Non-Mendelian inheritance

Non-Mendelian inheritance describes any pattern of inheritance where traits segregate in a manner that is not in accordance with Mendel's laws: i.e. the segregation ratio of the offspring significantly deviates from the expected Mendelian ratio. Although all inheritance in fungi, bacteria and viruses is non-Mendelian, the term generally applies to instances in eukaryotic reproduction where Mendelian inheritance is typically expected.

Types of non-Mendelian inheritance include:

  • Extranuclear inheritance, such as mitochondrial inheritance, may only occur down the maternal line because the cytoplasm of the maternal gamete carries these organelles. Mitochondrial diseases (commonly of the muscles and eye) can thus only be passed down the maternal line.
  • Gene conversion, which is a specific type of recombination, unlike chromosomal crossover during meiosis, where the DNA sequence of part of one chromosome is transferred to another, without a change in the sequence of the original chromosome.

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  • Infectious heredity, such as the inheritance of viral DNA in the cytoplasm, that is inherited only when a parent is infected and only down the line of that parent
  • Genomic imprinting, which is the epigenetic marking of DNA on chromosomes, according to whether they are maternal or paternal, that alters their level of gene expression
  • Trinucleotide repeat disorders, where dynamic mutation occurs: the greater the number of repeats, the greater the likelihood of further mutation. Individuals with sufficient repeats find themselves in the pre-mutation range, and may have affected offspring even though they are not affected themselves.