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Genetics: Analysis and Principles
Robert J. Brooker
CHAPTER 4
EXTENSIONS OF
MENDELIAN INHERITANCE
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Symbols for Alleles
Dominant alleles are usually indicated
either by an italic uppercase letter (D) or by
a an italic letter or group of letters followed
by a superscript + (Wr+).
Recessive alleles are usually indicated
either by an italic lowercase letter (d) or by
an italic letter or group of letters (Wr)
without the +.
If no dominance exists, italic uppercase
letters and superscripts are used to denote
alternative alleles (R1, R2; CW, CR).

Morgan’s Experiment


The chromosome theory of inheritance was
confirmed through studies carried out by
Thomas Hunt Morgan
Morgan tried to induce mutations into the fruit
fly Drosophila melanogaster

Treatments included



Rearing in the dark
X-rays
Radium
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3-82

After 2 years, Morgan finally obtained an
interesting result

A male fruit fly with white eyes rather than the
normal red eyes


Morgan reasoned that this white eyed male must have
arisen from a new mutation that converted a red-eyed
allele into a white-eyed allele
Morgan followed Mendel’s approach in
studying the inheritance of this white-eyed trait

He made crosses then analyzed their outcome
quantitatively
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3-83
Morgan’s first mutant
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Hypothesis

A quantitative analysis of genetic crosses may
reveal the inheritance pattern for the white eye
allele
Testing the Hypothesis

Refer to Figure 3.19
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3-84
Fig. 3.19 (TE Art)
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1. Cross the white-eyed
male to red-eyed
females.
Experimental level
Conceptual level
XWY x XW+X+
x
2. Record the results of the
F1generation. This involves
noting the eye color and
sexes of several thousand
flies.
XW+Y male offspring and XW+XW
female offspring, both with red eyes
x
XW+Y x XW+XW
F1 generation
3. Cross F1 offspring with each
other to obtain F2 offspring.
Also record the eyecolor and
sex of the F2 offspring.
1 XW+Y : 1 XWY : 1 XW+XW+ : 1 XW+XW
1 red-eyed male : 1 white-eyed male :
2 red-eyed females
F2 generation
4. In a separate experiment,
perform a testcross between
a white-eyed male and
a red-eyed female from the
F1generation.
Record the results.
XWY x XW+XW
x
From
F1 generation
1 XW+Y : 1 XWY : 1 XW+XW+ : 1 XW+XW
1 red-eyed male : 1 white-eyed male :
1 red-eyed females : 1 white-eyed female
The Data
Cross
Results
Original white eyed-male F1 generation:
to red-eyed females
All red-eyed flies
F1 male to F1 female
F2 generation:
2,459 red-eyed females
1,011 red-eyed males
0 white eyed-females
782 white-eyed males
Test Cross
Results
White-eyed male to
F1 female
129 red-eyed females
132 red-eyed males
88 white eyed-females
86 white-eyed males
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3-86
Interpreting the Data

The first cross yielded NO white-eyed females
in the F2 generation


These results indicate that the eye color alleles
are located on the X chromosome
Genes that are physically located on the X
chromosome are called X-linked genes or Xlinked alleles
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3-87

A Punnett square predicts the absence of
white-eyed females in the F2 generation


F1 male is Xw+Y
F1 female is Xw+Xw
+
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3-88

The testcross resulted in red-eyed females
and males, and white-eyed females and
males, in approximately equal numbers



This is consistent with an X-linked pattern of
inheritance
Male is XwY
F1 female is Xw+Xw
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3-90
Reciprocal crosses


Crosses between different strains in which
the sexes are reversed
These crosses reveal whether a trait is
carried on a sex chromosome or an
autosome

X-linked traits do not behave identically in
reciprocal crosses
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3-92

Consider the following two crosses:


Male is Xw+Y
Female is XwXw


Male is XwY
Female is Xw+Xw+
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3-93

When comparing the two Punnett squares, the
outcomes of the reciprocal cross did not yield
the same results
the male transmits X-linked genes only to his
daughters
 the female transmits X-linked genes to all her
children


This explains why X-linked traits do not
behave equally in reciprocal crosses
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3-94
Extensions of Mendelian Inheritance

Mendelian inheritance describes inheritance
patterns that obey two laws



Simple Mendelian inheritance involves



Law of segregation
Law of independent assortment
A single gene with two different alleles
Alleles display a simple dominant/recessive
relationship
Extensions of Mendelian inheritance are more
complex and may involve multiple alleles and/or
multiple genes
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
Prevalent alleles in a population are termed
wild-type alleles

These typically encode proteins that



Function normally
Are made in the right amounts
Alleles that have been altered by mutation
are termed mutant alleles



These tend to be rare in natural populations
They are likely to cause a reduction in the
amount or function of the encoded protein
Such mutant alleles are often inherited in a
recessive fashion
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4-6

Consider, for example, the traits that Mendel studied
Wild-type (dominant) allele Mutant (recessive) allele

Purple flowers
White flowers
Axial flowers
Terminal flowers
Yellow seeds
Green seeds
Round seeds
Wrinkled seeds
Smooth pods
Constricted pods
Green pods
Yellow pods
Tall plants
Dwarf plants
Another example is from Drosophila
Wild-type (dominant) allele Mutant (recessive) allele
Red eyes
White eyes
Normal wings
Miniature wings
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

Genetic diseases are caused by mutant alleles
In many human genetic diseases , the
recessive allele contains a mutation

This prevents the allele from producing a fully
functional protein
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4-8

In a simple dominant/recessive relationship, the
recessive allele does not affect the phenotype of the
heterozygote


So how can the wild-type phenotype of the heterozygote
be explained?
There are two possible explanations

1. 50% of the normal protein is enough to accomplish the
protein’s cellular function


Refer to Figure 4.1
2. The heterozygote may actually produce more than 50%
of the functional protein

The normal gene is “up-regulated” to compensate for the lack of
function of the defective allele
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4-9
Figure 4.1
Complete Dominance
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4-10
Lethal Alleles

Essential genes are those that are absolutely
required for survival

The absence of their protein product leads to a lethal
phenotype



It is estimated that about 1/3 of all genes are essential for
survival
Nonessential genes are those not absolutely
required for survival
A lethal allele is one that has the potential to
cause the death of an organism


These alleles are typically the result of mutations in
essential genes
They are usually inherited in a recessive manner
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4-11

Many lethal alleles prevent cell division


These will kill an organism at an early age
 Maintained in populations by heterozygotes
Some lethal allele exert their effect later in life

Huntington disease (dominant allele)



Characterized by progressive degeneration of the nervous
system, dementia and early death
Maintained in populations by late onset (age 30 to 50)
Conditional lethal alleles may kill an organism only
when certain environmental conditions prevail

Temperature-sensitive (ts) lethals


A developing Drosophila larva may be killed at 30 C
But it will survive if grown at 22 C
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4-12


A lethal allele may produce ratios that
seemingly deviate from Mendelian ratios
An example is the “creeper” allele in chicken


Creepers have shortened legs and must creep
along
Creeper chicken are heterozygous
Scot’s Dumpy
4-13
Creeper X Normal
Creeper X Creeper
1 creeper : 1 normal
1 normal : 2 creeper
Creeper is a dominant allele
Creeper is lethal in the
homozygous state
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4-14
Figure 4-4
Copyright © 2006 Pearson Prentice Hall, Inc.
Incomplete Dominance

In incomplete dominance the heterozygote
exhibits a phenotype that is intermediate
between the corresponding homozygotes

Example:


Flower color in the four o’clock plant
Two alleles


CR = wild-type allele for red flower color
CW = allele for white flower color
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4-15
CO 4
1:2:1 phenotypic
ratio NOT the 3:1
ratio observed in
simple Mendelian
inheritance
In this case, 50% of
the CR protein is not
sufficient to produce
the red phenotype
Figure 4.2
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4-16
Incomplete Dominance

Whether a trait is dominant or incompletely
dominant may depend on how closely the trait is
examined

Take, for example, the characteristic of pea shape


Mendel visually concluded that
 RR and Rr genotypes produced round peas
 rr genotypes produced wrinkled peas
However, a microscopic examination of round peas
reveals that not all round peas are “created equal”
 Refer to Figure 4.3
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4-17
Figure 4.3
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4-18
Multiple Alleles

Many genes have multiple alleles



Three or more different alleles
May display a hierarchy of dominance
May display codominance
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4-19

An interesting example is coat color in rabbits

Four different alleles


C (full coat color)
cch (chinchilla pattern of coat color)


ch (himalayan pattern of coat color)


Lack of pigmentation
The dominance hierarchy is as follows:


Pigmentation in only certain parts of the body
c (albino)


Partial defect in pigmentation
C > cch > ch > c
Figure 4.4 illustrates the relationship between
phenotype and genotype
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4-20
Fig. 4.4

The himalayan pattern of coat color is an
example of a temperature-sensitive
conditional allele

The enzyme encoded by this gene is functional
only at low temperatures



Therefore, dark fur will only occur in cooler areas of
the body
This is also the case in the Siamese pattern of coat
color in cats
Refer to Figures 4.4c and 4.5
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4-21


The ABO blood group provides another example of
multiple alleles
It is determined by the type of antigen present on
the surface of red blood cells


Antigens are substances that are recognized by
antibodies produced by the immune system
As shown in Table 4.3, there are three different
types of antigens found on red blood



Antigen A, which is controlled by allele IA
Antigen B, which is controlled by allele IB
Antigen O, which is controlled by allele i
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4-22


Allele i is recessive to both IA and IB
Alleles IA and IB are codominant

They are both expressed in a heterozygous individual
N-acetylgalactosamine
B
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4-23


The carbohydrate tree on the surface of RBCs is
composed of three sugars
A fourth can be added by the enzyme glycosyl
transferase

The i allele encodes a defective enzyme




The carbohydrate tree is unchanged
IA encodes a form of the enzyme that can add Nacetylgalactosamine to the carbohydrate tree
IB encodes a form of the enzyme that can add galactose
to the carbohydrate tree
Thus, the A and B antigens are different enough to
be recognized by different antibodies
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4-24


For safe blood transfusions to occur, the donor’s
blood must be an appropriate match with the
recipient’s blood
For example, if a type O individual received blood
from a type A, type B or type AB blood



Antibodies in the recipient blood will react with antigens in
the donated blood cells
This causes the donated blood to agglutinate
A life-threatening situation may result because of clogging
of blood vessels
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4-25
Table 4-1
Copyright © 2006 Pearson Prentice Hall, Inc.
Overdominance

Overdominance is the phenomenon in which a
heterozygote is more vigorous than both of the
corresponding homozygotes


It is also called heterozygote advantage
Example = Sickle-cell anemia



Autosomal recessive disorder
Affected individuals produce abnormal form of hemoglobin
Two alleles


HbA  Encodes the normal hemoglobin, hemoglobin A
HbS  Encodes the abnormal hemoglobin, hemoglobin S
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4-34
Fig. 4.9

HbSHbS individuals have red blood cells that deform
into a sickle shape under conditions of low oxygen
tension
 Refer to Figure 4.9
 This has two major ramifications

1. Sickling phenomenon greatly shortens the life span
of the red blood cells


2. Odd-shaped cells clump


Anemia results
Partial or complete blocks in capillary circulation
Thus, affected individuals tend to have a shorter
life span than unaffected ones
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4-35
• The sickle cell allele has been found at a fairly
high frequency in parts of Africa where malaria is
found
– How come?
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
Distribution of
malaria caused by
Plasmodium falciparum
(a protozoan)
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Malaria is caused by a protozoan,
Plasmodium


This parasite infects red blood cells
Red blood cells of heterozygotes, are likely to
rupture when infected by Pasmodium sp.


This prevents the propagation of the parasite
Therefore, HbAHbS individuals are “better”
than


HbSHbS, because they do not suffer from sickle
cell anemia
HbAHbA, because they are more resistant to
malaria
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4-36


At the molecular level, overdominance is due
to two alleles that produce slightly different
proteins
How can these two protein variants produce a
favorable phenotype in the heterozygote?
1. Disease resistance
 2. Homodimer formation
 3. Variation in functional activity

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4-37
Fig. 4.10(TE Art)
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Pathogen can
successfully
propagate
Pathogen
cannot
successfull
y
propagate
A1A1
A1A2
Normal homozygote
(sensitive to infection)
Heterozygote
(resistant to infection
(a) Disease resistance
A1
A1
A2
A2
A1
(b) Homodimer formation
E1
E2
27o-32oC
30o-37oC
(optimum
(optimum
temperature temperature
range)
range)
(c) Variation in functional activity
A2
Dominance Patterns Caused by
Heterodimerization
Dimer
A1:A1
A1:A2
A2:A2
Activity
+
+
+
+
-
++
-
-
-
Complete
Dominance
Complete
Dominance
WT
mut
Overdominance
Heterozygote
>homozygote
A1= WT; A2 = mutant


Overdominance is related to a common
mating strategy used by animal and plant
breeders
Two different highly inbred strains are
crossed



The hybrids may display traits superior to both
parents
This phenomenon is termed hybrid vigor or
heterosis
Heterosis is used to improve quantitative
traits such as size, weight and growth rate
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4-41
Incomplete Penetrance


In some instances, a dominant allele is not
expressed in a heterozygote individual
Example = Polydactyly




Autosomal dominant trait
Affected individuals have additional fingers
and/or toes
A single copy of the polydactyly allele is usually
sufficient to cause this condition
In some cases, however, individuals carry the
dominant allele but do not exhibit the trait
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4-42
Fig. 4.12
Figure 4.11
Inherited the polydactyly allele from
his mother and passed it on to a
daughter and son
Does not exhibit the trait himself
even though he is a heterozygote
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4-43
Incomplete Penetrance

The term indicates that a dominant allele does not
always “penetrate” into the phenotype of the
individual

The measure of penetrance is described at the
population level


If 60% of heterozygotes carrying a dominant allele
exhibit the trait allele, the trait is 60% penetrant
Note:

In any particular individual, the trait is either penetrant or
not
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4-44
Expressivity


Expressivity is the degree to which a trait is
expressed
In the case of polydactyly, the number of digits can
vary


A person with several extra digits has high expressivity
of this trait
A person with a single extra digit has low expressivity
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4-45
Expressivity of eyeless


The molecular explanation of expressivity and
incomplete penetrance may not always be
understood
In most cases, the range of phenotypes is thought
to be due to influences of the


Environment
and/or
Other genes
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4-46
Environment

Environmental
conditions may have a
great impact on the
phenotype of the
individual

Example 1

Snapdragon flower color
vs. Temperature and
degree of sunlight
Figure 4.13
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4-47
Environment

Example 2 = Phenylketonuria
 Autosomal recessive disorder in humans
 Caused by a defect in the gene that encodes the
enzyme phenylalanine hydroxylase


Converts phenylalanine to tyrosine
Affected individuals cannot metabolize
phenylalanine


Phenylalanine will thus accumulate
It ultimately causes a number of detrimental effects

Mental retardation, for example
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4-48
Environment

Example 2 = Phenylketonuria


Newborns are now routinely screened for PKU
Individuals with the disease are put on a strict
dietary regimen


These individuals tend to develop normally


Their diet is essentially phenylalanine-free
Refer to Figure 1.10
Thus the PKU test prevents a great deal of
human suffering

Furthermore, it is cost-effective
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4-49
Sex and Traits

The inheritance pattern of certain traits is
governed by the sex of the individual

These traits are of two main types


Sex-influenced
Sex-limited
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4-50
Sex-influenced Traits

Traits where an allele is dominant in one sex
but recessive in the opposite sex

Thus, sex influence is a phenomenon of
heterozygotes
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4-51
Sex-influenced Traits

Example: Pattern baldness in humans

Caused by an autosomal gene
 Heterozygotes: Allele B behaves as dominant
in males, but recessive in females
Genotype
Phenotype
in Females
Phenotype
in Males
BB
bald
bald
Bb
nonbald
bald
bb
nonbald
nonbald
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4-52
Sex-influenced Traits

Pattern baldness appears to be related to the
level of male sex hormones

In females, a rare tumor of the adrenal gland can
cause the secretion of large amounts of male sex
hormones


If this case a heterozygous Bb female will become
bald
When the tumor is removed surgically, her hair returns
to its normal condition
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4-53

The autosomal nature of pattern baldness has
been revealed by analysis of human pedigrees

Refer to Figure 4.15
Bald fathers can pass
the trait to their sons
Figure 4.15
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4-54
Sex-limited Traits

Traits that occur in only one of the two sexes

For example in humans


Breast development is normally limited to females
Beard growth is normally limited to males
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4-55

Another example: Feather plumage in chicken




Refer to Figure 4.16
Caused by an autosomal gene
Hen-feathering is controlled by a dominant allele
expressed in both sexes
Cock-feathering is controlled by a recessive allele only
expressed in males
Genotype
Phenotype
in Females
Phenotype
in Males
hh
hen-feathered
cock-feathered
Hh
hen-feathered
hen-feathered
HH
hen-feathered
hen-feathered
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4-56
Fig. 4.16
Sex-limited Traits

The pattern of hen-feathering depends on the
production of sex hormones

If the single ovary is surgically removed from a
newly hatched hh female

She will develop cock-feathering and look
indistinguishable from a male
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4-57
4.2 GENE INTERACTIONS


Gene interactions occur when two or more
different genes influence the outcome of a
single trait
Indeed, morphological traits such as height
weight and pigmentation are affected by many
different genes in combination with
environmental factors
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4-58


We will next examine three different cases, all
involving two genes that exist in two alleles
The three crosses we will perform can be illustrated
in this general pattern


If these two genes govern two different traits


AaBb X AaBb
 Where A is dominant to a and B is dominant to b
A 9:3:3:1 ratio is predicted among the offspring
However, the two genes in this section do affect the
same trait

The 9:3:3:1 ratio may be affected
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4-59
A Cross Involving a Two-Gene Interaction
Can Still Produce a 9:3:3:1 ratio

Inheritance of comb morphology in chicken



First example of gene interaction
Discovered by William Bateson and Reginald
Punnett in 1906
Comb types come in four different morphologies

Refer to Figure 4.17a
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4-60
Figure 4.17b
The crosses of Bateson and Punnett
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4-61


Thus, the F2 generation consisted of chickens with
four types of combs
 9 walnut : 3 rose : 3 pea : 1 single
Bateson and Punnett reasoned that comb
morphology is determined by two different genes





R (rose comb) is dominant to r
P (pea comb) is dominant to p
R and P combination: walnut comb
rrpp produces single comb
Note:

Mendel’s laws of segregation and independent assortment
still hold!
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4-62
A Cross Involving a Two-Gene Interaction
Can Produce a 9:7 ratio

Inheritance of flower color in the sweet pea


Also discovered by Bateson and Punnett
Lathyrus odoratus normally has purple flowers



Bateson and Punnett obtained several true-breeding
varieties with white flowers
They carried out the following cross
 P: True-breeding purple X true-breeding white
 F1: Purple flowered plants
 F2: Purple- and white-flowered in a 3:1 ratio
These results were not surprising
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4-63

But these results were
Figure 4.18
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4-64


Thus, the F2 generation contained purple and white
flowers in a ratio of 9 purple : 7 white
Flower color is determined by two different genes:




C (one purple-color-producing) allele is dominant to c
(white)
P (another purple-color-producing) allele is dominant to p
(white)
cc or pp masks P or C alleles, producing white color
Thus, a plant that is homozygous for either recessive white
allele, would develop a white flower
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4-65


The term epistasis describes the situation in which a
gene can mask the phenotypic effects of another gene
Epistatic interactions often arise because two (or more)
different proteins participate in a common cellular
function
Colorless
precursor
Enzyme C
The recessive c allele
encodes an inactive enzyme

Colorless
intermediate
Enzyme P
Purple
pigment
The recessive p allele
encodes an inactive enzyme
If an individual is homozygous for either recessive allele


It will not make any functional enzyme C or enzyme P
Therefore, the flowers remain white
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4-66
An example of epistasis- 9:3:4

BbCc
BbCc
Sperm
1⁄
BC
4
1⁄
4
bC
1⁄
4
1⁄
Bc
4
bc
Eggs
1⁄
1⁄
4
BC
BBCC
BbCC
BBCc
BbCc
4
bC
BbCC
bbCC
BbCc
bbCc
1⁄
1⁄
4
Bc
BBCc
BbCc
BBcc
4
bc
BbCc
bbCc
Bbcc
9⁄
16
3⁄
16
Bbcc
bbcc
4⁄
16
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
c/c “masks” B/therefore
C is epistatic to B
A Cross Involving a Two-Gene Interaction
Can Produce an 8:4:3:1 ratio

Inheritance of the Cream-Eye allele in
Drosophila



Discovered by Calvin Bridges
He identified a rare fly with cream-colored eyes, in
a true-breeding culture of flies with eosin eyes
This could be explained in one of two ways


1. A new mutation changed the eosin allele into a
cream allele
2. A mutation occurred in another gene that modified
the expression of the eosin allele
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4-67
The Hypothesis

Cream-colored eyes in fruit flies are due to the
effect of a second gene that modifies the
expression of the eosin allele
Testing the Hypothesis

Refer to Figure 4.19
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4-68
Figure 4.19
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4-69
The Data
Cross
Outcome
P cross:
Cream-eyed male X
wild-type female
F1: all red eyes
F1 cross:
F1 brother X F1 sister
F2: 104 females with red eyes
47 males with red eyes
44 males with eosin eyes
14 males with cream eyes
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Interpreting the Data
Cross
Outcome
P cross:
Cream-eyed male X
wild-type female
F1: all red eyes
F1 cross:
F1 brother X F1 sister
F2: 104 females with red eyes
47 males with red eyes
44 males with eosin eyes
14 males with cream eyes
F2 generation contains males with eosin eyes
This indicates that the cream allele is
not in the same gene as the eosin allele
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4-71
Interpreting the Data

One possibility is that the cream allele is an
autosomal recessive allele

C = Normal allele


ca = Cream allele


Does not modify the eosin phenotype
Modifies the eosin color to cream
Refer to Figure 4.19
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4-72
Figure 4.19
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4-73
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4-74

The specific modifier allele, ca, can modify the
phenotype of the eosin- but not the red-eye allele


The eosin can be modified only when the ca allele is
homozygous
The predicted 8:4:3:1 ratio agrees reasonably well
with the data of Bridges
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4-75