Re: Will a recessive allele becomes a dominant allele?

The degree of dominance shown by an allele can indeed change.  This
phenomenon was most famously shown by E.B. Ford in 1940.  Ford worked with
the moth Abraxas grossulariata, which can be yellow or white.  A
single gene (we'll call it L) controls the color.  In wild moths, L1 L1
moths were yellow, L2 L2 moths were white, and L1 L2 heterozygotes were a
pale yellow intermediate color.  In other words, this was an example of
incomplete dominance.  

Ford performed four generations of artificial selection in two moth
populations.  In one, he selected heterozygotes that were lighter in color;
in the other population he selected heterozygotes that were darker.  After
four generations, the allele L1 acted as a nearly complete recessive the
first population -- that is, the heterozygotes were white just like L2 L2
homozygotes.  In the other population the same allele, L1, had become
completely dominant, so that the heterozygotes were yellow just like L1 L1
homozygotes.  

Ford concluded that he had selected for different alleles of modifier genes
that code for products that affected the production of the yellow pigment,
in such a way that a formerly incompletely dominant allele could become
either dominant or recessive.  There's a nice brief review of work on the
evolution of dominance in a 1999 paper by Otto and Bourguet.  

Basically, you have to remember that an allele codes for a protein, often
an enzyme.   A single enzyme doesn't produce a phenotype by itself; it has
to have substrates to act on, and the product of the reaction it catalyzes
may then be the substrate for another reaction catalyzed by another enzyme.
 Thus there are genes that code for enzymes that affect how much substrate
there is to work with, and what happens to the product after it's made. 
Not surprisingly, some of these genes will affect whether the heterozygote
looks more like one homozygote or the other.  

As for the other answer, I'm not sure if it's phrased exactly the way your
teacher had it; it's a bit hard to assess as written.  Males are more
likely to express a trait caused by a recessive allele of a gene on the X
chromosome, as they only have one.

Ford EB. 1940. The theory of dominance. American Naturalist 64:560–566.
Otto SP, Bourguet D. 1999.  Balanced Polymorphisms and the Evolution of
Dominance. Am. Nat. 153: 561-574. 



Moderator's note:

Actually, the words "recessive" and "dominant" do not describe independent
properties of an allele; instead, they describe a relationship between two
alleles. I.e. A1 is dominant to A2. The dominant allele in one relationship
between two alleles can be the recessive allele in another relationship. for
example, A1 is dominant to A2, where A2 is recessive; but A2 is dominant to A3
(A2 is dominant).