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Genetics 10201232
Faculty of Agriculture &
Veterinary Medicine
Instructor: Dr. Jihad Abdallah
Topic 10: Non-Mendelian inheritance
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• Many genes do not follow Mendelian inheritance
– linked genes do not follow Mendel’s law of
independent assortment
– Other non-Mendelian inheritance patterns:
• Maternal effect
• Epigenetic inheritance
• Extranuclear inheritance
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Maternal Effect
– Inheritance pattern for certain nuclear genes in
which the genotype of the mother directly
determines the phenotype of her offspring
the genotypes of the father and the offspring do
not affect the phenotype of offspring
– Explained by the accumulation of gene products
provided by the mother to her developing eggs
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The genotype of the mother determines the
phenotype of the offspring for maternal effect
genes
A. E. Boycott (1920s)
• Was the first to study an example of maternal effect
in water snail (Limnea peregra)
– Shell and internal organs can be either right-
handed (Dextral) or left-handed (sinistral)
• Determined by cleavage pattern
of egg after fertilization
– Dextral orientation is more common and dominant
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• Boycott began with two different true-breeding
strains
– One dextral, one sinistral
• Dextral ♀ x sinistral ♂ dextral offspring
• Reciprocal cross sinistral offspring
• Contradict a Mendelian pattern of inheritance
Dextral
female
Sinistral
male
Sinistral
female
Dextral
male
All dextral All sinistral
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• Oogenesis in female animals
– Oocyte is formed
– Nourished by surrounding diploid maternal nurse
cells
• Receives gene products from nurse cells
• Genotype of nurse cells determines gene products in
oocyte
Female gametes receive gene products from
the mother that affect early development
stages of the embryo
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Epigenetic Inheritance
– Modification which occurs to a nuclear gene or
chromosome that alters gene expression.
– Occurs during spermatogenesis, oogenesis, and
early stages of embryogenesis
– Gene expression is altered
• May be fixed during an individual’s lifetime
– Expression is not permanently changed over
multiple generations
• DNA sequence is not altered
• When the individual makes gametes, the genes may
become activated and remain operative in the offspring
which receives it.
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• Two types of epigenetic inheritance will be
discussed:
1. Dosage compensation
2. Genomic imprinting
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Dosage Compensation
– Males and females of many species have
different numbers of sex chromosomes
– But the level of expression of many genes
on sex chromosomes is similar in both
sexes
– In mammals, it is initiated during early stages
of development
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Murray Barr and Ewart Bertram (1949)
• Identified a highly condensed structure in
interphase nuclei of somatic cells of
female cats
– This structure was absent in male cats
– “Barr body”
– Later identified as a highly condensed X
chromosome
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• X chromosome inactivation
– DNA in inactivated X chromosomes becomes
highly compacted
• A Barr body is formed
– Most genes cannot be expressed
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• XX females 1 Barr body
• XY males 0 Barr bodies
• XO females 0 Barr bodies (Turner
syndrome)
• XXX females 2 Barr bodies (Triple X
syndrome)
• XXY males 1 Barr body (Kleinfelter
syndrome)
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Genomic Imprinting
• Occurs during gamete formation (before
fertilization)
• Involves a single gene or chromosome
• Governs whether offspring express the
maternally- or the paternally-derived gene
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Genomic Imprinting
• Genomic imprinting involves the physical
marking of a segment of DNA
– Mark is retained and recognized throughout
the life of the organism inheriting the marked
DNA
– Resulting phenotypes display non-Mendelian
inheritance patterns
– Offspring expresses one allele, not both
– “Monoallelic expression”
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• Genomic imprinting in mice
– The Igf-2 gene encodes an insulin-like growth
factor
• Functional allele required for normal size
• Igf-2m allele encodes a non-functional protein
– Imprinting results in the expression of the
paternal allele only
• Paternal allele is transcribed
• Maternal allele is not transcribed (transcriptionally
silent)
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• The Igf-2 gene encodes an insulin-like growth factor
• Functional allele required for normal size
• Igf-2m allele encodes a non-functional protein
– Igf-2m Igf-2m ♀ x Igf-2 Igf-2 ♂ Normal offspring
– Igf-2m Igf-2m ♂ x Igf-2 Igf-2 ♀ Dwarf offspring
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– The imprint of the Igf-2 gene is erased during
gametogenesis
– A new imprint is then established
• Oocytes possess an imprinted gene that is
silenced
• Sperm possess a gene that is not silenced
– The phenotypes of offspring are determined
by the paternally derived allele
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• Genomic imprinting
– Involves differentially methylated regions
(DMRs) located near imprinted genes
• Maternal or paternal copy is methylated,
not both
– Methylation generally inhibits expression
• Can enhance binding of transcription-
inhibiting proteins and/or inhibit binding of
transcription-enhancing proteins
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– Methylation occurs during gametogenesis
• Methylated in oocyte or sperm, not both
– Imprinting is maintained in the somatic cells
of the offspring
– Imprinting is erased during gametogenesis in
these offspring
• New imprinting established
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Extranuclear Inheritance
• Most genes are found in the nucleus
• Some genes are found outside of the
nucleus (mitochondria and chloroplasts)
– Resulting phenotypes display non-Mendelian
inheritance patterns
• “Extranuclear inheritance”
• “Cytoplasmic inheritance”
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• Mitochondria and chloroplasts possess
DNA
– Circular chromosomes resemble smaller
versions of bacterial chromosomes
– Located in the nucleoid region of the organelles
• Multiple nucleoids often present
• Each can contain multiple copies
of the chromosome
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• Mitochondrial genome size varies greatly
among different species
– 400-fold variation in mitochondrial
chromosome size
• Mitochondrial genomes of animals tend to be fairly
small
• Mitochondrial genomes of fungi, algae, and protists
tend to be intermediate in size
• Mitochondrial genomes of plants tend to be fairly
large
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• Human mitochondrial DNA is called mtDNA
– Circular chromosome 17,000 base pairs in
length
• Less than 1% of a typical bacterial chromosome
– Carries relatively few genes
• Genes encoding rRNA and tRNA
• 13 genes encoding proteins
functioning in ATP generation
via oxidative phosphorylation
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• Chloroplast genomes tend to be larger
than mitochondrial genomes
– Correspondingly greater number of genes
– ~100,000 – 200,000 bp in length
– Ten times larger than the mitochondrial
genome of animal cells
• The inheritance pattern of extranuclear
genetic material displays non-Mendelian
inheritance
– Mitochondria and plastids do not segregate
into gametes as do nuclear chromosomes
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• Pigmentation in Mirabilis jalapa
– The four-o’clock plant
– Pigmentation is determined by chloroplast
genes
• Green phenotype is the wild-type condition
– Green pigment is formed
• White phenotype is due to a mutation in a
chloroplast gene
– Synthesis of green pigment is diminished
• Cells containing both types of chloroplasts
“Heteroplasmy” display green coloration because
the normal chloroplasts produce the green pigment
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• Pigmentation in Mirabilis jalapa
– Pigmentation in the offspring depends solely
on the maternal parent
• “Maternal inheritance”
• Chloroplasts are inherited only through the
cytoplasm of the egg
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• Symbiosis involves a close relationship between two species where at least one member benefits
– Endosymbiosis involves such a relationship where one organism lives inside the other
• Mitochondria and chloroplasts were once free-living bacteria
– Engulfed and retained by early eukaryotes
(Endosymbiosis)
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