Question

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

Answer 1

If you do a Punnett square, this will make a lot more sense. Say the alleles for hartnups disease is hh (both recessive). If you have a mom with hh alleles and a dad with hh alleles, all offspring will be hh(which means all offspring will have the disease).

Now keep in mind that there's a difference between phenotype (what's actually expressed/can be noticed) and genotype (what's in your genes). A parent may not have the disease but could be a carrier (which means there's a potential of passing on the "disease genes" to the offspring). Say the mom is Hh, and the dad is Hh (both are carriers). Draw the Punnett square, and you should find there's a 1 in 4 chance that the offspring will be hh (and have the disease).

Hopefully that all makes sense. Good luck!

Answer 2

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.