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Chapter 9 Patterns of Inheritance. Heredity : The transmission of traits from one generation to another. Variation : Offspring are different from their parents and siblings. Genetics : The scientific study of heredity and hereditary variation.

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Chapter 9 Patterns of Inheritance

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Chapter 9

Patterns of Inheritance

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

Variation: Offspring are different from their parents and siblings.

Genetics: The scientific study of heredity and hereditary variation.

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

I. History of Genetics

  • Prehistoric Times:Little is known about when humans first recognized the importance of heredity.

    • Domestication and breeding of horses, cattle, and various breeds of dogs around 8000 and 1000 B.C.

    • Cultivation of many plants (corn, wheat, and rice) around 5000 B.C. in Mexico and other regions.

    • Artificial pollination of date palms by Assyrians around 850 B.C.

I. History of Genetics

  • Greek Influence:

    • Pythagoras: Greek philosopher speculated around 500 B.C. that human life begins with male and female fluids, or semens, originating in body parts.

    • Hippocrates: Around 500-400 B.C., Theory of pangenesis. “Humors” from an individual’s body collect in their semen, and are passed on to next generation. Humors could be healthy or diseased. Acquired characteristics could be inherited.

    • Aristotle: 384-322 B.C. Postulated that semens were purified blood and that blood was the element of heredity. The potential to produce body features was inherited, not the features themselves.

I. History of Genetics

  • Blending Hypothesis: In 1800s biologists and plant breeders suggested that traits of parents mix to form intermediate traits in offspring.


    Red flower x White flowerPink flower

    Tall height x Short heightMedium height

    Blue bird x Yellow birdGreen birds

    Fair skin x dark skinMedium skin color

    If blending always occurred, eventually all extremecharacteristics would disappear from the population.

  • Gregor Mendel: Established genetics as a science in 1860s. Considered the founder of modern genetics.

II. Modern Genetics

Began as a science in 1860s.

  • Gregor Mendel:An Austrian monk, who was a farmer’s son. He was trained in mathematics, chemistry, and physics.

    • Studied the breeding patterns of plants for over 10 years.

    • Artificially crossed peas, watermelons, and other plants.

    • Kept meticulous records of thousands of breedings and resulting offspring.

    • Rejected blending hypothesis, and stressed that heritable factors (genes) retain their individuality generation after generation.

II. Modern Genetics

Gregor Mendel:

  • Calculated the mathematical probabilities of inheriting many genetic traits.

  • Published results in 1866. They were largely ignored due to fervor surrounding Darwin’s publications on evolution.

  • Discouraged by the lack of attention from the scientific community, he quit his work and died a few years later.

  • Importance of Mendel’s work was not appreciated until early 1900s when his paper was rediscovered.

III. Mendel’s Experiments

  • Used “true-breeding” or purebred plant varieties for seven pea characteristics. Self-pollination produces all identical offspring.

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

    Trait Varieties

    Flower colorPurple or white

    Seed colorYellow or green

    Seed shapeRound or wrinkled

    Pod colorGreen or Yellow

    Pod shapeSmooth or constricted

    Flower positionAxial or terminal

    Plant heightTall or short

Seven Pea Characteristics Studied by Mendel

The Pea Flower Has Both Male and Female Parts

Mendel Used Artificial Fertilization to Cross Different

Varieties of Peas

III. Mendel’s Experiments

Question: What will we obtain when we cross a pea plant with purple flowers with one with white flowers?

Possible outcomes:

1. If blending hypothesis is true, then plants would be an intermediate color, e.g.: light purple.

2. Someplants will be purple, others will be white.

3.All plants will be purple or all plants will be 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 Results

All plants displayed one trait only.

Trait VarietiesOffspring

Flower colorPurple or white100% Purple

Seed colorYellow or green100% Yellow

Seed shapeRound or wrinkled100% Round

Pod colorGreen or Yellow100% Green

Pod shapeSmooth or constricted100% Smooth

Flower positionAxial or terminal100% Axial

Plant heightTall or short100% Tall

The trait that prevailed was dominant, the other recessive.

IV. Mendel’s Conclusions

1. Results indicate that blending hypothesis is not true.

2. Only one of the two traits appeared in the first generation. He called this the dominant trait. He called the trait that disappeared the recessive trait.

Mendel then asked the following questions:

  • What has happened to the recessive (white) trait?

  • Has it been lost?

  • Has it been altered?

  • Do the crossbred plants carry genetic information for the recessive trait?

Recessive Traits Reappear in Second Generation

IV. Mendel’s Conclusions

1. Results indicate that the recessive trait is intact.

2. The crossbred plants with purple flowers must be carrying the genetic information to produce white flowers.

3. The crossbred plants with purple flowers are genetically different from the purebred plants, even though they look the same.

IV. Mendel’s Conclusions

4. Must distinguish between:

Phenotype:Physical appearance of individual.

Example:Two phenotypes for flower color.

  • Purple flowers

  • White flowers.

    Genotype:Genetic makeup of an individual.

    Not all purple flowers are genetically identical.

IV. Mendel’s Conclusions

5. Each individual carries two genes for a given genetic trait. One gene comes from the individual’s mother, the other from the father.

There are two alternative forms of genes or hereditary units.

The alternative forms of these hereditary units are called alleles.

P: Allele for purple flowers

p: Allele for white flowers

IV. Mendel’s Conclusions

6. In a given individual, the two genes for a given trait may be the same allele (form of a gene) or different.


PurplePP (Homozygous dominant)

PurplePp (Heterozygous dominant)

Whitepp (Homozygous recessive)

Homologous Chromosomes Bear the Two Alleles for Each Characteristic

Phenotype and Genotype of Mendel’s Pea Plants

IV. Mendel’s Conclusions

7. How can we explain the consistent 3:1 phenotypic ratio in the F2 generation?

During gamete formation, the two alleles for a given trait separate (Principle of segregation). Egg or sperm cells only contain one allele for a given trait.

When a sperm and egg come together during fertilization, each one contributes one allele to the offspring, which restores the pair of alleles.

Principle of Segregation: Each Parent or Gamete

Contributes One Allele to Offspring

Punnet Square:

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

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

P p


pPp pp


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 Traits

Question: What will we obtain in F2 generation, when we cross a pea plant with round yellow peas (RRYY) with one with wrinkledgreen peas (rryy)?

F1 Generation will all be round yellow (RrYy).

Possible outcomes of F2 Generation:

1. If the two traits are inherited as a package (RY and ry), then will only get yellow round and greenwrinkled peas.

2.If two traits are inherited independently, will get:

  • Not only yellow round and greenwrinkled peas.

  • But also yellowwrinkled and green round peas

Principle of Independent Assortment is Revealed by

Tracking Two Characteristics

V. Dihybrid Cross Conclusions

1.Principle of Independent Assortment: Genetic traits are inherited independently of one another. One trait does not affect the inheritance of the other.

2. Heterozygous individuals with yellow round peas (RrYy) from the F1 generation, will produce four types of gametes:

1/4 RY1/4 rY1/4 Ry1/4 ry

instead of only two:

1/2 RY1/2 ry

V. Dihybrid Cross Conclusions

3. The offspring of a dihybrid cross displays a 9:3:3:1 phenotypic ratio:

9/16 Yellow Round (Y-R-)

3/16 Green Round (yyR-)

3/16 YellowWrinkled (Y-rr)

1/16 GreenWrinkled (yyrr)

VI. Principles of Mendelian Genetics

1. There are alternative forms of genes, the units that determine heritable traits.

These alternative forms are called alleles.


Pea plants have one allele for purple flower color, and another for white color.

VI. Principles of Mendelian Genetics

2. For each inherited characteristic, an individual has two genes: one from each parent.

In a given individual, the genes may be the same allele (homozygous) or they may be different alleles (heterozygous).

VI. Principles of Mendelian Genetics

3. When two genes of a pair are different alleles, only one is fully expressed (dominant allele). The other allele has no noticeable effect on the organism’s appearance (recessive allele).


Purple allele for flower color is dominant

White allele for flower color is recessive

VI. Principles of Mendelian Genetics

4. A sperm or egg cell (gamete) only contains one allele or gene for each inherited trait.

Principle of Segregation: Alleles segregate (separate) during gamete formation.

(When? During meiosis I)

During fertilization, sperm and egg each contribute one allele to the new organism, restoring the allele pair.

VI. Principles of Mendelian Genetics

5. Principle of Independent Assortment: Two different genetic characteristics are inherited independently of each other.*

*As long as they are on different chromosomes.

Mendel did not know about meiosis, but meiosis explains this observation.


How are chromosomes shuffled during meiosis I?

VII. Human Genetics

Inheritance of human traits.

Most genetic diseases are recessive.

Dominant TraitsRecessive Traits

Widow’s peakStraight hairline

Freckles No freckles

Free earlobeAttached earlobe

NormalCystic fibrosis


NormalTay-Sachs disease

Normal Albinism

Normal hearingInherited deafness

Huntington’s DiseaseNormal

DwarfismNormal height

VII. Other Types of Inheritance

A. Incomplete Dominance:

For some characteristics, the F1 hybrids of a true-breed cross have an intermediate phenotype between that of parents.

Incomplete dominance does not support blending, because the parental alleles are not lost.


Snapdragon flower color

Hypercholesteremia in humans

Incomplete Dominance: Offspring of True Bred Cross

Have Intermediate Phenotypes

VII. Other Types of Inheritance

B. Multiple Alleles and Codominance:

For some characteristics, there are more than 2 alleles.

Example: ABO blood type.

There are three alleles that control blood type in humans.

IA: Red blood cells have carbohydrate A.

IB: Red blood cells have carbohydrate B.

i: No carbohydrate on red blood cells.

B. Multiple Alleles and Codominance:

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

IA and IB are codominant and dominant over i.

IA = IB > i

GenotypeBlood Type (Phenotype)

IA IB AB (Universal acceptor)





ii O (Universal donor)

Multiple Alleles: ABO Blood Groups

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

C. Pleiotropy:

One gene affects more than 1 characteristic.


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

A: Normal hemoglobin

a: Sickle cell hemoglobin

Alleles display incomplete dominance:


AA Normal

Aa Sickle cell trait (Healthy. Malaria resistance)

aa Sickle cell anemia

C. Pleiotropy:

Individuals with sickle cell anemia (Genotype: aa) have abnormal hemoglobin, which causes many different health problems:

  • Breakdown of red blood cells

    • Weakness

    • Anemia

  • Clogging of blood vessels

    • Heart failure

    • Pain and fever

    • Organ damage (brain, spleen, etc.)

    • Paralysis

    • Rheumatism

  • Accumulation of red blood cells in spleen/spleen damage

Pleiotropy: One Gene Affects Multiple Traits

VII. Other Types of Inheritance

D. Polygenic Inheritance:

Some genetic characteristics are controlled by two or more genes:


  • Human skin color: At least three genes.

  • Human eye color: At least two genes.

  • Human height

    The alleles usually have an additive effect, resulting in multiple phenotypes.

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

Polygenic Inheritance: Human Skin Color

is Determined by Several Genes

Chromosome Behavior Accounts for Mendel’s Findings

VII. Other Types of Inheritance

E. Linkage:

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

These traits tend to be inherited together or display linkage.

Linked genesdo notfollow Mendel’s principle of independent assortment.

Crossing over produces new combinations of alleles on chromosomes.

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

Linkage: Crossing Over Causes New Combinations of Genes

VII. Other Types of Inheritance

F. Sex-linked Inheritance:

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

These genes are inherited differently than genes on autosomes.

Females (XX)Males (XY)

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

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

Sex Chromosomes Determine an Individual’s Sex

X-Y System in mammals:

Other Systems:

VII. Other Types of Inheritance

F. Sex-linked Inheritance:

X Chromosome Genes:

  • Hemophilia

  • Color blindness

  • Muscular dystrophy

  • Severe combined immunodeficiency syndrome (SCID)

    Y chromosome Genes:

  • Testis determining factor (TDF)

  • Coarse earlobe hair

VII. Other Types of Inheritance

F. Sex-linked Inheritance:

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

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

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

F. Sex-linked Inheritance:


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

Male GenotypeMale Phenotype

XHY (Hemizygous)Normal

XhY (Hemizygous) Hemophiliac

Female GenotypeFemale Phenotype


XHXh(Heterozygous)Normal carrier


F. Sex-linked Inheritance:


What kind of children will be born from the marriage of a normal man (XHY) and a normal woman who is a carrier of the hemophilia gene (XHXh)?



Xh XH Xh XhY

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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