Transcript non-Mendelian inheritance

Genetics: Analysis and Principles
Robert J. Brooker
CHAPTER 7
NON-MENDELIAN
INHERITANCE
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INTRODUCTION
• Mendelian inheritance patterns involve genes that
– Directly influence the outcome of an organism’s traits
and
– Obey Mendel’s laws
• Most genes in eukaryotic species follow a
Mendelian pattern of inheritance
– However, there are many that don’t
• Linkage can be considered as non-Mendelian
inheritance
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INTRODUCTION
• Patterns of inheritance that deviate from a
Mendelian pattern:
– Maternal effect and epigenetic inheritance
• Involve genes in the nucleus
– Extranuclear inheritance
• Involves genes in organelles other than the nucleus
– Mitochondria
– Chloroplasts
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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
• This phenomenon is due to the accumulation of
gene products that the mother provides to her
developing eggs
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
The first example of a maternal effect gene was
discovered in the 1920s by 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
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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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Reciprocal cross
reciprocal cross is a
breeding experiment
designed to test the role of
parental sex on a given
inheritance pattern. All parent
organisms must be true
breeding to properly carry out
such an experiment. In one
cross, a male expressing the
trait of interest will be crossed
with a female not expressing
the trait. In the other, a
female expressing the trait of
interest will be crossed with a
male not expressing the trait.
A 3:1 phenotypic ratio would
be predicted by a Mendelian
pattern of inheritance
Figure 7.1
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Alfred Sturtevant later explained the inconsistency
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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CO 7
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Parental
generation
Fig. 7.1(TE Art)
DD
dd
dd
DD
F1 generation
Dd
All dextral
F2 generation
Dd
All sinistral
Males and females
1 DD
2 Dd
1 dd
All dextral
Cross to each other
F3 generation
Males and females
3 dextral
1 sinistral

Note that the phenotype of each generation
depends on the maternal genotype of the
previous generation

DD or Dd mothers produce dextral offspring
dd mothers produce sinistral offspring
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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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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
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D gene products cause egg cleavage that
promotes a right-handed body plan
Figure 7.2
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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
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
Maternal effect genes encode RNA or proteins that
play important roles in the early steps of
embryogenesis
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For example
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Cell division
Cleavage pattern
In Drosophila, geneticists have identified several
dozen maternal effect genes

These have profound effects on the early stages of
development
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7.2 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
• Epigenetic changes are caused by DNA and
chromosomal modifications
– These can occur during oogenesis, spermatogenesis or
early embryonic development
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Dosage Compensation
• The purpose of dosage compensation is to offset
differences in the number of active sex
chromosomes
• Dosage compensation has been studied extensively
in mammals, Drosophila and Caenorhabditis elegans
• Depending on the species, dosage compensation
occurs via different mechanisms
– Refer to Table 7.1
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Fig.
7.3
Barr body is a highly
condensed X chromosome
• The mechanism of X inactivation, also known as
the Lyon hypothesis, is schematically illustrated in
Figure 7.4
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The example involves a white and black variegated
coat color found in certain strains of mice
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A female mouse has inherited two X chromosomes
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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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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
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• 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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7.3 EXTRANUCLEAR INHERITANCE
• Extranuclear inheritance refers to inheritance
patterns involving genetic material outside
the nucleus
• The two most important examples:
mitochondria and chloroplasts
• These organelles are found in the cytoplasm
– Extranuclear inheritance = cytoplasmic inheritance
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• 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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• The main function of mitochondria is oxidative
phosphorylation
– A process used to generate ATP (adenosine triphosphate)
• ATP is used as an energy source to drive cellular reactions
• The genetic material in mitochondria is referred to as mtDNA
• The human mtDNA consists of only 17,000 bp (Figure 7.14)
– 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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