Editing Mendelian inheritance (section)

==={{anchor|Law of Dominance and Uniformity}}Law of Dominance and Uniformity===
[[File:Dominant-recessive inheritance P - F1 - F2.png|thumb|F<sub>1</sub> generation: All individuals have the same genotype and same phenotype expressing the dominant trait (<span style="color:#990000;">red</span>).<br />F<sub>2</sub> generation: The phenotypes in the second generation show a 3 : 1 ratio.<br />In the genotype 25 % are homozygous with the dominant trait, 50 % are heterozygous  [[genetic carrier]]s of the recessive trait, 25 % are homozygous with the recessive genetic trait and [[Gene expression|expressing]] the recessive character.]]
[[File:Intermediate inheritance P - F1 - F2.png|thumb|In [[Mirabilis jalapa]] and [[Antirrhinum majus]] are examples for intermediate inheritance.<ref name="Mendelian Genetics">Biology University of Hamburg: ''[http://www1.biologie.uni-hamburg.de/b-online/e08/08a.htm Mendelian Genetics]''</ref><ref>[[Neil A. Campbell]], [[Jane B. Reece]]: Biologie. Spektrum-Verlag Heidelberg-Berlin 2003, {{ISBN|3-8274-1352-4}}, page 302–303.</ref> As seen in the F<sub>1</sub>-generation, heterozygous plants have "<span style="color:magenta;">light pink</span>" flowers—a mix of "<span style="color:#990000;">red</span>" and "white". The F<sub>2</sub>-generation shows a 1:2:1 ratio of <span style="color:#990000;">red</span>: <span style="color:magenta;">light pink</span>: <span style="color:black;">white.</span>]]
If two parents are mated with each other who differ in one [[genetic trait|genetic characteristic]] for which they are both [[homozygous]] (each pure-bred), all offspring in the first generation (F<sub>1</sub>) 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 F<sub>1</sub>-generation.<ref>Ulrich Weber: Biologie Gesamtband Oberstufe, 1st edition, Cornelsen Verlag Berlin 2001, {{ISBN|3-464-04279-0}}, page 170 - 171.</ref>

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 F<sub>1</sub> 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 F<sub>1</sub> 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 F<sub>1</sub>-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.<ref name="Mendelian Genetics"/><ref>Biologie Schule - kompaktes Wissen: [http://www.biologie-schule.de/uniformitaetsregel.php Uniformitätsregel (1. Mendelsche Regel)]</ref><ref>Frustfrei Lernen: [https://www.frustfrei-lernen.de/biologie/uniformitaetsregel.html Uniformitätsregel (1. Mendelsche Regel)]</ref><ref>Spektrum Biologie: ''[http://www.spektrum.de/lexikon/biologie/unvollstaendige-dominanz/68637 Unvollständige Dominanz]''</ref><ref>Spektrum Biologie: ''[https://www.spektrum.de/lexikon/biologie/intermediaerer-erbgang/34304 Intermediärer Erbgang]''</ref>