The fact that there appears to be a genetic component to inflammatory bowel disease, but that it does not show clear Mendelian inheritance ratios suggests any of the following, EXCEPT: the gene for the disease has incomplete penetrance. the gene for the disease has limited expressivity. the disease is polygenic. the gene for the disease is recessive.

Option D

Explanation:

(the gene for the disease has incomplete penetrance. the gene for the disease has limited expressivity. the disease is polygenic) - all these are examples of non-mendelian inheritance which include incomplete penetrance, polygenic inheritance etc. These do not follow the mendelian pattern of inheritance.

(the gene for the disease is recessive.)- this shows the mendelian pattern of inheritance... Dominant and recessive characteristics are examples that show Mendelian inheritance.

your answer is "the 1950's"