Re: pink vs. orange

A few things to consider.
A pure pink iris results from careful selection from cultivars , parents etc, selecting to eliminate apricot and peach overtones. This removal of these overtones is tricky, and is the result of of modifier or minor genes. Crossing two pinks from unrelated lines results in a large proportion of seedlings with apricot and peach tones. These peach and apricot tones don't seem to have anything to do with the alternative yellow, but is involved with how complete is the blockage of the conversion of lycopene to carotene. According to the current theories of carotene production, there are two conversions, going on, and these are competing. One leading to alpha-carotene (mid yellow) and from there to lutein (a pale yellow). The other branch leads to beta-carotene (intense yellow) and subsequently to a number of light yellow pigments.
Thus to get a pure pink, both of these conversions need to be blocked. One leakage would likely lead to apricot, the other to peach. This is an educated guess (at best) or wild speculation at worst. Take your choice. In any case don't take this a gospel.
The breeding for orange takes a different route, selecting for different modifies, and involves the alternative yellow.
Even when crossing two pinks we can, and do often get a number of apricots and peaches, having nothing to do with alternative yellow.
So when crossing a pink with and orange, there will be a number of peach and apricot toned cultivars, which are basically poor quality pinks. This is a result of losing a number of the modifier genes needed to produce pure pinks. But, these re not ORANGE. The alternative yellow gene will not show up in these crosses unless it is carried as a recessive in the pink cultivar.
I'm preparing some presentation material to post on iris-photos to show some of this.
For the most part, the alternative yellow genes won't show up to any degree in breeding that doesn't involve any cultivars from pink or orange ancestors. To check for lycopene put beard in alcohol and view beard after alcohol has dissolved carotene.
Why this is not something to worry about in regular yellow is two fold. One, it wouldn't have much distribution in regular yellow. Two it may not even work if we don't t have tttt in plants. This last is a speculation.
What you get in any cross depends on how closely the appearance (phenotype) matches the genetic (genotype) and the number and type of recessive genes a plant contains. this can be determined by what it produces in test crosses or speculation can be made from studying the ancestor charts.
The plastids are the receptacles of the cartenoid pigments (except perhaps when alternative yellow is involved). this is relatively fixed for iris as far as I can tell. So there is only a fixed amount of what amount of cartenoid pigment you can store in the petals. Not enough beta-carotene to produce orange, and not enough lycopene to produce red.
In addition, there is a method of storing lycopene in tomatoes that involve storing a crystalline structure, a process that does not seem to be available to iris. In addition, tomatoes, carrots etc, have a huge number of layers of cells to store the carotenoid pigments. flower petals have only three layers of cells. (granted that this mesophyll layer is a jumble and it can be argued that there are four layers) In addition to storing more pigment, these multiple layers of cells also prevent light from passing through and diluting colour. I haven't seen a dark enough yellow, orange or pink iris to prevent light from passing through, and have never seen a carrot or tomato that allows light to pass through it.
I misspoke when I said pink and yellow are alleles. They are not alleles, but independent functions. The gene producing the pigments for pink and yellow would better be called "Carotene" genes, referred to as C for the dominant production of carotene. This would refer to the gene which allows for the conversion of neurosporene to lycopene which then converts to gamma and then beta-carotene.
The t gene, when present in four copies in a tetraploids, (or two in diploids) prevents the up conversion of lycopene to beta-carotene and the accumulation of lycopene
Christian reread the previous posts and then ask one question at a time and I will do my best to answer. 

Chuck Chapman

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