Myostatin Deficiency

Myostatin (also known as growth differentiation factor 8) is a secreted TGF beta protein family member that inhibits muscle differentiation and growth. Myostatin is produced primarily in skeletal muscle cells, circulates in the blood and acts on muscle tissue, by binding a cell-bound receptor called the activin type II receptor.^[1][2] In humans, myostatin is encoded by the MSTN gene.^[3]

Animals lacking myostatin or animals treated with substances such as follistatin that block the binding of myostatin to its receptor have significantly larger muscles. 

Myostatin is active in muscles used for movement (skeletal muscles) both before and after birth. This protein normally restrains muscle growth, ensuring that muscles do not grow too large. Mutations that reduce the production of functional myostatin lead to an overgrowth of muscle tissue. Myostatin-related muscle hypertrophy has a pattern of inheritance known as incomplete autosomal dominance. People with a mutation in both copies of the MSTN gene in each cell (homozygotes) have significantly increased muscle mass and strength. 

Source: http://en.wikipedia.org/wiki/Myostatin            

The following are pics of animals and humans with Myostatin Deficiency:

Source: 

http://www.who-sucks.com/people/monstrous-myostatin-misfortunes-a-collection-of-myostatin-deficiency-pictures

Well, I guess eugenicists win this round.