page_title: Mendelian inheritance
section_title: Law of Dominance and Uniformity
hierarchical_section_title: Mendelian inheritance / Mendel's genetic discoveries / Law of Dominance and Uniformity
caption_attribution_description: English: Crossing of two different homozygous individuals and their first and second progeny generation in dominant recessive inheritance
context_page_description: Mendelian inheritance is a type of biological inheritance that follows the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 and popularised by William Bateson. These principles were initially controversial. When Mendel's theories were integrated with the Boveri–Sutton chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics. Ronald Fisher combined these ideas with the theory of natural selection in his 1930 book The Genetical Theory of Natural Selection, putting evolution onto a mathematical footing and forming the basis for population genetics within the modern evolutionary synthesis.
context_section_description: If two parents are mated with each other who differ in one genetic characteristic for which they are both homozygous (each pure-bred), all offspring in the first generation (F₁) are equal to the examined characteristic in genotype and phenotype showing the dominant trait. This uniformity rule or reciprocity rule applies to all individuals of the F₁-generation.
  The principle of dominant inheritance discovered by Mendel states that in a heterozygote the dominant allele will cause the recessive allele to be "masked": that is, not expressed in the phenotype. Only if an individual is homozygous with respect to the recessive allele will the recessive trait be expressed. Therefore, a cross between a homozygous dominant and a homozygous recessive organism yields a heterozygous organism whose phenotype displays only the dominant trait.
  The F₁ offspring of Mendel's pea crosses always looked like one of the two parental varieties. In this situation of "complete dominance," the dominant allele had the same phenotypic effect whether present in one or two copies.
  But for some characteristics, the F₁ hybrids have an appearance in between the phenotypes of the two parental varieties. A cross between two four o'clock (Mirabilis jalapa) plants shows an exception to Mendel's principle, called incomplete dominance. Flowers of heterozygous plants have a phenotype somewhere between the two homozygous genotypes. In cases of intermediate inheritance (incomplete dominance) in the F₁-generation Mendel's principle of uniformity in genotype and phenotype applies as well. Research about intermediate inheritance was done by other scientists. The first was Carl Correns with his studies about Mirabilis jalapa.
caption: F1 generation: All individuals have the same genotype and same phenotype expressing the dominant trait (red).
  F2 generation: The phenotypes in the second generation show a 3 : 1 ratio.
  In the genotype 25 % are homozygous with the dominant trait, 50 % are heterozygous  genetic carriers of the recessive trait, 25 % are homozygous with the recessive genetic trait and expressing the recessive charakter.
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