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 Gregor Mendel's discovery

Before Mendel
(according to Francis Galton, 1822 - 1911):

Mendel's principle of inheritance (1865):

combination of autosomal genes

1916, X/Y - chromosomal inheritance was discovered
(Th. H. Morgan / Calvin B. Bridges)

Sex is determined by the male sperm cell. The Mendelian laws are modified in respect to the inheritance of the sex chromosomes (X,Y) by this particular male position.

Because of the limitations set by the gender, there are only 4 possible combinations of X/Y chromosomes : No sons carrying the paternal X1, and no daughters carrying the paternal Y0 !

Pedigrees drawn with PED 4.2