NonMendelian Inheritance

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Non-Mendelian Inheritance

• Chapter 7

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Non-Mendelian inheritance

• Maternal effect
• Epigenetic inheritance
• Extranuclear inheritance

• Inheritance patterns that deviate from a
  Mendelian pattern
• Genotypes of offspring do not directly govern the
  phenotype in ways predicted by Mendel
• Due to specific timing of nuclear gene expression
  and nuclear gene inactivation
• Inheritance of and influence on traits by
  extranuclear genetic material

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Maternal effect

• Inheritance pattern observed for nuclear genes
• Genotype of the mother directly determines the
  phenotypic traits of the offspring
• Genotypes of the neither individual itself, nor
  the father participate in the phenotype
• Due to the mother providing gene products to the
  developing eggs

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Maternal effect

• Ist studied by A. E. Boycott in 1920s using
  Limnea peregra (water snails)
• Shell shape can be either
• Right hand facing (dextral)
• Left hand facing (sinistral)
• Direction is decided on by the egg cleavage
  pattern immediately after fertilization
• Genetic crosses were completed to examine the
  transmission of this trait

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Inheritance pattern of snail coiling
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Oogenesis in female animals

• Oocyte is surrounded by nurse cells during
  maturation
• Nurse cells are diploid
• If nurse cells are heterozygotic, their genes are
  activated to produce mRNA and protein
• Gene products are transported to the oocyte
• It makes does not matter what the oocyte allele
  is- just what the gene products the nurse cells
  are producing

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Mechanism of maternal effect
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Maternal effect genes and gene products

• Maternal effect genes encode RNA and proteins
  critical in embryogenesis
• Participate in cell division, cleavage
  patterning, and body axis orientation
• Mutations in maternal effect alleles can often be
  severe/ even lethal
• Many studies have been done in Drosophila and the
  maternal effect on antero-posterior and
  dorso-ventral axis patterning (will cover in
  chapter 23)

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Epigenetic inheritance

• Modification is made to nuclear genes or
  chromosomes, altering gene expression
  transiently, but not permanently change DNA
  sequence
• Modifications occur during oogenesis,
  spermatogenesis, early embryogenesis- permanently
  effecting the traits of the individual
• Two examples
• Dosage compensation
• Genomic imprinting

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Dosage compensation

• Mechanism to offset differences in sex
  chromosomes between males and females
• Required to equilibrate the level of expression
  in both sexes even though the male and female
  complement of sex chromosomes are different
• Termed in 1932 by Hermann Muller in response to
  eye color mutations in Drosophila (on X
  chromosome)

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Drosophila dosage compensation

• X linked gene leading to apricot eye color is a
  similar phenotype in homzygous females and
  hemizygous males
• Heterozygous females (apricot and deletion) have
  a paler color- one copy in females does not equal
  one copy in males
• Copy number is compensated by increased
  expression level in males
• Dosage compensation does not occur in all X
  linked genes - why?

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Types of dosage compensation
Sex chromosomes
Females Males
One X chromosome is inactivated Paternally
derived X chromosome is inactivated Expression of
X chromosome in males is increased 2x Expression
level of both X chromosomes is decreased 50 in
hermaphrodites
Placental Mammals Marsupial Mammals Drosophilia
melanogaster Caenorhabditis elegans
XX XY
XX XY
XX XY
XX XO
Process is unclear for birds and fish
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Random X inactivation

• Theory proposed in 1961 by Mary Lyon, Liane
  Russell
• First evidence was cytological- 1949 Murray Barr
  and Ewart Bertram
• Condensed structure observed in somatic cell
  nuclei during interphase, only in female cats

Barr Body
Highly condensed X chromosome
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Calico cat X inactivation

• All calico cats are females
• Heterozygous for X-linked gene with an orange or
  black allele (white coloring is due to a separate
  gene)
• Orange and black patches are distributed randomly
• X inactivation of one of the two alleles in
  somatic cells

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Mechanism of X- inactivation

• Lyon Hypothesis
• Examined in mice with variegated coat color
• Inherit allele for white coat color from mother
  (Xb), black coat color from father (XB)
• Patches of epithelial tissue derived from
  embryonic cell in which one of the X chromosomes
  were inactivated
• Compaction of DNA during inactivation prevent
  gene expression

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Mechanism of X inactivation
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Experiment 7A

• Test of Lyon hypothesis at the cellular level
• Use an gene on the X chromosome that encodes
  glucose-6-phosphate dehydrogenase
• There are two alleles that produce protein
  variants that run either fast or slow when
  subjected to gel electrophoresis
• Heterozygous adult female produce both enzyme
  variants, while males produce only one

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Experimental technique

• Isolate tissue from a heterozygous female
• Culture on solid media to produce colonies -
  groups of cells that originated from one single
  progenitor cell
• Identify whether these clonal populations express
  only one G-6-PD variant

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Hypothesis

• Single somatic cells from a heterozygous female
  should only produce one variant of the G-6-PD
  enzyme

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Experimental set up
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Experimental set up (cont.)
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Data
Result single somatic clones only express one
form of the enzyme
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How does X inactivation occur?

• Human cells are able to count the X
  chromosomes, and allow only 1 to remain active
• In females- 2 Xs are counted, one is inactivated
• In males- 1 X is counted, none are inactivated
• If there is an abnormality in the number of sex
  chromosomes, counting still occurs, and more or
  less Barr bodies are produced

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X- inactivation center (Xic)

• Region on the X chromosome plays a critical role
  in X inactivation (process still not fully
  understood)
• The number of Xics are counted
• If a chromosome is missing an Xic- no X
  chromosomes are inactivated - this is embryonic
  lethal!

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Xist gene
Successful compaction requires first the
activation of Xist gene on inactivated X
chromosome Xist gene product is an untranslated
RNA molecule that coats the X chromosome to be
inactivated The promotes binding of other
proteins to the chromosome and compaction into a
Barr body
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Xce region
There are multiple Xce alleles Heterozygous
females with a strong Xce allele will favor the
other X chromosome for inactivation, skewing the
inactivation (usually not to more than 70) Tsix
gene produces an RNA complementary to Xist RNA
(antisense) Expression of Tsix is thought to
prevent inactivation during embryonic development
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Stages of X inactivation

• Initiation
• One X is targeted for inactivation
• One X is chosen to remain active
• Spreading
• Chosen X is inactivated
• Expression of Xist, coating of X, condensation
• begins near X-inactivation center and spreads
  outward
• Maintenance
• Inactivated X chromosome maintained during
  somatic divisions

Embryonic stages
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Initiation Occurs during embryonic development.
The number of X inactivation centers (Xics)
are counted and one of the X chromosomes remains
active and the other is targeted for inactivation.
To be inactivated
Xic
Xic
Spreading Occurs during embryonic development.
It begins at the Xist and progresses toward both
ends until the entire chromosome is inactivated.
The Xist gene encodes an mRNA that coats the X
chromosome and promotes its compaction into a
Barr body.
Xic
Xic
Further spreading
Barr body
Maintenance Occurs from embryonic development
through adult life. The inactivated X chromosome
is maintained as such during subsequent cell
divisions.
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Escape X inactivation

• Some genes are expressed on inactivated X
  chromosome
• Xist
• Pseudoautosomal genes also found on Y chromosome

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Genomic Imprinting

• Segment of DNA is marked
• Mark is retained and recognized throughout the
  life of the organism inheriting the marked DNA
• Causes non-Mendelian patterns, due to the ability
  to distinguish between maternally and paternally
  inherited alleles
• Offspring express one of the marked alleles, not
  both (monoallelic expression)

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Imprinting example IgF-2 allele

• Encodes murine growth hormone- insulin-like
  growth factor 2
• Imprinting results in expression of paternal
  allele, but NOT maternal
• Paternal allele is transcribed, maternal allele
  is transcriptionally silent
• Mutant of Igf-2 (Igf-2m) can cause dwarfism but
  only if inherited from the male parent

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Igf-2 imprinting in mouse
mother
father
mother
father
Igf-2m Igf-2m x Igf-2 Igf-2
Igf-2m Igf-2m x Igf-2 Igf-2
Igf-2m Igf-2
Igf-2 Igf-2m
silent
expressed
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3 stages of imprinting

• Establishment of imprint during gametogenesis
• Maintenance of imprint
• Erasure and re-establishment of imprint in germ
  cells

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Imprinting via DNA methylation

• DMR (differentially methylated regions) near
  imprinted genes
• Methylated in sperm or oocytes, not both
• Methylation results in inhibition of gene
  expression (most of the time) via enhancing the
  binding of inhibitors or inhibiting the binding
  of enhancers
• But, as usual, there are interesting exceptions

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H19 and Igf-2 expression in humans

• 2 imprinted human genes
• Controlled by the same DMR
• DMR region also contains regulatory binding sites
  for transcription of both H19 and Igf-2 genes
• Highly methylated on paternal chromosome
• Maternal chromosomal region is unmethylated

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H19 and Igf-2 expression in humans
Only Igf-2 mRNA expressed believed methylation
prevents an inhibitor of Igf-2 from Binding to
DMR region
Only H19 mRNA expressed
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Human disorders as a result of imprinting

• Prader-Willi syndrome and Angelman syndrome
• Prader-Willi patients- reduced motor function,
  obsesity and mental deficiencies
• Angelman patients- hyperactive, unusual seizures,
  repetitive symmetrical muscle movements, mental
  deficiencies
• Both due to small deletion of chromosome 15
• If inherited from paternal parent- Prader-Willi
• If inherited from maternal parent- Angelman

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Angelman syndrome

• Results from the lack of expression of a single
  gene UBE3A, located in this region of chromosome
  15
• Paternal allele is silenced- therefore if the
  inherited maternal chromosome is lacking this
  region- there is an overall lack of expression

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Prader-Willi syndrome

• Genes responsible not yet determined
• Although there are several known imprinted genes
  in this region that would be good candidates,
  including SNRPN, involved in gene splicing

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Extranuclear Inheritance

• Organellar genetic material
• Mitochondria and chloroplasts have genetic
  material
• Located inside the nucleoid
• Genetic material is circular, double stranded DNA
• There is variation in size and number of copies
  of this DNA

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mtDNA

• Size of mtDNA varies between organisms (yeast 75
  kB, pea 110 kB, human 16.5 kB)
• Encode ribosomal and tRNA, required for synthesis
  of proteins inside mitochondria
• Encode 13 polypeptides involved in oxidative
  phosphorylation, to allow synthesis of ATP

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Chloroplast genomes

• cpDNA range 120-217 kB in length
• Genes encoded by cpDNA encode factors for
  transcription, translation, photosynthesis and
  electron transport

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Endosymbiont theory

• Theory that the ancient origin of plastids was
  when a primordial bacterium took up residence
  in a eukaryotic cell

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Evidence for endosymbiont therory

• Both mitochondria and chloroplasts have their
  own DNA, that replicates independent of nuclear
  genome
• mtDNA and cpDNA not organized into nucleosomes by
  histones (like nuclear DNA)
• mtDNA utilize bacterial N-formyl methionine and
  tRNAfMet
• Bacterial translation inhibitors function on
  mtDNA and cpDNA, but not nuclear DNA

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Uniparental inheritance of mt DNA

• Experimentally defined in interspecies crosses
  between Xenopus laevis and Xenopus borealis
• Utilize a probe that recognizes mtDNA-
  hybridizing best to only one species
• When the two frog species were crossed, the F1
  hybrid had only one type of mtDNA, matching the
  maternal parent

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Inheritance of cpDNA

• Monitor the presence of proteins in tobacco
  plants (Nicotiana sp)
• Proteins of distinct species are distinguishable
  when examined by gel electrophoresis
• Rubisco (ribulose bisphosphate carboxylase) is
  comprised of 55 kDa large subunit, 12 kDa small
  subunit
• Large subunit is maternally inherited, while the
  small subunit is biparentally inherited
• This indicates the cooperation of nuclear and
  cpDNA gene products

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Inheritance of cpDNA

• Chlamydomonas has single chloroplast
• Identification of strain that was resistant to
  streptomycin smr
• This resistance was inherited from the mt mating
  type

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Extrachromosomal inheritance

• During mitosis, organelles are partitioned
  randomly, with number of organelles not
  distributing equally to progeny cells
• Ex. Pigmentation of Mirabilis jalapa is solely
  due to maternal parent (choroloplasts in egg)

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Organelle Transmission
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Mitochondrially encoded disorders

• Lebers hereditary optic neuropathy (LHON)
• Improper function of the mitochondrial electron
  transport chain causes degeneration of the optic
  nerve
• LHON only passed from mother to offspring
• Not all offspring show evidence of disease, and
  severity among offspring is variable

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Chromosome Reproduction and Inheritance

• Chapter 3

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Cell types

• Prokaryote
• Circular chromosome in cytoplasm
• Cytoplasm is surrounded by plasma membrane
• Gram-negative bacteria have a secondary outer
  membrane

• Eukaryote
• Compartmentalized cells
• Contain membrane bound organelles
• Golgi apparatus
• ER
• Mitochondria
• Chloroplast (in plants)
• Nucleus has 2 membranes, harbors genetic material
  on chromosomes

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Cell types
Prokaryotic cell
Eukaryotic cell
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Eukaryotic chromosome inheritance

• Most species are diploid- pairs of chromosomes
• Pair of chromosomes are called homologues

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Bacterial cell division

• Bacteria reproduce by asexual binary fission
• Genetic material is duplicated (circular
  chromosomes)
• Utilize binary fission to divide - equally
  distributing genetic/ chromosomal copies between
  mother and daughter cells

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Eukaryotic cell division

• More complex cell cycle
• Composed of 4 phases
• G1
• G0
• S (replication)
• G2
• M(mitosis)

Interphase
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Eukaryotic cell cycle
G0- postponing decision to divide
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Events of G1

• Cell prepares to divide
• During this phase cell reaches restriction point-
  committed to cell division

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S synthesis phase

• Chromosomes are duplicated
• Chromosomal copies are called chromatids
• Sister chromatids are joined via kinetochore
  protiens at a region called the centromere

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M phase

• Occurs after G2 (when cell accumulates products
  required for mitosis)
• Purpose equally distribute and sort chromosomes
  equally into 2 nuclei
• Sorting called mitosis
• Mitosis first observed by Walter Fleming in 1870s
• In salamander larval epithelial cells saw
  threads that divide and part, moving to
  separate daughter cells

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Mitosis
Compton Lab, Dartmouth College
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Mitotic stages

• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase

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Prophase

• Occurs after genetic material has been
  replicated, joined as sister chromatids
• Nuclear membrane breaks down into vesicles
• Chromosomal condensation
• Formation of the mitotic spindle
• Required for directing the movements of
  chromosomes during later stages of mitosis

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Microtubules

• Heterodimer of alpha and beta tubulin

Alberts, 2002
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Mitotic spindle
Microtubules (MTs) formed by polymerization of
tubulin
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Types of microtubules

• Aster
• Polymerize away from chromosomes
• Critical for positioning of spindle apparatus in
  the cell
• Polar
• Originate at the centrosome towards metaphase
  plate
• Kinetochore
• Make attachment with the kinetochore structure on
  the chromosome

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Prometaphase

• Nuclear membrane is absent
• Spindle fibers are interacting with sister
  chromatids
• Kinetochore microtubules attempt to capture
  kinetochore

Alberts, 2002
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Prometaphase

• Mitiotic spindle begins to form
• Sister chromatids are tugged back and forth
  between poles

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Metaphase

• Occurs when all pairs of sister chromatids are
  captured and align at the metaphase plate
• Cells are ready to be distributed into daughter
  cells

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Anaphase

• Sister chromatid associations are separated
• Each chromosome, moves toward pole it is linked
  to
• This movement is powered by shortening of
  kinetochore microtubules and the lengthening of
  polar microtubules, pushing against one another

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Telophase

• Chromosomes are located at poles of daughter
  cells
• Reformation of nuclear membrane in 2 distinct
  cells
• Cleavage furrow is formed, constricting to
  separate the cells during cytokinesis

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Results from mitosis

• 2 daughter cells, genetically identical (other
  than a rare, random mutation) to one another
• Process completed in somatic cell replication

Compton Laboratory, Dartmouth College