The inheritance of flower doubleness and nectary spur in Pelargonium x hortorum Bailey

Summary The formation of single flowers of 5 petals and 5 sepals is determined by the homozygous recessive state, dd, of the doubleness gene, D/d, which is epistatic to modifying genes determining flower type. In the presence of the dominant allele, i.e. genotypes DD or Dd, the flowers are semi-double or double. Owing to the D allele alone, the single frequency of 5 petals and 5 sepals is doubled to 10 petals and 10 sepals, of which up to 5 are petaloid, to give a semi-double flower. In addition, in the presence of the D allele, three modifying loci M1/m1, M2/m2, and M3/m3 are activated to give a series of distinct doubles with integral multiples of the basic perianth number. The homozygous recessive genes m1m1 and m2m2 both add an increment of 10 perianth parts, and m3m3 adds an increment of 20 perianth parts. In heterozygotes, M1m1, M2m2 and M3m3, the dominant alleles inhibit the incremental effect of their corresponding recessive alleles. The single flower cultivars investigated probably have the genotype dd, M1M1, M2M2, M3M3 and the semi-double cultivars the genotype Dd, M1m1, M2M2, M3M3.The single flowers have a nectariferous spur, characteristic of the genus, adnate to the pedicel. As the spur is absent from semi-double and double flowers, its presence is assumed to be either a pleiotropic effect of the single flower gene, or to be controlled by an unidentified gene tightly linked with it.