_Research indicates that the Leptin gene was associated with marbling and, more generally, fat deposition:

• Department of Animal Science, University of Saskatchewan (nd). DNA Studies of Leptin and Meat Quality (2006) Available at: http://homepage.usask.ca/~schmutz/leptin.html 

• T. W. Geary, E. L. McFadin, M. D. MacNeil, E. E. Grings, R. E. Short, R. N. Funston and Leptin as a predictor of carcass composition in beef cattle.  Available at: http://jas.fass.org/content/81/1/1.full.pdf+html

• Mitchell, Clifford (2005)  NEW DNA TEST COULD PROVIDE KEY TO PROFIT.  Cattle Today.  Available at: http://www.cattletoday.com/archive/2005/September/CT418.shtml      "Testing for the leptin gene in livestock is a new cutting edge proposition associated with DNA gene markers. The IGENITY-L™ test, provided by Merial, allows producers to utilize yet another tool in beef production to improve carcass merit.  “Leptin is a protein produced in white fat tissue and the more leptin produced, the stronger the signal is sent to the brain to reduce appetite,” Tate says. “The leptin protein is directly related to appetite, energy utilization and fat deposition.”
  

• Herd, D.B., and L.R. Sprott. “Body Condition, Nutrition and Reproduction of Beef Cows.” Texas Agricultural Extension Service, College Station, Texas: The Texas A&M University System.   "TT cattle may be ‘easier fleshing’ (apt to carry more external body fat). Cattle with greater fleshing ability are much preferred from a maternal standpoint. The amount of external fat is the primary determinant of a cows body condition score (BCS). On average, females with a higher BCS will produce heavier calves, have a shorter calving interval, and more longevity"
• Genomics: The new tool for the 21st Century*
  The Effect of Leptin on Average Daily Gain and Feed Efficiency:
  Leptin positive animals finish for slaughter approximately 37 days faster and had an ADG of 3.323 compared with non-positive animals that had an ADG of 2.937.
  
  Leptin positive animals also exhibit a greater feed efficiency (6.4) compared to non-positive animals (8.2).  
  (Quantum Management Protocol)
  
  Yield Grade:
  Although the results vary in terms of the proportion that grade AAA or higher, the trend has always been the same:  TT cattle have a much higher chance of AAA, or in the USA system "choice".
  (Department of Animal and Poultry Science, University of Saskatchewan)
  
  Milk Production:
  Animals with the TT gentype produced more milk (3.3 lbs/day) than animals with the CC genotype. 
  (Van Eenennaam, A. 2006)
  
  
  The Effect of Leptin on Lactating Beef Cows and Calf Weaning Weights: