According to medical science,marriage between blood relations can result in genetic disorder in the progeny.?

Most species have mechanisms that are designed to avoid inbreeding. In most mammal species males migrate from the place of their birth and join other unrelated groups or move to new territories and in birds the females tend to do the same.

Occasional inbreeding is not harmful even to humans, it repeated inbreeding that causes problems. In wild species where mating opportunities are limited because the population size is too small, they do incur problems because of repeated inbreeding. However, if a fraction of the population survives, the deleterious alleles are purged and then the genetic health of the population improves even with reduced genetic variation.

Inbreeding does affect animals actually. Think of pure-breed dogs. Humans went out of their way to mate dogs for 'purity' of the breed, which often meant mating relations. As a result, many of the pure breeds have health problems.

maybe its just in humans?

Not everyone is affected its only the people with susceptible genes who are affected because of consaginous marriage