To investigate the possible presence of introns in three newly discovered genes (X, Y, and Z), you perform an experiment in which the restriction enzyme HaeIII is used to cleave either the DNA of each gene or the cDNA made by copying its mRNA with reverse transcriptase. The resulting DNA fragments are separated by gel electrophoresis, and the presence of fragments in the gels is detected by hybridizing to a radioactive DNA probe made by copying the intact gene with DNA polymerase in the presence of radioactive substrates.
The following results are obtained:

Source of DNA Number of Fragments After Source Electrophoresis
Gene X DNA 3
cDNA made from mRNA X 2
Gene Y DNA 4
cDNA made from mRNA Y 2
Gene Z DNA 2
cDNA made from mRNA Z 2

(a) What can you conclude about the number of introns present in gene X?
(b) What can you conclude about the number of introns present in gene Y?
(c) What can you conclude about the number of introns present in gene Z?