The present invention relates generally to genetic sequences and their complementary forms capable of facilitating the modification of a phenotype of a plant. More particularly, the present invention relates to polynucleotide sequences defining B-type genes, to the proteins encoded thereby, to methods for isolating such polynucleotides and to nucleic acid constructs for the expression thereof. The present invention further provides cells, particularly transformed bacterial or plant cells and to differentiated tissue including whole plants and their progeny comprising cells which contain these nucleic acid constructs or parts of the constructs. Plants and parts of plants, such as flowering and reproductive parts including seeds, also form part of the present invention. The genetic sequences of the present invention may be used inter alia for the production of plants and, in particular, oil palm plants, which have modified phenotypes and/or which exhibits more highly desired characteristics such as, for example, male sterility or plants in which the sex ratio may be manipulated, and for the diagnosis and, preferably, elimination of the mantled phenotype.