Non-Mendelian inheritance

Non-Mendelian inheritance is any pattern in which traits do not segregate in accordance with Mendel's laws. These laws describe the inheritance of traits linked to single genes on chromosomes in the nucleus. In Mendelian inheritance, each parent contributes one of two possible alleles for a trait. 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. There are several situations in which the proportions of phenotypes observed in the progeny do not match the predicted values. Non-Mendelian inheritance plays a role in several disease which affected the processes. 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 used for further genetic experiments. Antirrhinum majus also shows intermediate inheritance of the pigmentation of the blossoms. 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.