Mutation can also take place during seed formation but only with very low frequency. Although meristem culture is only an artificial way of vegetive propagation there are a few differences. In meristem culture the neat order of layers is easily disturbed. This leads to chimaeral rearrangements with much higher frequency than in a garden. Also the mutation frequency might be higher as the whole metabolism can be upset, but usually it will be a rare event.
Yellow plants are, as far as is known, nearly always due to a nuclear mutation.
White sectors can, in my opinion, be due to change both in the DNA of the nucleus and the chloroplast. A yellow plant is a heterozygote with one gene for yellow and one for green whereby the yellow is dominant. This dominance is peculiar and it seems that the mutant protein upsets the organization in a chloroplast giving a yellow color to it. Recombination in a yellow plant can result in a plant with a green or a white edge. In a white-edged plant the gene for green has been exchanged for a gene for yellow and two genes for yellow may give rise to white.