyR-) Round (yyR-)

3/16 3/16 YellowYellow WrinkledWrinkled (Y-rr) (Y-rr)

1/16 1/16 GreenGreen WrinkledWrinkled (yyrr) (yyrr)

VI. Principles of Mendelian GeneticsVI. Principles of Mendelian Genetics

1. 1. There are alternative forms of There are alternative forms of genesgenes, the units , the units

that determine heritable traits. that determine heritable traits.

These alternative forms are called These alternative forms are called allelesalleles..

Example:Example:

Pea plants have one Pea plants have one alleleallele for purple flower for purple flower

color, and another for white color.color, and another for white color.


2. 2. For each inherited characteristic, an For each inherited characteristic, an

individual has individual has two genestwo genes: one from each : one from each

parent. parent.

In a given individual, the genes may be the In a given individual, the genes may be the

same allele (same allele (homozygoushomozygous) or they may be ) or they may be

different alleles (different alleles (heterozygousheterozygous).).


3. 3. When two genes of a pair are different alleles, When two genes of a pair are different alleles,

only one is fully expressed (only one is fully expressed (dominant alleledominant allele). ).

The other allele has no noticeable effect on the The other allele has no noticeable effect on the

organism’s appearance (organism’s appearance (recessive allelerecessive allele).).

ExampleExample::

Purple allele for flower color is Purple allele for flower color is dominantdominant

White allele for flower color is White allele for flower color is recessiverecessive


4. 4. A sperm or egg cell (gamete) A sperm or egg cell (gamete) only contains one only contains one

alleleallele or gene for each inherited trait. or gene for each inherited trait.

Principle of SegregationPrinciple of Segregation: Alleles : Alleles segregatesegregate

(separate) during gamete formation.(separate) during gamete formation.

(When? During meiosis I)(When? During meiosis I)

During During fertilizationfertilization, sperm and egg each , sperm and egg each

contribute one allele to the new organism, contribute one allele to the new organism,

restoring the allele pair.restoring the allele pair.


5. 5. Principle of Independent AssortmentPrinciple of Independent Assortment: Two : Two

different genetic characteristics are inherited different genetic characteristics are inherited

independentlyindependently of each other.* of each other.*

*As long as they are on different chromosomes.*As long as they are on different chromosomes.

Mendel did not know about meiosis, but meiosis Mendel did not know about meiosis, but meiosis

explains this observation.explains this observation.

Why? Why?

How are chromosomes shuffled during meiosis I?How are chromosomes shuffled during meiosis I?

VII. Human GeneticsVII. Human GeneticsInheritance of human traits. Inheritance of human traits.

Most genetic diseases are recessive.Most genetic diseases are recessive.

Dominant TraitsDominant Traits Recessive TraitsRecessive Traits

Widow’s peakWidow’s peak Straight hairlineStraight hairline

Freckles Freckles No frecklesNo freckles

Free earlobeFree earlobe Attached earlobeAttached earlobe

NormalNormal Cystic fibrosisCystic fibrosis

NormalNormal PhenylketonuriaPhenylketonuria

NormalNormal Tay-Sachs diseaseTay-Sachs disease

Normal Normal AlbinismAlbinism

Normal hearingNormal hearing Inherited deafnessInherited deafness

Huntington’s DiseaseHuntington’s Disease NormalNormal

DwarfismDwarfism Normal heightNormal height

VII. Other Types of InheritanceVII. Other Types of Inheritance

A. A. Incomplete DominanceIncomplete Dominance::

For some characteristics, the F1 hybrids of a true-For some characteristics, the F1 hybrids of a true-

breed cross have an breed cross have an intermediate phenotypeintermediate phenotype

between that of parents. between that of parents.

Incomplete dominance Incomplete dominance does not support blendingdoes not support blending, ,

because the parental alleles are not lost. because the parental alleles are not lost.

ExamplesExamples::

Snapdragon flower colorSnapdragon flower color

Hypercholesteremia in humansHypercholesteremia in humans

Incomplete Dominance: Offspring of True Bred CrossHave Intermediate Phenotypes


B. B. Multiple Alleles and CodominanceMultiple Alleles and Codominance::

For some characteristics, there are more than 2 For some characteristics, there are more than 2 alleles.alleles.

ExampleExample: ABO blood type.: ABO blood type.

There are three alleles that control blood type in There are three alleles that control blood type in humans.humans.

IIAA: : Red blood cells have carbohydrate A.Red blood cells have carbohydrate A.

IIBB:: Red blood cells have carbohydrate B.Red blood cells have carbohydrate B.

i:i: No carbohydrate on red blood cells.No carbohydrate on red blood cells.

B. B. Multiple Alleles and CodominanceMultiple Alleles and Codominance::

Codominance:Codominance: When both alleles are present, they When both alleles are present, they are both fully expressed.are both fully expressed.

IIAA and I and IBB are codominant and dominant over i. are codominant and dominant over i.

IIAA = I = IBB > i > i

GenotypeGenotype Blood Type (Phenotype)Blood Type (Phenotype)

IIAA I IBB AB ( AB (Universal acceptorUniversal acceptor))

IIAA I IAA A A

IIAAii A A

IIBB I IBB B B

IIBB I I B B

iiii O ( O (Universal donorUniversal donor))

Multiple Alleles: ABO Blood Groups

Blood type O: Universal donor. Blood type AB: Universal acceptor

C. C. PleiotropyPleiotropy::

One gene affects more than 1 characteristic.One gene affects more than 1 characteristic.

ExampleExample: :

Sickle cell anemia. There are two alleles that Sickle cell anemia. There are two alleles that determine hemoglobin sequence.determine hemoglobin sequence.

A: Normal hemoglobinA: Normal hemoglobin

a: Sickle cell hemoglobina: Sickle cell hemoglobin

Alleles display Alleles display incomplete dominanceincomplete dominance::

GenotypeGenotype PhenotypePhenotype

AAAA Normal Normal

AaAa Sickle cell trait Sickle cell trait (Healthy. Malaria (Healthy. Malaria

resistance)resistance)

aaaa Sickle cell anemia Sickle cell anemia

C. C. PleiotropyPleiotropy::

Individuals with sickle cell anemia (Genotype: aa) Individuals with sickle cell anemia (Genotype: aa) have abnormal hemoglobin, which causes many have abnormal hemoglobin, which causes many different health problems:different health problems: Breakdown of red blood cellsBreakdown of red blood cells

WeaknessWeakness AnemiaAnemia

Clogging of blood vesselsClogging of blood vessels Heart failureHeart failure Pain and feverPain and fever Organ damage (brain, spleen, etc.)Organ damage (brain, spleen, etc.) ParalysisParalysis RheumatismRheumatism

Accumulation of red blood cells in spleen/spleen damageAccumulation of red blood cells in spleen/spleen damage

Pleiotropy: One Gene Affects Multiple Traits


D. D. Polygenic InheritancePolygenic Inheritance::

Some genetic characteristics are controlled by two Some genetic characteristics are controlled by two or more genes:or more genes:

Examples:Examples: Human skin color: At least three genes.Human skin color: At least three genes. Human eye color: At least two genes.Human eye color: At least two genes. Human heightHuman height

The alleles usually have an The alleles usually have an additiveadditive effect, resulting in effect, resulting in

multiple phenotypesmultiple phenotypes. .

Phenotypes for skin color can range from very dark to Phenotypes for skin color can range from very dark to

very light.very light.

Polygenic Inheritance: Human Skin Color is Determined by Several Genes

Chromosome Behavior Accounts for Mendel’s Findings


E. E. LinkageLinkage::

Some genetic characteristics are controlled by two Some genetic characteristics are controlled by two genes that are on the genes that are on the samesame chromosome. chromosome.

These traits tend to be inherited together or These traits tend to be inherited together or display display linkagelinkage..

Linked genesLinked genes do notdo not follow Mendel’s principle offollow Mendel’s principle of independent assortment.independent assortment.

Crossing overCrossing over produces new combinations of alleles produces new combinations of alleles on chromosomes.on chromosomes.

Linkage: Genes on the Same Chromosome Tend to be Inherited Together

Linkage: Crossing Over Causes New Combinations of Genes


F. F. Sex-linked InheritanceSex-linked Inheritance::

Some genetic characteristics are controlled by Some genetic characteristics are controlled by genes that are on the sex chromosomes.genes that are on the sex chromosomes.

These genes are inherited differently than genes on These genes are inherited differently than genes on autosomes.autosomes.

Females (XX)Females (XX) Males (XY)Males (XY)

The X chromosome is much larger than the Y The X chromosome is much larger than the Y chromosome, and contains many more genes.chromosome, and contains many more genes.

The Y chromosome is very small and contains very The Y chromosome is very small and contains very few genes. few genes.

Sex Chromosomes Determine an Individual’s Sex

X-Y System in mammals: Other Systems:



X Chromosome Genes:X Chromosome Genes: Hemophilia Hemophilia Color blindnessColor blindness Muscular dystrophyMuscular dystrophy Severe combined immunodeficiency syndrome (SCID)Severe combined immunodeficiency syndrome (SCID)

Y chromosome Genes:Y chromosome Genes: Testis determining factor (TDF)Testis determining factor (TDF) Coarse earlobe hairCoarse earlobe hair



Women can be homozygous or heterozygous for Women can be homozygous or heterozygous for sex-linked traits.sex-linked traits.

Men only have one X chromosome, so they are Men only have one X chromosome, so they are hemizygoushemizygous for sex-linked traits. for sex-linked traits.

For this reason, males are more susceptible to X-For this reason, males are more susceptible to X-linked diseases.linked diseases.


Examples:Examples:

Hemophilia is a recessive X-linked disorder, in which affected Hemophilia is a recessive X-linked disorder, in which affected individuals’ blood does not clot normally. Males and females individuals’ blood does not clot normally. Males and females inherit the trait differently.inherit the trait differently.

Male GenotypeMale Genotype Male PhenotypeMale Phenotype

XXHHYY (Hemizygous) (Hemizygous) NormalNormal

XXhhYY (Hemizygous) (Hemizygous) HemophiliacHemophiliac

Female GenotypeFemale Genotype Female PhenotypeFemale Phenotype

XXHHXXHH (Homozygous)(Homozygous) Normal Normal

XXHHXXhh (Heterozygous)(Heterozygous) Normal carrierNormal carrier

XXhhXXhh (Homozygous)(Homozygous) HemophiliacHemophiliac


Problem:Problem:

What kind of children will be born from the marriage of a What kind of children will be born from the marriage of a normal man (Xnormal man (XHHY) and a normal woman who is a carrier of Y) and a normal woman who is a carrier of the hemophilia gene (Xthe hemophilia gene (XHHXXhh)?)?

XXHH Y Y

XXHH X XHH X XHH X XHHY Y

XXhh XXHH X Xhh X XhhYY

Daughters: All normal. 50% carriers and 50% homozygous.Sons: 50% normal, 50% hemophiliacs.

Sex Linked Traits are Inherited in a Unique Pattern

Color Blindness is a Sex-Linked Trait in Humans

Hemophilia: A Sex Linked Disorder in Royal Family of Russia

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Chapter 9 Patterns of Inheritance