In 1892, while at the University of Tübingen, Correns began to experiment with trait inheritance in plants. On January 25, 1900, he published his first paper, “G. Mendel’s Law Concerning the Behavior of the Progeny of Racial Hybrids”, in which he restated Mendel’s results and his law of segregation and law of independent assortment. Although the paper cited both Charles Darwin and Mendel, Correns did not fully recognise the relevance of genetics to Darwin’s ideas.
In attempting to determine the extent to which Mendel’s laws are valid, Correns undertook a classic study on heredity in the four-o’clock plant (Mirabilis jalapa). The blotchy leaves of these variegated plants show patches of green and white tissue, but some branches carry only green leaves and others carry only white leaves. Whether a tissue is green or white depends on whether there are green or white chloroplasts in the cytoplasm of its cells. Flowers appear on all types of branches, and Correns performed a variety of crosses.
Two features of his results were surprising. First, unlike what Mendel had observed, Correns found that there was a difference between reciprocal crosses, that is, leaf color depended greatly on which parent (i.e., flower’s branch) had which trait. Such results are normally encountered only for sex-linked genes, but Correns’ results cannot be explained by sex linkage. Secondly, the phenotype of the maternal parent was solely responsible for determining the phenotype of all progeny, that is, the phenotype of the male parent appeared to be irrelevant, making no contribution to the progeny at all! Although the white progeny plants did not live long because they lacked chlorophyll, the other types of progeny did survive and could be used in further generations of crosses. The same patterns of maternal inheritance always appeared in these subsequent generations.
Maternal inheritance can be explained if the chloroplasts are somehow genetically autonomous and furthermore, are never transmitted via the sperm. This is reasonable since the chloroplasts come exclusively from the mother in most angiosperms. In his 1909 paper, Correns established variegated leaf color as the first conclusive example of cytoplasmic inheritance (cases in which certain characteristics of the progeny are determined by factors in the cytoplasm of the female sex cell), also known as extrachromosomal or non-Mendelian inheritance.
Unfortunately, most of Correns’ work went unpublished and was destroyed in the Berlin bombings of 1945.