Incomplete dominance (partial or mosaic dominance) given by Correns 1903 is the phenomenon, where none of the two contrasting alleles or factors is dominant. The expression of the character in a hybrid (F1) individual is intermediate or a fine mixture of the expression of the two factors. eg, Mirabilis jalapa (four O′ clock plant) and Antirrhinum majus (dog flower or snapdragon). There are 2 types of flower colour in pure state red and white. When the two types of plants are crossed, the hybrid or plants of F1 generation have pink flower. If the later are selfed, the plants of F2 generation are of three types-red, pink and white in ratio of 1:2:1.
Pisum sativum obeys law of dominance, in which both contrasting alleles (recessive and dominant) are present and able to express dominant one at F1 hybrid.
In F1 generation, offsprings are tall though they have received both the factors ( T and t ). It is because of the dominant nature of factor for tall and recessive nature of factor for dwarf. On selfing, the recessive trait reappears in the F2 generation showing that it is suppressed in F1 generation and not lost.
Neurospora crassa was used by Beadle and Tatum for studying mutation. They selected this fungus as it can be readily grown in the laboratory on a defined medium (a medium that contains only known substances, rather than some uncharacterised cell extract). They then exposed its spores to X-ray and allowed the progeny to grow on a complete medium (a medium that contains all necessary nutrients and would, therefore, supplies these nutrients to growing fungi).
