I’m not experience in genetic code in hamster. All I know is the very basics from studying biology. But this link will help explain genetic codes in hamsters -
Genetics and Inheritance and also this link has Syrian colours
Syrian. This explains how the National Hamster Club writes genetic codes
If you breed two Rex carriers (Rxrx) there is 25% chance of the pups will be Rex (rxrx). As Rex, rx, is a recessive gene this is why it written lower case the babies will need two of these genes to express this phenotype so they will be rxrx. Any dominant gene written with an uppercase letter I.e Rx (normal hair) a baby will only need one of these genes to express this trait they will never express the recessive trait. 50% chance of the pups will be Rex carriers (Rxrx) and 25% chance will not be Rex or Rex carriers (RxRx).