Good Morning,
The P gene effects the Black and Brown coloring in the Hamsters coat. It also as Hekomi said turns Black/dark brown eyes deep red as adults often so dark they appear at first glance to be black. However as young pups the red eye coloring is like the DEW colored eyes. Cinnamon carried often effects the coat coloring as well producing a much richer Cinnamon like glow to what the show standard requires.
Since Cinnamon doesn't show much effect on BEC, and Yellow coats the only visible effect on Cream is the color of the Eyes becomes red, and the apricot color coat is slightly lightened.
Masquarade a REC LH
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On Agouti Yellow the Black fur becomes dark cinnamon brown and the eyes become Red and the color is called Honey.
Shiloh and Iris's SH Honey Band female 4 week old pup
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The effects on Golden, and Black are really noticed as the eyes become red, the ears become flesh grey, and the Coat of the Golden becomes a gingery Orange brown with light slate grey cheek flashes, and light slate grey under coats instead of Black Cheek Flashes, and dark slate grey under coats. The Golden Brown coat becomes a rich orange/ ginger color and has no ticking, and is of course called Cinnamon.
Orangey an Umbrous Cinnamon SH female
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This happens because of the p gene's effects on Black and Brown fur. Now Cinnamon turns Black to a dove grey color and is why a Hamster with two genes of Black and two genes of Cinnamon is called Dove. The eyes on Doves starts out very bright red but turns into a Claret red (deep dark red) as and adult.
1 1/2 year old Dovey a SH Dove who was also Umbrous
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The only way I knew Dovey was Umbrous was her producing Sable pups bred to a Cream mate.
With Umbrous and Black Eyed Cream, Cinnamon turns the brown of a Sable to the Orange brown of Mink, and of course changes the eyes from Black to Claret Red as adults.
Spudlet a LH Mink male who introduced me to the wonderful color of Mink
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As for size and body/head type there really shouldn't be so much difference when we all strive to meet one standard. I use the NHC standards and strive to produce a Hamster who could be shown in the UK and do well. You can go on the web site for NHC and read the standards that the NHC has set for what they view as the perfect Hamster. Always remember there is no such Hamster as the perfect Hamster but you can produce a Hamster who is very very close to perfect. The standard states the head should be rounded not pointed like a Rats head. The skull should be wide and ears should set wide on the skull. Eyes should be round and large not small and pea eyed.
Size is a big one as I personally have seen what effect on health and life span big does. Bigger is not always better, and I pray that in Hamsters they don't turn to a Hamster so big it's whole body and organs can't sustain it's health. I used to breed and show Cardigan Welsh Corgi Dogs where the Bigger is better goal has just about destroyed the health of today's Cardigan Corgis. With bigger size came in serious Disc issues, Lameness from growth plates that grew deformed from trying to carry the heavier weight. Heart failure because the correct size heart couldn't keep up with the demands of the bigger body. The biggest one earlier death due to the body not able to sustain itself and dying far sooner then it should.
I have had/ and currently Have Females that weigh in at 195g at only 4 months old. Shiloh is my biggest male at 180g.
However I feel weight is not a good scale(no pun intended) to use to determine proper size of a Hamster. A bigger structured Hamster will have heavier bones and if not in shape will weigh a lot more then it's body really should be at. You can take and average sized Hamster pup and Protein pump it up so it will seem bigger and will be bigger but at what cost to the life span of that Hamster. You have to look at Syrian Hamsters in the wild to really see what size and weight a Syrian Hamster was designed to be, and I can assure you there will not be any 300g wild Syrian hamsters out there. Again this is just my personal opinion and you may feel a big Hamster is the perfect Hamster. I like my Hamsters to be around 175g to 195g for Females and 155g to 180g for males.
Any bigger and there comes a cage issue in the USA as there truly isn't any commercially sold cage able to fit a Hamster any bigger.