Carl Correns

Carl Erich Correns (September 10, 1864 - February 14, 1933) was a German botanist and geneticist, who is notable primarily for his independent discovery of the principles of heredity, and for his rediscovery of Gregor Mendel's earlier paper on that subject, which he achieved simultaneously but independent of the botanists Erich Tschermak von Seysenegg and Hugo de Vries.

By a quirk of history Correns was a student of Nageli, a renowned botanist with whom Mendel corresponded about his work with peas but who failed to understand how significant Mendel's work was. Tschermak was a grandson of a man who taught Mendel botany during his student days in Vienna.

Early life and education
Carl Correns was born September of 1864 in Munich. Orphaned at an early age, he was raised by an aunt in Switzerland. He entered the University of Munich in 1885. While there, he was encouraged to study botany by Karl Nägeli, a botanist whom Mendel corresponded with on the subject of his pea plant experiments. After completing his thesis, Correns became a tutor at the University of Tübingen and in 1913 he became the first director of the newly founded Kaiser-Wilhelm-Institut für Biologie in Berlin-Dahlem.

Key experiments and findings
Carl Correns conducted much of the foundational work for the field of genetics at the turn of the 19th century. He rediscovered and independently verified the work of Mendel in a separate model organism. He also discovered cytoplasmic inheritance, an important extension of Mendel's theories, which demonstrated the existence of extra-chromosomal factors on phenotype. Most of Correns' work went unpublished however, and was destroyed in the Berlin bombings of 1945.

Rediscovery of Mendel
In 1892, while at the University of Tübingen, Correns began to experiment with trait inheritance in plants. He focused mainly on the hawkweed plant experiments that Mendel carried out, not being aware of the pea plant results. Correns published his first paper on January 25, 1900, which cited both Charles Darwin and Mendel, though without fully recognising the relevance of genetics to Darwin's ideas. In Correns' paper, "G.Mendel's Law Concerning the Behavior of the Progeny of Racial Hybrids", he restated Mendel's results and his law of segregation and law of independent assortment.

Cytoplasmic inheritance
After rediscovering Mendel's laws of heredity, which apply to chromosomal inheritance, he undertook experiments with the four o'clock (Mirabilis jalapa) to investigate apparent counterexamples to Mendel's laws in the heredity of variegated (green and white mottled) leaf color. Correns found that, while Mendelian traits behave independently of the sex of the source parent, leaf color depended greatly on which parent had which trait. For instance, pollinating an ovule from a white branch with pollen from another white area resulted in white progeny, the predicted result for a recessive gene. Green pollen used on a green stigma resulted in all green progeny, the expected result for a dominant gene. However, if green pollen fertilized a white stigma, the progeny were white, but if the sexes of the donors were reversed (white pollen on a green stigma), the progeny were green.

This non-mendelian inheritance pattern was later traced to a gene named iojap which codes for a small protein required for proper assembly of the chloroplast ribosome. Even though iojap assorts according to Mendel's rules, if the mother is homozygous recessive, then the protein is not produced, the chloroplast ribosomes fail to form, and the plasmid becomes non-functional because the ribosomes cannot be imported into the organelle. The progeny could have functional copies of iojap, but since the chloroplasts come exclusively from the mother in most angiosperms, they would have been inactivated in the previous generation, and so will give white plants. Conversely, if a white father is paired with a green mother with functional chloroplasts, the progeny will only inherit functional chloroplasts, and will thus be green. In his 1909 paper, he established variegated leaf color as the first conclusive example of cytoplasmic inheritance.