What advantages do pea plants and fruit flies have for studies of inheritance? Why aren’t humans equally suitable?

Gregor Mendel never saw a gene, yet he concluded that some inherited factors were responsible for the patterns of inheritance he observed in peas. Similarly maps of Drosophila chroma zones were conceived by observing the patterns of inheritance of linked genes, not by observing the genes directly. Is it legitimate for biologists to claim the existence of objects and processes they cannot actually see? How do scientists know whether an explanation is correct?

In c. elegans, genetics model organism, movement problems (unc) and small body size (sma) are encoded by two mutant alleles that are recessive to those that produce wild-type traits (unc+ and sma+). A worm homozygous for movement problems and small body is crossed with a worm homozygous for the wild-type traits. The F1 have normal movement and normal body size. The F1 are then crossed with worms that have movement problems and small body size in a testcross. The progeny of this testcross is: Normal movement, normal body size            210 Movement problems, normal body size          9 Normal movement, small body size                 11 Movement problems, small body size           193   a)From the test cross results, can you tell if the two genes are on the same chromosome or not? Explain your reasoning.   b)What phenotypic proportions would be expected if the genes for round eyes and white body were located on different chromosomes?  (please explain hot to get to these conclusions)

If I were working with the fruit fly Drosophilaand want to determine whether a newly found characteristic is dominant or recessive. What experiment  will help me determine  the dominance of the trait.