Non-Mendelian inheritance

If the genotypes of both parents in a genetic cross are known, Mendel's laws can be used to determine the distribution of phenotypes expected for the population of offspring.

Certain inherited diseases and their presentation display non-Mendelian patterns, complicating making predictions from family history.

Mendel's principle of segregation of genes applies too, as in the F2-generation homozygous individuals with the phenotypes of the P-generation[clarification needed] appear.

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.

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.

[citation needed] Genetic traits located on gonosomes sometimes show specific non-Mendelian inheritance patterns.

Individuals can develop a recessive trait in the phenotype dependent on their sex—for example, colour blindness and haemophilia (see gonosomal inheritances).

[9] While working with Mirabilis jalapa, Correns observed that leaf colour was dependent only on the genotype of the maternal parent.

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[10] ultimately led to the discovery of genetic material in the mitochondria, the mitochondrial DNA.

[citation needed] According to the endosymbiont theory, mitochondria and chloroplasts were once free-living organisms that were each taken up by a eukaryotic cell.

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.

[citation needed] It is the transmission of this organellar DNA that is responsible for the phenomenon of extranuclear inheritance.

Thus, the phenotype of traits linked to genes found in either chloroplasts or mitochondria are determined exclusively by the maternal parent.

For example, at least three genes are involved in making the reddish-brown pigment in the eyes of fruit flies.

The broad variety of skin colour in humans comes about partly because at least four different genes probably control this trait.

[13] 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.

[citation needed] One of the most well-studied examples of infectious heredity is the killer phenomenon exhibited in yeast.

[citation needed] Heritable traits that result from infection with foreign particles have also been identified in Drosophila.

[15] Although this process is usually associated with viruses, recent research has shown that the Wolbachia bacterium is also capable of inserting its genome into that of its host.

[citation needed] Genes are imprinted differently depending on the parental origin of the chromosome that contains them.

[20] Mutations that occur early on in development will affect a greater number of cells and can result in an individual that can be identified as a mosaic strictly based on phenotype.

Individuals with a number of repeats that falls in the premutation range have a good chance of having affected children.

Mirabilis jalapa
Carl Correns
Co-dominant expression of genes for plumage colours.
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 tomcat with that allele is born orange. This allele is epistatic over some other coat color genes. [ 5 ] [ 6 ]
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 they each have an allele for orange from the father and only homozygotes become orange.
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).