Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 15.5 Some Inheritance Patterns Are Exceptions to Standard Mendelian Inheritance
Although most genetic inheritance follows Mendel’s laws, some patterns deviate due to special cases such as genomic imprinting and extranuclear (cytoplasmic) inheritance. These exceptions highlight the influence of parental origin and non-nuclear genes in heredity.
Genomic Imprinting
- In most traits, the allele’s origin (maternal or paternal) doesn’t affect its expression
- In genomic imprinting, the expression of an allele depends on the parent it was inherited from
- Imprinting involves the silencing or activation of certain genes during gamete formation
- Result: only one allele (either maternal or paternal) is expressed in the offspring
- This imprint is erased and reset in each generation according to the sex of the individual forming gametes
- In mammals, about 100 human genes and 120–180 mouse genes are known to be imprinted
Example: Igf2 Gene in Mice
- Igf2 is a growth factor gene
- Only the paternal allele is expressed; maternal allele is silenced
- Heterozygotes show different phenotypes depending on whether the mutant allele is inherited from the father (dwarf) or mother (normal)
- Mechanism: DNA methylation of cytosines affects gene expression by altering chromatin structure
- Imprinted genes often play roles in development, metabolism, temperature regulation, and may be linked to certain cancers
- Abnormal imprinting or having zero or two active copies of an imprinted gene can lead to developmental failure
Inheritance of Organelle Genes
- Not all genes are located in the nucleus—mitochondria and chloroplasts have their own DNA
- These extranuclear genes are inherited only from the mother in most animals and plants
- Mitochondria in sperm are usually destroyed in the egg after fertilization
Mitochondrial Diseases in Humans
- Caused by defects in mitochondrial genes (often in the electron transport chain or ATP synthase)
- Affect energy-demanding tissues: nervous system and muscles
- Examples:
- Mitochondrial myopathy: muscle weakness and fatigue
- Leber’s hereditary optic neuropathy: sudden vision loss in young adults
- Mitochondrial DNA mutations may also contribute to common conditions like diabetes, heart disease, Alzheimer’s, and aging
- Inheritance solutions: experimental “three-parent” embryos—nuclear DNA from one woman placed into a donor egg with healthy mitochondria
Chloroplast Gene Inheritance in Plants
- First discovered in 1909 by Carl Correns in variegated-leaf plants
- Leaf coloration patterns depend only on the maternal parent
- Caused by plastid DNA mutations affecting pigment production
- Offspring inherit plastids from the egg; sperm contributes little cytoplasm
- Leaf color patterns vary depending on the distribution of wild-type vs. mutant plastids in cells
In a Nutshell
Some genes are inherited in non-Mendelian ways. Genomic imprinting causes genes to be expressed differently depending on the parent of origin. Organelles like mitochondria and plastids carry their own DNA, inherited maternally and influencing traits and diseases outside the nucleus.