Patterns of Inheritance

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.


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.


Blending Hypothesis: In 1800s biologists and plant

breeders suggested that traits of parents mix to form

intermediate traits in offspring.

Parents Offspring

Red flower x White flower Pink flower

Tall height x Short height Medium height

Blue bird x Yellow bird Green birds

Fair skin x dark skin Medium skin color

If blending always occurred, eventually all extreme

characteristics 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 color Purple or white

Seed color Yellow or green

Seed shape Round or wrinkled

Pod color Green or Yellow

Pod shape Smooth or constricted

Flower position Axial or terminal

Plant height Tall 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. Some plants 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 Varieties Offspring

Flower color Purple or white 100% Purple

Seed color Yellow or green 100% Yellow

Seed shape Round or wrinkled 100% Round

Pod color Green or Yellow 100% Green

Pod shape Smooth or constricted 100% Smooth

Flower position Axial or terminal 100% Axial

Plant height Tall or short 100% 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


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.


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.


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


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

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

different.

Phenotype Genotype:

Purple PP (Homozygous dominant)

Purple Pp (Heterozygous dominant)

White pp (Homozygous recessive)

Homologous Chromosomes Bear the

Two Alleles for Each Characteristic

Phenotype and Genotype of Mendel’s Pea Plants


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

P PP Pp

p 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 Traits

Question: What will we obtain in F2 generation, when

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

with one with wrinkled green 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 green wrinkled peas.

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

Not only yellow round and green wrinkled peas.

But also yellow wrinkled 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 RY 1/4 rY 1/4 Ry 1/4 ry

instead of only two:

1/2 RY 1/2 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 Yellow Wrinkled (Y-rr)

1/16 Green Wrinkled (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.

Example:

Pea plants have one allele for purple flower

color, and another for white color.


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).


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).

Example:

Purple allele for flower color is dominant

White allele for flower color is recessive


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.


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.

Why?

How are chromosomes shuffled during meiosis I?

VII. Human Genetics

Inheritance of human traits.

Most genetic diseases are recessive.

Dominant Traits Recessive Traits

Widow’s peak Straight hairline

Freckles No freckles

Free earlobe Attached earlobe

Normal Cystic fibrosis

Normal Phenylketonuria

Normal Tay-Sachs disease

Normal Albinism

Normal hearing Inherited deafness

Huntington’s Disease Normal

Dwarfism Normal 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.

Examples:

Snapdragon flower color

Hypercholesteremia in humans

Incomplete Dominance: Offspring of True Bred Cross

Have Intermediate Phenotypes


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

Genotype Blood Type (Phenotype)

IA IB AB (Universal acceptor)

IA IA A

IAi A

IB IB B

IB I B

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.

Example:

Sickle cell anemia. There are two alleles that

determine hemoglobin sequence.

A: Normal hemoglobin

a: Sickle cell hemoglobin

Alleles display incomplete dominance:

Genotype Phenotype

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


D. Polygenic Inheritance:

Some genetic characteristics are controlled by two

or more genes:

Examples:

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


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 genes do not follow 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


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:



X Chromosome Genes:

Hemophilia

Color blindness

Muscular dystrophy

Severe combined immunodeficiency syndrome (SCID)

Y chromosome Genes:

Testis determining factor (TDF)

Coarse earlobe hair



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.


Examples:

Hemophilia is a recessive X-linked disorder, in which

affected individuals’ blood does not clot normally. Males

and females inherit the trait differently.

Male Genotype Male Phenotype

XHY (Hemizygous) Normal

XhY (Hemizygous) Hemophiliac

Female Genotype Female Phenotype

XHXH (Homozygous) Normal

XHXh (Heterozygous) Normal carrier

XhXh (Homozygous) Hemophiliac


Problem:

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 Y

XH XH XH XHY

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

Patterns of Inheritance

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