Garrod's concept regarding the relationship between the gene and metabolism can best be described as:
a. one gene - one polypeptide
b. one mutant gene - one metabolic block
c. one gene - one metabolism
d. one gene - one transcript

b. one mutant gene - one metabolic block

Explanation:

Garrod studied inborn errors of metabolism and explained the molecular causes of various genetic diseases such as albinism and alkaptonuria.

Accordingly, the individuals homozygous for the recessive allele exhibit metabolic defect responsible for the disease.

For example, alkapton is normally degraded into maleylacetoacetic acid by the reaction catalyzed by the enzyme homogentisic acid oxidase. A mutation in the respective gene produces nonfunctional enzyme and leads to alkaptonuria as the individuals are not able to degree alkapton in absence of functional enzyme.

Hence, one mutation blocks one metabolic pathway and leads to a genetic disorder.