Incomplete dominance In cases of intermediate inheritance due to incomplete dominance, the principle of dominance discovered by Mendel does not apply. Nevertheless, the principle of uniformity works, as all offspring in the F1-generation have the same genotype and same phenotype. Mendel's principle of segregation of genes applies too, as in the F2-generation homozygous individuals with the phenotypes of the P-generation appear. Intermediate inheritance was first examined by Carl Correns in Mirabilis jalapa he used for further genetic experiments. Antirrhinum majus also shows intermediate inheritance of the pigmentation of the blossoms. Co-dominance Co-dominante expression of genes for plumage colours. In cases of co-dominance, the genetic traits of both different alleles of the same gene-locus are clearly expressed in the phenotype. For example, in certain varieties of chicken, the allele for black feathers is co-dominant with the allele for white feathers. Heterozygous chickens have a colour described as "erminette", speckled with black and white feathers appearing separately. Many human genes, including one for a protein that controls cholesterol levels in the blood, show co-dominance too. People with the heterozygous form of this gene produce two different forms of the protein, each with a different effect on cholesterol levels. Genetic linkage When genes are located on the same chromosome and no crossing over took place before the segregation of the chromosomes into the gametes, the genetic traits will be inherited in connection, because of the genetic linkage. These cases constitute an exception to the<unk>  rule of independent assortment. Multiple alleles In<unk>  inheritance, genes have only two alleles, such as a and A. Mendel consciously chose pairs of genetic traits, represented by two alleles for his inheritance experiments. In nature, such genes often exist in several different forms and are therefore said to have multiple alleles. An individual, of course, usually has only two copies of each gene, but many different alleles are often found within a population. A rabbit's coat color is determined by a single gene that has at least four different alleles. They display a pattern of a dominance-hierarchy that can produce four coat colors. In the genes for the dog coat colours there are four alleles on the Agouti-locus. The allele "aw" is dominant over the alleles "at" and "a" but recessive under "Ay". Many other genes have multiple alleles, including the human genes for ABO blood type. Epistasis In the genepool of cats (Felis catus) there is a recessive allele for orange coat on the X-Chromosome. In a male the Y-Chromosome cannot compensate this, so a hemizygous tomcat is born orange. This allele is epistatic over some other coat color genes. A heterozygous cat with kittens from an orange tomcat: 50 % are orange, 50 % can produce eumelanin. Here the segregation of her two alleles, one dominant for the ability to produce eumelanin, one recessive for orange, was crucial for the colour of the kittens. With the young males it is decisive which of the two X-Chromosomes they received from the mother, because the Y-Chromosome does not contain a corresponding allele from the father. In the young females it is also decisive which X-Chromosome they got from the mother, because the allele for orange is recessive, so that only homozygotes become orange. If one or more genes cannot be expressed because of another genetic factor hindering their expression, this epistasis can make it impossible even for dominant alleles on certain other gene-loci to have an effect on the phenotype. An example in dog coat genetics is the homozygosity with the allele "e e" on the Extension-locus making it impossible to produce any other pigment than pheomelanin. Although the allele "e" is a recessive allele on the extension-locus itself, the presence of two copies leverages the dominance of other coat colour genes. Domestic cats have a gene with a similar effect on the X-chromosome. Sex-linked inheritance Genetic traits located on gonosomes sometimes show specific<unk>  patterns. Individuals can develop a recessive trait in the phenotype dependent on their sex—for example, colour blindness and haemophilia (see gonosomal inheritances). As many of the alleles are dominant or recessive, a true understanding of the principles of<unk>  inheritance is an important requirement to also understand the more complicated inheritance patterns of sex-linked inheritances. Extranuclear inheritance Example of a pedigree for a genetic trait inherited by mitochondrial DNA in animals and humans. Offspring of the males with the trait don't inherit the trait. Offspring of the females with the trait always inherit the trait (independently from their own sex). Extranuclear inheritance (also known as cytoplasmic inheritance) is a form of<unk>  also first discovered by Carl Correns in 1908. While working with Mirabilis jalapa, Correns observed that leaf colour was dependent only on the genotype of the maternal parent. Based on these data, he determined that the trait was transmitted through a character present in the cytoplasm of the ovule. Later research by Ruth Sager and others identified DNA present in chloroplasts as being responsible for the unusual inheritance pattern observed. Work on the poky strain of the mould Neurospora crassa begun by Mary and Hershel Mitchell ultimately led to the discovery of genetic material in the mitochondria, the mitochondrial DNA. According to the endosymbiont theory, mitochondria and chloroplasts were once free-living organisms that were each taken up by a eukaryotic cell. Over time, mitochondria and chloroplasts formed a symbiotic relationship with their eukaryotic hosts. Although the transfer of a number of genes from these organelles to the nucleus prevents them from living independently, each still possesses genetic material in the form of double stranded DNA. It is the transmission of this organellar DNA that is responsible for the phenomenon of extranuclear inheritance. Both chloroplasts and mitochondria are present in the cytoplasm of maternal gametes only. Paternal gametes (sperm for example) do not have cytoplasmic mitochondria. Thus, the phenotype of traits linked to genes found in either chloroplasts or mitochondria are determined exclusively by the maternal parent. In humans, mitochondrial diseases are a class of diseases, many of which affect the muscles and the eye. Polygenic traits Many traits are produced by the interaction of several genes. Traits controlled by two or more genes are said to be polygenic traits. Polygenic means "many genes" are necessary for the organism to develop the trait. For example, at least three genes are involved in making the reddish-brown pigment in the eyes of fruit flies. Polygenic traits often show a wide range of phenotypes. The broad variety of skin colour in humans comes about partly because at least four different genes probably control this trait.<unk> -random segregation<unk> -random segregation of chromosomes is a deviation from the usual distribution of chromosomes during meiosis and in some cases of mitosis. Gene conversion Gene conversion can be one of the major forms of<unk> . Gene conversion arises during DNA repair via DNA recombination, by which a piece of DNA sequence information is transferred from one DNA helix (which remains unchanged) to another DNA helix, whose sequence is altered. This may occur as a mismatch repair between the strands of DNA which are derived from different parents. Thus the mismatch repair can convert one allele into the other. This phenomenon can be detected through the offspring<unk><unk>  ratios, and is frequently observed, e.g., in fungal crosses. Infectious heredity Another form of<unk>  is known as infectious heredity. Infectious particles such as viruses may infect host cells and continue to reside in the cytoplasm of these cells. If the presence of these particles results in an altered phenotype, then this phenotype may be subsequently transmitted to progeny. Because this phenotype is dependent only on the presence of the invader in the host cell's cytoplasm, inheritance will be determined only by the infected status of the maternal parent. This will result in a uniparental transmission of the trait, just as in extranuclear inheritance. One of the most well-studied examples of infectious heredity is the killer phenomenon exhibited in yeast. Two double-stranded RNA viruses, designated L and M, are responsible for this phenotype. The L virus codes for the capsid proteins of both viruses, as well as an RNA polymerase. Thus the M virus can only infect cells already harbouring L virus particles. The M viral RNA encodes a toxin that is secreted from the host cell. It kills susceptible cells growing in close proximity to the host. The M viral RNA also renders the host cell immune to the lethal effects of the toxin. For a cell to be susceptible it must therefore be either uninfected or harbour only the L virus. The L and M viruses are not capable of exiting their host cell through conventional means. They can only transfer from cell to cell when their host undergoes mating. All progeny of a mating involving a doubly infected yeast cell will also be infected with the L and M viruses. Therefore, the killer phenotype will be passed down to all progeny. Heritable traits that result from infection with foreign particles have also been identified
Non-Mendelian inheritance
Structural inheritance
Heteroplasmy
Allelic heterogeneity
Genetic heterogeneity
Variegation 
Germline mosaicism
Expressivity 