Ali Drew, of Christchurch, asks :-

When thinning a row of carrots some were the usual orange colour but several were yellow and one was purple. How does this happen from the same packet of seeds?

Bill Griffin, a plant scientist at Crop and Food Research, Lincoln, responded.

New varieties of carrots are bred to improve characteristics which people want - including how good they taste, whether they are easy to grow and colour. Carrots are an outbreeding species (seeds are only formed following pollination from another carrot plant), so it is difficult to maintain lines of seed, or varieties, which all the look the same: It is hard to produce what scientists call a 'homozygous' line. One way carrot breeders get around this is to develop hybrid seed (the first generation seed from a cross) varieties. This first generation is all genetically identical, so when you plant a packet of seed, most of the seeds which grow will be the hybrid variety (the picture you see on the front of the seed packet).

However, the hybrid seed production system is not perfect. Sometimes the outcrossing is not complete and a few self-pollinated seeds may form which look like the two parents involved, or pollen from another parent may contaminate the system and also form seeds which do not look like the target hybrid. When this foreign pollen involves differences like carrot colour, these strangers are very obvious in your resulting crop.