Why two parent with hartnups disease will not have any normal children but two without hartnups disease could have child with hartnups disease

It depends because the parents without could have a recessive allele or be heterozygous for a certain trait. The genotypes of the parents would be Hh x Hh. Therefore having a 25% chance of having a child with hartnup's disease
             Hope that helped.

Used to water and cool climates, probably not thick coated or fluffy. they most likely have thin coats and have to survive to find food. sorry if this wasn't the answer you were looking for..: )