The laying of brown-shelled eggs in the Marans is due to the inescapable presence of hereditary genes that are very complex and, to date, undiscovered (“the genetic linkage” of this characteristic is therefore unknown).
The complexity of the inheritance of extra-russet-red egg colour would result from the presence of several genes and not of only one, of which some genes would be dominant whereas others would be recessive.
Given these conditions, the heredity of dark brown eggshells is automatically reproduced only if the various genes responsible for the brown shells are joined together in the genotype of the subject considered.
We are thus well in the presence of a breed characteristic for which the laws of its genetics still hold many secrets.
We can note in addition that the genes causing “creamy-white” eggs (these even not decoded to date and perhaps quite as complex?) would be rather dominant compared to those causing “brown eggs.”
Moreover, if the report which is made of a greater effectiveness in the transmission of the genes for dark eggs by cocks more than by hens is proved to be a reality with concrete cases in the Marans, then it would be possible to imagine that at least one of the unknown genes causing extra-russet-red eggs would be “sex-linked”, i.e. present in the double state in the cock.
Consequently, in ameliorative crossings for extra-russet-red eggs, when a purebred cock is mated to a non-purebred hen, the cock would visibly transmit his dark-egg genes to all his descendants, while a purebred hen, if mated to a non-purebred cock, would have visible evidence of the dark-egg genes in only with half of her descendants.
This could explain the impression of a greater effectiveness of the cocks to transmit the characteristic of dark eggs in the first generation.
Attention must be paid as this situation is indeed misleading; it describes only one coupling of subjects which one can suppose non-purebred, at least to some extent, for genes related to the characteristic of “brown eggs.” With time, the recessive brown egg genes will reappear.