1. Some inheritance patterns are exceptions to standard Mendelian inheritance
Chapter 15, Section 5

2. Normal Igf2 allele is expressed.
Genomic Imprinting
Paternal chromosome
Maternal chromosome
Normal-sized mouse (wild type)
Normal Igf2 allele is not expressed.
For a few mammalian traits (2-3 dozen), the phenotype depends on which parent passed along the alleles for those traits.
Such variation in phenotype is called genomic imprinting.
Genomic imprinting occurs during gamete formation and results in the silencing of a particular allele of certain genes.
Genes are directly imprinted in sperm and eggs, so a zygote expressed only one allele of an imprinted allele.
Imprints are transmitted to all body cells during development.
In a given species, the imprinted genes are always imprinted in the same way.
(a) Homozygote
Mutant Igf2 allele inherited from mother
Mutant Igf2 allele inherited from father
Normal-sized mouse (wild type)
Dwarf mouse (mutant)
Normal Igf2 allele is expressed.
Mutant Igf2 allele is expressed.
Mutant Igf2 allele is not expressed.
Normal Igf2 allele is not expressed.
(b) Heterozygotes

3. Genomic Imprinting cont.
It appears that imprinting is the result of the methylation (addition of —CH3) of cysteine nucleotides.
Such methylation may silence the allele; however, for a few genes, methylation has been shown to activate expression of the allele.
Depending on the original function of the condensed DNA in regulating allele expression, the result is either silencing or activation of a given allele.
Genomic imprinting is thought to affect only a small fraction of mammalian genes, but most imprinted genes are critical for embryonic development.

4. Inheritance of Organelle Genes
Extranuclear genes (or cytoplasmic genes) are found in organelles in the cytoplasm.
Mitochondria, chloroplasts, and other plant plastids carry small circular DNA molecules.
Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg.
The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant.
Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systems.
For example, mitochondrial myopathy and Leber’s hereditary optic neuropathy.