11-Non-Mendelian Inheritance
BIOL 1113 Organisms and their Environment
Non-Mendelian Inheritance
Source: Rooker et al, Chapter 18
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Overview of Inheritance
Mendelian Inheritance
Genes are transmitted unchanged across cell divisions and generations, barring rare mutations.
Obey Mendel’s laws of segregation: Each gamete receives one of the two alleles.
For multiple-gene crosses, they follow Mendel’s law of independent assortment, allowing predictions about phenotype.
Non-Mendelian Inheritance
Epigenetics: Changes in gene expression passed to offspring without altering the gene sequence.
Examples: Genomic imprinting, X-chromosome inactivation, environmental signals.
Genes outside the nucleus do not follow Mendel's second law (e.g., mitochondria, chloroplasts).
Some gene pairs are linked on the same chromosome, thus assort together.
Examples: Linkage and recombination.
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Epigenetics Defined
Study of changes in gene expression inherited cell-to-cell or across generations that do not involve a gene sequence change.
Examples of epigenetic inheritance in species: Yeast, honey bees, Darwin’s finches, Toadflax/Butter and Eggs.
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Epigenetic Mechanisms
DNA Methylation: Addition of methyl groups to nucleotides, affecting gene expression.
Histone Modifications: Modifications to histones that determine DNA condensation and accessibility.
RNA Interference: Regulatory mechanism impacting gene expression.
Functions: Gene silencing, heritable changes, reversible.
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DNA Methylation
Methylation most often silences gene expression; acetylation usually enhances expression.
Can be heritable through mitosis and sometimes meiosis.
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Histone Modifications
Histones condense DNA within the nucleus.
Methylation leads to gene silencing; acetylation promotes gene activation.
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Honey Bees and Epigenetics
Queens and workers differentiated by diets and DNA methylation patterns.
Queens: Fed royal jelly, inhibiting DNA methylation; enhanced growth and larger size.
Workers: Fed pollen; high DNA methylation levels, resulting in smaller size.
Over 550 genes differently methylated between workers and queens.
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Cycloidea Gene and Toadflax
The cycloidea gene influences flower symmetry.
Peloria mutants exhibit significant methylation leading to phenotypic changes.
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Genomic Imprinting
Changes in gene expression due to epigenetic marks from either the mother or father.
Offspring expresses either the maternal or paternal gene copy, but not both.
Approximately 150-1000 imprinted genes in mammals.
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Example of Imprinting: Igf2
Insulin-like growth factor-2 required for growth.
Only paternal copy expressed in mice; absence leads to dwarfism.
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X-Chromosome Inactivation
Random inactivation of one X chromosome in females during embryonic development.
Ensures equal expression of X-linked genes in XX and XY individuals.
Barr body visible in some cells as a sign of inactivation.
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X Inactivation Patterns
Randomization occurs in embryonic cells but not in placental cells, where maternal X is expressed, preventing immune rejection.
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Agouti Gene in Mice
Agouti gene influences coat color determined by DNA methylation patterns.
Offspring coat color varies with parental diets rich in B vitamins influencing methylation levels.
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Environmental Effects and Epigenetics: Bisphenol A
BPA disrupts methylation patterns during fetal development, affecting gene expression in varying tissues and sexes.
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Project Ice Storm Findings
Study on stress effects on epigenetics in offspring from mothers pregnant during the Quebec ice storm.
Differential DNA methylation found on 1675 sites across 22 chromosomes.
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Extranuclear Inheritance: Organelles
Chloroplasts and mitochondria, with distinct genomes, highlight examples of non-Mendelian inheritance.
They rely on nuclear genes for certain proteins.
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Maternal Inheritance in Chloroplasts
Example in Four o'clock plants: Leaf color determined by maternal chloroplast origin.
Mutation affects pigment synthesis, reflecting maternal genetic contribution.
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Sea Slug and Chloroplasts
Elysia chlorotica incorporates algal chloroplasts, enabling photosynthesis without reliance on its nuclear genes.
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Mitochondrial Inheritance
Some mitochondrial diseases demonstrate maternal inheritance patterns, such as Leber’s hereditary optic neuropathy and myoclonic epilepsy.
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Three Parent Babies
New reproductive techniques involving one nucleus from one mom and mitochondria from another to prevent mitochondrial diseases.
Issues regarding the interaction between nuclear DNA and mitochondrial DNA remain.
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Genetic Mapping and Linkage
Linkage concerns genes close together assorting together, violating the principle of independent assortment due to strong recombination tendencies.
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Drosophila melanogaster Studies
Thomas Hunt Morgan’s experiments challenged Mendelian genetics, indicating genes on the same chromosome tend to be inherited together.