Non-Mendelian Inheritance

Introduction to Non-Mendelian Inheritance

Non-Mendelian inheritance refers to genetic patterns that do not conform to the classic rules established by Gregor Mendel. These patterns involve exceptions to Mendel's laws of segregation and independent assortment, and often include influences from cytoplasmic genes, epigenetic modifications, or parental origin effects.

• Mendelian Rules:

  1. Offspring traits are directly determined by their own genotype.

  2. Genes are passed unaltered except for rare mutations.

  3. Genes obey the law of segregation.

  4. Genes obey the law of independent assortment.

• Non-Mendelian Patterns:

  • Maternal effect (breaks rule 1): Offspring phenotype determined by mother's genotype.

  • Epigenetic inheritance (breaks rule 2): Gene expression altered by modifications (e.g., methylation), not DNA sequence.

  • Extranuclear inheritance (breaks rule 3): Genes in mitochondria/chloroplasts, not nucleus.

  • Linkage (breaks rule 4): Genes close together on the same chromosome are inherited together (covered in Ch. 6).

Maternal Effect

Definition and Mechanism

The maternal effect is an inheritance pattern where the genotype of the mother directly determines the phenotype of her offspring, regardless of the offspring's own genotype. This is due to gene products (mRNA or proteins) deposited in the egg by maternal nurse cells during oogenesis.

• Example: Shell coiling in snails (Limnaea peregra), where dextral (right-handed) coiling is dominant over sinistral (left-handed).

• Key Point: The offspring's phenotype matches the mother's genotype, not its own.

Snail Coiling Inheritance Pattern

• Reciprocal crosses between DD (dextral) and dd (sinistral) females produce F1 offspring with the same genotype (Dd), but their phenotype matches the mother's genotype.

• F2 generation: All dextral if mother is Dd, regardless of offspring genotype.

• F3 generation: 3:1 dextral:sinistral ratio appears, delayed by one generation.

Cellular Mechanism

• Nurse cells (maternal, diploid) surround the oocyte and transfer gene products into the egg.

• Eggs from DD or Dd mothers receive D gene products, resulting in dextral offspring.

• Eggs from dd mothers receive only d gene products, resulting in sinistral offspring.

• The orientation of the first mitotic spindle in the embryo determines the adult shell coiling direction.

Epigenetic Inheritance

Definition and Mechanisms

Epigenetic inheritance involves heritable changes in gene expression that do not alter the DNA sequence. These changes are reversible and often involve DNA methylation or chromatin modification.

• Occurs during gametogenesis or early embryonic development.

• Key phenomena: Dosage compensation and genomic imprinting.

Dosage Compensation

Dosage compensation equalizes the expression of sex chromosome genes between males and females. Mechanisms vary by species:

• In birds (ZZ male, ZW female), dosage compensation is incomplete; many Z-linked genes are not compensated.

X-Chromosome Inactivation (XCI) in Mammals

• One X chromosome in female somatic cells is condensed into a Barr body and inactivated (Lyon hypothesis).

• Random inactivation leads to mosaic phenotypes (e.g., calico cats, variegated mice).

• Inactivation is permanent in somatic cells and passed to daughter cells during mitosis.

• Experimental evidence: Heterozygous females for G-6-PD enzyme show clones expressing only one allele, confirming XCI.

Molecular Mechanism of XCI

• X-inactivation center (Xic): Region on X chromosome essential for inactivation.

• Xist gene: Produces RNA that coats the X chromosome, initiating inactivation.

• Stages: Nucleation (choice and initiation), spreading (Xist RNA coats chromosome), maintenance (inactivation preserved in all daughter cells).

• Some genes (pseudoautosomal) escape inactivation.

Genomic Imprinting

Definition and Example

Genomic imprinting is an epigenetic phenomenon where a gene is marked (imprinted) in a parent-of-origin-specific manner, resulting in monoallelic expression in the offspring.

• Example: Igf2 gene in mice. Only the paternal allele is expressed; the maternal allele is silenced.

• Reciprocal crosses with the same genotype can yield different phenotypes depending on which parent transmits the functional allele.

Stages of Imprinting

• Establishment: Imprint is set during gametogenesis (sperm or egg formation).

• Maintenance: Imprint is preserved during embryogenesis and in adult somatic cells.

• Erasure and Reestablishment: Imprint is erased and reset in the germ cells of the next generation.

Molecular Mechanism: DNA Methylation

• Imprinting involves methylation of an imprinting control region (ICR) near the gene.

• Methylation typically inhibits transcription.

• Pattern of methylation is sex-specific and reestablished each generation.

Imprinting and Human Disease

• Prader-Willi syndrome (PWS): Deletion on paternal chromosome 15; maternal copy is imprinted (silenced).

• Angelman syndrome (AS): Deletion on maternal chromosome 15; paternal copy is imprinted (silenced).

• Different genes are responsible for each syndrome within the same chromosomal region.

Extranuclear (Cytoplasmic) Inheritance

Definition and Organelles Involved

Extranuclear inheritance involves genes located outside the nucleus, primarily in mitochondria and chloroplasts. These organelles contain their own circular DNA and are inherited through the cytoplasm.

• Also called cytoplasmic inheritance.

• Organellar genomes are located in nucleoids within the organelle.

Genetic Composition of Organelles

Mitochondrial and Chloroplast Genomes

• Mitochondrial DNA (mtDNA): Small, circular; human mtDNA is 17,000 bp, encodes 13 polypeptides, rRNAs, and tRNAs.

• Chloroplast DNA (cpDNA): Larger; tobacco cpDNA is 156,000 bp, encodes 110-120 genes for photosynthesis and gene expression.

Maternal Inheritance in Plants

• Example: Four o'clock plant (Mirabilis jalapa) leaf color is determined solely by the maternal parent.

• Green, white, or variegated leaves result from the type of chloroplasts inherited from the egg.

• Heteroplasmy: Presence of both normal and mutant chloroplasts in a cell leads to variegated phenotype.

Patterns of Organelle Inheritance

Mechanisms of Maternal Mitochondrial Inheritance

• Lack of entry: Sperm mitochondria do not enter the egg (e.g., Chinese hamster).

• Destruction before fertilization: Sperm mitochondrial DNA destroyed by endonuclease (e.g., Drosophila).

• Destruction after fertilization: Paternal mitochondria targeted for destruction by ubiquitin (e.g., most mammals).

Human Mitochondrial Diseases

• Transmitted maternally via the egg cytoplasm.

• Can also arise from somatic mutations accumulating with age.

• Mitochondrial DNA is prone to damage due to high oxygen consumption and limited repair mechanisms.

• Heteroplasmy: Cells may contain both normal and mutant mitochondria; disease manifests when mutant mitochondria exceed a threshold proportion.

Three Parent Babies

• Technique to prevent transmission of mitochondrial diseases.

• Nuclear DNA from mother with mitochondrial mutation is transferred to a donor egg with healthy mitochondria, then fertilized by father's sperm.

• Resulting child has nuclear DNA from both parents and mitochondrial DNA from donor.

Endosymbiosis Theory

Origin of Mitochondria and Chloroplasts

The endosymbiosis theory proposes that mitochondria and chloroplasts originated from free-living bacteria that were engulfed by ancestral eukaryotic cells.

• Chloroplasts: Originated from cyanobacteria.

• Mitochondria: Originated from Gram-negative nonsulfur purple bacteria.

• Evidence: Organelles have circular chromosomes, gene sequences similar to bacteria, and many original genes have been lost or transferred to the nucleus.

• Significance: Acquisition of mitochondria enabled efficient ATP synthesis; chloroplasts enabled photosynthesis.

Summary Table: Key Non-Mendelian Inheritance Mechanisms

Additional info: Some explanations and tables have been expanded for clarity and completeness based on standard genetics knowledge.