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Wheeler High School
The Center for Advanced Studies in Science, Math &
Technology
Chapter 9: Extranuclear Inheritance
Post-AP DNA/Genetics – Ms. Hager
Extranuclear Inheritance
• You are accustomed to discussing
inheritance as being transferred from the
nuclear genes located on chromosomes
from your parents.
– This is not always the case…
Several Varieties of Extranuclear
Inheritance
• Organelle heredity – DNA contained from
mitochondria or chloroplasts determines
phenotype of offspring
• Infectious heredity – comes about from the
symbiotic (parasitic) relationship associated with a
microorganism; inherited phenotype is affected by
the presence of the microorganism living in the
cell’s cytoplasm
• Maternal effect – nuclear gene products are
stored in the egg and then transmitted through the
ooplasm to the offspring
Organelle Heredity – Chloroplast &
Mitochondria
• Analysis of organelle DNA is much more
complex than nuclear DNA
WHY?
– Function of organelle dependent of gene
products of nuclear DNA and organelle DNA
• Figuring out where mutations occur is difficult.
– Lots of organelles in the cell and only one or
two may have mutation
• Therefore the mutant phenotype will not be seen.
Chloroplasts: Variegation in Four
O’Clock Plants
• Mirabilis jalapa
• Some of the plant’s branches have white
leaves, others have green leaves and some
are variegated.
White leaves lack chlorophyll.
Therefore one can conclude
that these leaves do not contain
chloroplasts.
Chloroplasts: Variegation in Four
O’Clock Plants
•How is this trait
inherited?
•Inheritance is
determined based on the
location of the ovule,
regardless of the
phenotype of the source
of pollen.
Chloroplasts: Variegation in Four
O’Clock Plants
What conclusions can be drawn from this
observation?
Inheritance in the 4 O’Clock must transmitted
through the cytoplasm of the maternal
parent because the pollen, which contributes
little or no cytoplasm to the zygote, had no
apparent influence on the progeny
phenotypes.
Chloroplast Mutations in
Chlamydomonas
Chlamydomonas is an excellent model system
for studying organelle heredity because it
has a single large chloroplast that exhibits a
uniparental inheritance pattern.
*Circular dsDNA
Chlamydomonas
•The strR phenotype is
transmitted only through the
mt+ parent.
•After the zygote has gone
through meiosis and haploid
cells are produced, it is clear
that the genetic information in
the chloroplast of progeny cells
is derived only from the mt+
parent.
Figure 9-2
Copyright © 2006 Pearson Prentice Hall, Inc.
Mitochondrial Mutations
• Mitochondria, like chloroplasts, also have a
distinct genes.
• Also like chloroplasts, mitochondrial
mutations are transmitted through the
cytoplasm.
Mitochondrial Mutations: The Case
of poky (mi-1) in Neurospora
•1952 Mitchell and Mitchell
studied pink bread mold.
•Discovered a slow-growing
strain & named it poky.
•Slow growth is associated
with impaired mitochondrial
function.
• absence proteins needed for
e- transport; therefore aerobic
respiration is very slow
Chloroplast DNA
• Chloroplast DNA ranges from 100 to 225 kb
in length, and the genes carried on the DNA
encode products involved in photosynthesis
and translation.
Notice that chloroplast DNA
is circular. What does this
suggest?
Mutations in mtDNA Cause Human
Disorders
•Heteroplasmy is the
condition in which a
deleterious mutation
arises in an organelle,
such that an adult will
have cells with a variable
mixture of normal and
abnormal organelles.
•For a human disorder to
be attributed to
mitochondrial DNA, the
inheritance must exhibit a
maternal inheritance
pattern, the disorder must
reflect a deficiency in the
bioenergetic function of
the organelle, and there
must be a specific
mutation in a
mitochondrial gene.
Mutations in mtDNA Cause Human
Disorders
•
•
Human mitochondria contains 16,569 bp
Produces 13 proteins required for cellular
respiration
– Deletions can cause misfolded proteins which
can, in turn, can cause ATP issues
•
•
No histone proteins, therefore no structural
protection from mutations
Lots of free radicals (the ‘left-overs’ of
cellular respiration) accumulate and can
cause mutations
Mutations in mtDNA Cause Human
Disorders
• mtDNA is materially inherited
– Why do you think that is?
• There are lots of mtDNA in human cells,
therefore the impact of a mutation can be
‘diluted’
– Variation in genetic content of organelles is
called heteroplasmy!
Mutations in mtDNA Cause Human
Disorders
Criteria For mtDNA Mutation Diagnosis
1. Inheritance pattern must be maternal rather
than Mendelian
2. Disorder must reflect deficiency in the
bioenergetic function of organelle
3. Must be a mutation in 1 or more of the mt
genes
Mutations in mtDNA Cause Human
Disorders
• Three disorders arising from mtDNA are:
1) myoclonic epilepsy and ragged red
fiber disease (MERRF)
2) Leber’s hereditary optic neuropathy
(LHON)
3) Kearns– Sayre syndrome (KSS).
MERRF
Myoclonic Epilepsy
• Only offspring of affected mothers inherit this
• Symptoms:
–
–
–
–
–
Ataxia (lack of muscle coordination)
Deafness
Dementia
Epileptic seizures
“ragged-red” because there are blotchy red patches due
to proliferation of mitochondria
– Neurological symptoms (since the brain has a high
energy demand)
Figure 9-9a
Copyright © 2006 Pearson Prentice Hall, Inc.
LHON
Leber’s hereditary optic neuropathy
•
•
•
•
Maternal inheritance
mtDNA lesions
Blindness by age of 27
Abnormal oxidative phosphorylation (final
pathway in cell respiration)
– Point mutations in gene that codes for NADH
dehydrognease
• Many cases are sporadic
• Includes a lack of muscular control
KSS
Kearns– Sayre syndrome
•
•
•
•
Loss of vision
Hearing loss
Heart conditions
Deletions in mtDNA
– Deletions increase as severity of symptoms
increase
• Usually symptom-free as children
The Maternal Effect
• In maternal effect, an offspring’s phenotype
for a specific trait is under the control of
nuclear gene products present in the egg.
– Classic Example:
• Shell coiling in snails
• dd = sinistral (left-handed)
• Dd & DD = dextral (right-handed)
A maternal effect is
evident in generations II
and III, where the
genotype of the maternal
parent, rather than the
offspring’s own genotype,
controls the phenotype of
the offspring. The
photograph illustrates a
mixture of right- vs. lefthanded coiled snails.
Figure 9-13
Copyright © 2006 Pearson Prentice Hall, Inc.
Any Questions?
Figure 9-13-01
Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 9-13-02
Copyright © 2006 Pearson Prentice Hall, Inc.