Doubled haploids
Both the pollen and the egg are haploid. Sometimes, immature pollen can be induced to grow into a haploid plant. The egg cell can sometimes be induced to differentiate into a haploid embryo by pollinating with a non-compatible species (eg pollinating barley with maize).
Haploid plants are convertible into diploid ones by treating with the drug colchicine. In a doubled haploid, every gene is homozygous.