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.
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
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.
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
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
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
instead of only two:
1/2 RY
1/2 ry
1/4 Ry
1/4 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.
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).
Example:
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.
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 truebreed 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
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 = I B > 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

Weakness

Anemia
 Clogging
of red blood cells
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:
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
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 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
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
 Severe
dystrophy
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 Xlinked diseases.
F. Sex-linked Inheritance:
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
F. Sex-linked Inheritance:
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