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Genetics: Analysis and Principles
Robert J. Brooker
CHAPTER 7
NON-MENDELIAN
INHERITANCE
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INTRODUCTION

In this chapter we will discuss additional (even
bizarre) patterns of inheritance that deviate from a
Mendelian pattern

Maternal effect and epigenetic inheritance
 Involve genes in the nucleus
 X-inactivation (Dosage compansation)
 Genomic imprinting

Extranuclear inheritance

Involves genes in organelles other than the nucleus


Mitochondria
Chloroplasts
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7-3
7.1 MATERNAL EFFECT

Maternal effect refers to an inheritance pattern for
certain nuclear genes in which the genotype of
the mother directly determines the phenotype of
her offspring


Surprisingly, the genotypes of the father and
offspring themselves do not affect the phenotype of
the offspring
This phenomenon is due to the accumulation of
gene products that the mother provides to her
developing eggs
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7-4

The first example of a maternal effect gene was
discovered in the 1920s by A. E. Boycott

He was studying morphological features of the
water snail, Limnaea peregra

In this species, the shell and internal organs can be
arranged in one of two directions





Right-handed (dextral)
Left-handed (sinistral)
The dextral orientation is more common and dominant
The snail’s body plan curvature depends on the cleavage pattern
of the egg immediately after fertilization
Figure 7.1 describes Boycott’s experiment
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7-5
Reciprocal cross
A 3:1 phenotypic ratio would
be predicted by a Mendelian
pattern of inheritance
Figure 7.1
7-6


Alfred Sturtevant later explained the incongruity
with Mendelian inheritance

Snail coiling is due to a maternal effect gene that exists
as dextral (D) and sinistral (d) alelles

The phenotype of the offspring depended solely on the
genotype of the mother
His conclusions were drawn from the inheritance
patterns of the F2 and F3 generations
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7-7
Reciprocal cross
F1 mothers are Dd
The dominant allele, D,
caused ALL the F2
offspring to be dextral
F2 mothers include 3 with
the D allele and 1 with the
d allele
This explains this 3:1 ratio
Figure 7.1
7-8

Thus, in this example



DD or Dd mothers produce dextral offspring
dd mothers produce sinistral offspring
The phenotype of the progeny is determined by
the mother’s genotype NOT phenotype

The genotypes of the father and offspring do not affect
the phenotype of the offspring
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7-9

The non-Mendelian inheritance pattern of
maternal effect genes can be explained by the
process of oogenesis in female animals



Maturing animal oocytes are surrounded by maternal
cells that provide them with nutrients
These nurse cells are diploid, whereas the oocyte
becomes haploid
In the example of Figure 7.2a


A female is heterozygous for the snail-coiling maternal
effect gene
The haploid oocyte received the d allele in meiosis
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7-10
The gene products are a reflection of the genotype of the mother
They are transported to the cytoplasm of the oocyte where they
persist for a significant time after the egg has been fertilized
Thus influencing the early developmental stages of the embryo
Figure 7.2
7-11
D gene products cause egg cleavage that
promotes a right-handed body plan
Figure 7.2
7-12
d gene products
cause egg cleavage
that promotes a lefthanded body plan
Even if the egg is fertilized
by sperm carrying the
D allele
The sperm’s genotype is
irrelevant because the
expression of the sperm’s
gene would be too late
Figure 7.2
7-13

Maternal effect genes encode RNA and proteins
that play important roles in the early steps of
embryogenesis

For example-Cell division, Cleavage pattern, Body Axis
determination

Accumulation before fertilization allows these steps
to proceed very quickly after fertilization

Therefore defective alleles in maternal gene effects
tend to have a dramatic effect on the phenotype of
the individual

In Drosophila, geneticists have identified several dozen
maternal effect genes

These have profound effects on the early stages of development
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7-14
7.2 EPIGENETIC INHERITANCE

Epigenetic inheritance refers to a pattern in which
a modification occurs to a nuclear gene or
chromosome that alters gene expression



However, the expression is not permanently changed
over the course of many generations
That is because the DNA sequence does not change
Epigenetic changes are caused by DNA and
chromosomal modifications

These can occur during oogenesis, spermatogenesis or
early embryonic development
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7-15
The left micrograph shows the Barr body on the periphery of a human
nucleus after staining with a DNA-specific dye
The fur pattern of a calico cat
Dosage Compensation

The purpose of dosage compensation is to offset
differences in the number of active sex
chromosomes

Depending on the species, dosage compensation
occurs via different mechanisms

Refer to Table 7.1
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7-16
7-17


Dosage compensation is not well understood in
some species, such as birds and fish
In birds, the sex chromosomes are the




Z, a large chromosome containing many genes
W, a microchromosome containing few genes
Males are ZZ; females are ZW
It appears that the Z chromosome in males does not
undergo condensation like one of the X
chromosomes in female mammals



Different studies have shown presence and absence of
dosage compensation in birds
May occur only on specific genes
May be accomplished through histone modification
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7-18
Dosage compensation in mammals

In 1949, Murray Barr and Ewart Bertram identified a
highly condensed structure in the interphase nuclei
of somatic cells in female cats but not in male cats



This structure became known as the Barr body (Figure
7.3a)
In 1960, Susumu Ohno correctly proposed that the
Barr body is a highly condensed X chromosome
In 1961, Mary Lyon proposed that dosage
compensation in mammals occurs by the
inactivation of a single X chromosome in females
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7-19
The left micrograph shows the Barr body on the periphery of a human
nucleus after staining with a DNA-specific dye

The mechanism of X inactivation, also known as
the Lyon hypothesis, is schematically illustrated in
Figure 7.4

The example involves a white and black variegated
coat color found in certain strains of mice

A female mouse has inherited two X chromosomes


One from its mother that carries an allele conferring white coat
color (Xb)
One from its father that carries an allele conferring black coat
color (XB)
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7-20
The epithelial cells
derived from this
embryonic cell will
produce a patch of
white fur
At an early stage of
embryonic development
While those from
this will produce a
patch of black fur
Figure 7.4
7-21

During X chromosome inactivation, the DNA
becomes highly compacted


Most genes on the inactivated X cannot be expressed
When this inactivated X is replicated during cell
division

Both copies remain highly compacted and inactive

In a similar fashion, X inactivation is passed along
to all future somatic cells

Another example of variegated coat color Is found
in calico cats

Refer to Figure 7.3b
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7-22
The fur pattern of a calico cat
The Lyon Hypothesis Put to the Test
Experiment 7A


In 1963, Ronald Davidson, Harold Nitowsky and
Barton Childs set out to test the Lyon hypothesis at
the cellular level
To do so they analyzed the expression of a human
X-linked gene

The gene encodes glucose-6-phosphate dehydrogenase
(G-6-PD), an enzyme used in sugar metabolism
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7-23

Biochemists had found that individuals vary with
regards to the G-6-PD enzyme

This variation can be detected when the enzyme is
subjected to agarose gel electrophoresis

One G-6-PD allele encodes an enzyme that migrates very
quickly


Another allele encodes an enzyme that migrates slowly


The “fast” enzyme
The “slow” enzyme
The two types of enzymes have minor differences in their
structures

These do not significantly affect G-6-PD function
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7-24


Thus heterozygous adult females produce both types of
enzymes
Hemizygous males produce either the fast or the slow type
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7-25
Interpreting the Data

These results are consistent with the hypothesis
that


X inactivation has already occurred in any given
epithelial cell
AND
This pattern of inactivation is passed to all of the cell’s
progeny
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7-30
X Inactivation Depends on Xic,
Xist, TsiX and Xce

Researchers have found that mammalian cells can count their
X chromosomes and allow only one of them to remain active

Additional X chromosomes are converted to Barr bodies
Sex Chromosome
Composition
Number of
Barr bodies
Normal female
XX
1
Normal male
XY
0
Turner syndrome (female)
X0
0
Triple X syndrome (female)
XXX
2
Klinefelter syndrome (male)
XXY
1
Phenotype
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7-31

The genetic control of inactivation is not entirely
understood at the molecular level



However, a short region on the X chromosome termed
the X-inactivation center (Xic) plays a critical role
For inactivation to occur, each X chromosome must
have a Xic region (Figure 7.7, slide 7-35)
The Xic region contains a gene named Xist (for Xinactive specific transcript)


The Xist gene is only expressed on the inactive X
chromosome
It does not encode a protein


It codes for a long RNA, which coats the inactive X chromosome
Other proteins will then bind and promote chromosomal
compaction into a Barr body
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7-32
Promotes
compaction
Prevents
compaction
May regulate the transcription of
the Xic region
Thereby influences the choice of
the active X chromosome
Figure 7.7
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7-35

A gene designated TsiX also plays a role in
chromosome choice



It is located in the Xic region
It is expressed only during early embryonic development
It encodes an RNA complementary to Xist RNA


Termed antisense RNA (where Xist RNA is the sense RNA)
Tsix antisense RNA is believed to bind to Xist sense RNA
and inhibit its function



In other words, TsiX RNA prevent X chromosome inactivation
The choice of which X is inactivated involves a complex interplay
between Xist and Tsix
The exact mechanism is not understood
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7-33

A second region termed the X chromosome
controlling element (Xce) affects the choice of the X
chromosome to be inactivated



Xce is close to and may even overlap Xic
Xce may bind proteins that regulate the genes of the Xic
A female heterozygous for different Xce alleles will
have a skewed X-inactivation

The X chromosome that carries a strong Xce allele is
more likely to remain active than one with a weak Xce
allele
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7-34
Promotes
compaction
Prevents
compaction
May regulate the transcription of
the Xic region
Thereby influences the choice of
the active X chromosome
Figure 7.7
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7-35

The process of X inactivation can be divided into
three stages

Initiation


Spreading


The chosen X chromosome is inactivated
Maintenance


One of the X chromosomes is targeted to be inactive
The inactivated X chromosome is maintained as such during
future cell divisions
Refer to Figure 7.8
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7-36
Initiation. X counting
Spreading
The Barr body is
replicated and both
copies remain
compacted
Figure 7.8
7-37

A few genes on the inactivated X chromosome are
expressed in the somatic cells of adult female
mammals


These genes escape the effects of X inactivation
They include


Xist
Pseudoautosomal genes



Dosage compensation in this case is unnecessary because
these genes are located both on the X and Y
Up to a quarter of X genes in humans may escape full
inactivation
The mechanism is not understood

May involve loosening of chromatin in specific regions
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7-38
Genomic Imprinting

Genomic imprinting is a phenomenon in which
expression of a gene depends on whether it is
inherited from the male or the female parent

Imprinted genes follow a non-Mendelian pattern of
inheritance

Depending on how the genes are “marked”, the offspring
expresses either the maternally-inherited or the
paternally-inherited allele


Not both
This is termed monoallelic expression
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7-39

Let’s consider the following example in mice:

The Igf-2 gene encodes a growth hormone called insulinlike growth factor 2


Imprinting results in the expression of the paternal but
not the maternal allele



The paternal allele is transcribed into RNA
The maternal allele is not transcribed
Igf-2m is a mutant allele that yields a partially defective
protein


A functional Igf-2 gene is necessary for a normal size
This may cause a mouse to be dwarf depending on whether it
inherits the mutant allele from its father or mother
Refer to Figure 7.9
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7-40
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7-41

Genomic imprinting occurs in several species
including mammals, insects and plants

It may involve





A single gene
A part of a chromosome
An entire chromosome
Even all the chromosomes from one parent
It can be used for X inactivation in some species
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7-44
Imprinting and DNA Methylation

Genomic imprinting must involve a marking process

At the molecular level, the imprinting of several
genes is known to involve differentially methylated
regions (DMRs)


These are located near the imprinted genes
They are methylated either in the oocyte or sperm


Not both
They contain binding sites for one or more transcriptional
factors
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7-45

For most genes, methylation at a DMR results in
inhibition of gene expression

Methylation could



Enhance the binding of proteins that inhibit transcription
and/or
Inhibit the binding of proteins that enhance transcription
Because of this, imprinting is usually described as
a process that silences gene expression by
preventing transcription


However, this is not always the case
Refer to Figure 7.11
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7-46

Let’s consider two imprinted genes in humans,
H19 and Igf-2

They lie close to each other on human chromosome 11
Appear to be controlled by the same DMR

This DMR




Is ~ 2000 bp
Contains binding sites for proteins that regulate the transcription
of both genes
Is highly methylated on the paternally inherited chromosome

Methylation silences the H19 gene
and activates the Igf-2 gene
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7-47
Igf-2 gene
silenced
Only binds to
unmethylated
DMR
Only binds to
methylated
DMR
Figure 7.11a
H19 gene
activated
H19 gene Igf-2 gene
silenced
activated
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7-48
Figure 7.11b Methylation patterns in somatic cells and gametes of male
and female offspring
Haploid female gametes transmit
an unmethylated DMR
Haploid male gametes transmit
a methylated DMR
7-49

To date, imprinting has been identified in dozens of
mammalian genes

However, the biological significance of genomic imprinting is
still a matter of speculation
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7-50

Imprinting does play a role in the inheritance of certain
human diseases such as Prader-Willi syndrome (PWS) and
Angelman syndrome (AS)

PWS is characterized by




AS is characterized by





Reduced motor function
Obesity
Mental deficiencies
Hyperactivity
Unusual seizures
Repetitive symmetrical muscle movements
Mental deficiencies
Most commonly, PWS and AS involve a small deletion in
chromosome 15


If it is inherited from the mother, it leads to AS
If it is inherited from the father, it leads to PWS
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7-51

Researchers have discovered that this region contains
closely linked but distinct genes


AS results from the lack of expression of a single gene,
UBE3A



These are maternally or paternally imprinted
UBE3A encodes a protein called EA-6P that transfers small ubiquitin
molecules to certain proteins to target their degradation
The gene is paternally imprinted (silenced)
PWS results (most likely) from the lack of expression of a
single gene, designated SNRNP

SNRNP encodes a small nuclear ribonucleoprotein which is a
complex that controls gene splicing


This protein is necessary for the synthesis of critical proteins in the brain
The gene is maternally imprinted (silenced)
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7-52
Figure 7.12
7-53
7.3 EXTRANUCLEAR
INHERITANCE


Extranuclear inheritance refers to inheritance
patterns involving genetic material outside the
nucleus
The two most important examples are due to
genetic material within organelles


Mitochondria and chloroplasts
These organelles are found in the cytoplasm

Therefore, extranuclear inheritance is also termed
cytoplasmic inheritance
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7-54

The genetic material of mitochondria and
chloroplasts is located in a region called the nucleoid



The genome is composed of a single circular
chromosome containing double-stranded DNA
Note:




Refer to Figure 7.13
A nucleoid can contain more than one chromosome
An organelle can contain more than one nucleoid
Chloroplasts tend to have more nucleoids per
organelle than mitochondria
Refer to Table 7.3
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7-55
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7-56

Besides variation in copy number, the sizes of
organellar genomes also vary greatly among
different species



There is a 400-fold variation in the size of mitochondrial
genomes
There is also a substantial variation in size of chloroplast
genomes
In general, mitochondrial genomes are



Fairly small in animals
Intermediate in size in fungi, algae and protists
Fairly large in plants
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7-57

The main function of mitochondria is oxidative
phosphorylation

A process used to generate ATP (adenosine triphosphate)



The genetic material in mitochondria is referred to as mtDNA
The human mtDNA consists of only 17,000 bp (Figure 7.14)


ATP is used as an energy source to drive cellular reactions
It carries relatively few genes
 rRNA and tRNA genes
 13 genes that function in oxidative phosphorylation
Note: Most mitochondrial proteins are encoded by genes in
the nucleus

These proteins are made in the cytoplasm, then transported into the
mitochondria
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7-58
Necessary for synthesis of proteins
inside the mitochondrion
Function in oxidative phosphorylation
Figure 7.14
7-59

The main function of chloroplasts is photosynthesis

The genetic material in chloroplasts is referred to as cpDNA


The cpDNA of tobacco plant consists of 156,000 bp


It is typically about 10 times larger than the mitochondrial genome of
animal cells
It carries between 110 and 120 different genes
 rRNA and tRNA genes
 Many genes that are required for photosynthesis
As with mitochondria, many chloroplast proteins are encoded
by genes in the nucleus

These proteins contain chloroplast-targeting signals that direct them
from the cytoplasm into the chloroplast
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7-60
Genes designated
ORF (open
reading frame)
encode
polypeptides with
unknown
functions
Figure 7.15 A genetic map of the tobacco chloroplast genome
7-61
Maternal Inheritance in the Fouro’clock Plant

Carl Correns discovered that pigmentation in
Mirabilis jalapa (the four o’clock plant) shows a nonMendelian pattern of inheritance


Leaves could be green, white or variegated (with both
green and white sectors)
Correns determined that the pigmentation of the
offspring depended solely on the maternal parent
and not at all on the paternal parent

This is termed maternal inheritance


different than maternal effect
Refer to Figure 7.16
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7-62
Figure 7.16
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7-63

In this example, maternal inheritance occurs because the
chloroplasts are transmitted only through the cytoplasm of
the egg


The pollen grains do not transmit chloroplasts to the offspring
The phenotype of leaves can be explained by the types of
chloroplasts found in leaf cells
 Green phenotype is the wild-type


White phenotype is the mutant


Due to normal chloroplasts that can make green pigment
Due to a mutation that prevents the synthesis of the green
pigment
A cell can contain both types of chloroplasts


A condition termed heteroplasmy
In this case, the leaf would be green
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7-64

Figure 7.17 provides a cellular explanation for the
variegated phenotype in Mirabilis jalapa

Consider a fertilized egg that inherited two types
of chloroplast


Green and white
As the plant grows, the chloroplasts are
irregularly distributed to daughter cells

Sometimes, a cell may receive only white chloroplasts


Such a cell will continue to divide and produce a white sector
Cells that contain only green chloroplasts or a
combination of green and white will ultimately produce
green sectors
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7-65
Figure 7.17
7-66
The Petite Trait in Yeast

Mutations that yield defective mitochondria are
expected to make cells grow much more slowly

Boris Ephrussi and his colleagues identified
Saccharomyces cerevisiae mutants that have such
a phenotype


These were called petites because they formed small
colonies on agar plates
Wild-type strains formed larger colonies
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7-67

Biochemical and physiological evidence indicated
that petite mutants had defective mitochondria

Genetic analyses showed that petite mutants are
inherited in different ways

Two main types of mutants were identified

1. Segregational mutants




Have mutations in genes located in the nucleus
Segregate in a Mendelian manner in meiosis
Refer to Figure 7.18a
2. Vegetative mutants



Have mutations in genes located in the mitochondrial genome
Show a non-Mendelian pattern of inheritance
Refer to Figure 7.18b
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7-68

Yeast come in two mating types: a and a

Thus, Euphressi was able to cross yeast belonging to two different
strains
Zygote then
meiosis
Each resulting tetrad
shows a 2:2 ratio of
wild-type to petite
This result is typical of
Mendelian inheritance
Figure 7.18
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7-69

Euphressi discovered two types of vegetative petites

Neutral and Suppressive
Zygote then
Each resulting tetrad
shows a 4:0 ratio of
wild-type to petite
meiosis
Zygote then
These results contradict the normal
2:2 ratio expected for the segregation
of Mendelian traits
meiosis
Each resulting tetrad
shows a 0:4 ratio of
wild-type to petite
Figure 7.18
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7-70


Researchers later found that

Neutral petites lack most of their mitochondrial DNA

Suppressive petites lack only small segments of mtDNA
When two yeast cells are mated, offspring inherit
mitochondria from both parents
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Figure 7.18
Progeny have both “wild type”
and “neutral petite” mitochondria
 They display a normal
phenotype because of the wild
type mitochondria

Progeny have both “wild type” and “suppressive
petite” mitochondria
 So how come only petite colonies are produced?
 Two possibilities
 i. Suppressive petite mitochondria could
replicate faster than wild-type mitochondria
 ii. Recombination between wild-type and
petite mtDNA may ultimately produce defects
in the wild-type mitochondria

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Inheritance of Chloroplasts in Algae

The unicellular alga Chlamydomonas reinhardtii is
a model organism

It contains a single chloroplast

Occupies ~ 40% of the cell’s volume

Most strains are sensitive to the antibiotic
streptomycin (sms)

In 1954, Ruth Sager identified a mutant that was
resistant to streptomycin (smr)
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
Like yeast, Chlamydomonas can be found in two
mating types


Mating type is due to nuclear inheritance


It segregates in a 1:1 manner
Sager and her colleagues discovered that
“resistance to streptomycin” was not inherited in a
Mendelian manner


mt+ and mt–
smr was inherited from the mt+ parent but not from the
mt– parent
In subsequent studies, they mapped several
genes, including the smr gene, to the chloroplast
chromosome
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Because the
mt+ strain
was smr
Figure 7.19
Because the
mt+ strain
was sms
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The Pattern of Inheritance of
Organelles

The pattern of inheritance of mitochondria and
chloroplasts varies among different species
 Heterogamous species

Produce two kinds of gametes



Female gamete  Large
 Provides most of the cytoplasm of the zygote
Male gamete  Small
 Provides little more than a nucleus
In these species, organelles are typically inherited
from the mother
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7-76
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The Pattern of Inheritance of
Organelles

Species with maternal inheritance may, on
occasion, exhibit paternal leakage
 The paternal parent provides mitochondria
through the sperm

In the mouse, for example, 1-4 paternal mitochondria
are inherited for every 100,000 maternal mitochondria
per generation of offspring
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7-78
Human Mitochondrial Diseases

Human mtDNA is transmitted from mother to offspring via
the cytoplasm of the egg


Several human mitochondrial diseases have been
discovered


Therefore, the transmission of human mitochondrial diseases
follows a strict maternal inheritance pattern
These are typically chronic degenerative disorders affecting the
brain, heart, muscles, kidneys and endocrine glands
Example: Leber’s hereditary optic neuropathy (LHON)



Affects the optic nerve
May lead to progressive loss of vision in one or both eyes
LHON is caused by mutations in several different mitochondrial
genes
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7-79
The Endosymbiosis Theory

The endosymbiosis theory describes the
evolutionary origin of mitochondria and chloroplasts

These organelles originated when bacteria took up
residence within a primordial eukaryotic cell




chloroplasts originated as cyanobacterium
mitochondria originated as Gram-negative nonsulfur purple
bacteria
During evolution, the characteristic of the intracellular bacterial cell
gradually changed to that of the organelle
The endosymbiotic origin of organelles is supported
by several observations

These include


Organelles have circular chromosomes (like bacteria)
Organelle genes are more similar to bacterial genes than to those
found within the nucleus
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7-80
The eukaryotic cell
was now able to
undergo
phtosynthesis
Figure 7.20
The eukaryotic cell
was now able to
synthesize greater
amounts of ATP
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The bacterial cells
may have gained a
more stable
environment with
more nutrients
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The Endosymbiosis Theory

During the evolution of eukaryotic species, most genes
originally found in the bacterial genome have been lost or
transferred to the nucleus


The gene transfer has primarily been unidirectional


From the organelles to the nucleus
In addition, gene transfer can occur between organelles


Modern day mitochondria and chloroplasts have lost most of the genes
still found in present-day cyanobacteria and purple bacteria
Between two mitochondria, two chloroplasts or a mitochondrion and a
chloroplast
The biological benefits of gene transfer remain unclear
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Symbiotic Infective Particles

There are a few rare cases where infectious
particles establish a symbiotic relationship with their
host

These have provided interesting and even bizarre
examples of extranuclear inheritance
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Symbiotic Infective Particles

One such example is the killer trait in the protozoan
Paramecia aurelia


Killer paramecia contain kappa particles in their cytoplasm
Kappa particles have their own DNA which contains



Kappa particles are infectious


A gene that encodes the toxin paramecin, which kills other
paramecia
Genes that provide the killer paramecia with resistance to
paramecin
They can infect nonkiller strains and convert them into killer strains
Infective particles have also been identified in
Drosophila melanogaster
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