Mihai Cartoaje schreef
Solomon, Eldra Pearl, et al. Biology. 3rd edition. Sunders, Forth
Worth, 1993.

Page 233: "If genes occupy the same locus on each of a pair of
chromosomes, they are said to be alleles."

+ + +
That is somewhat cryptic, and can indeed be misread.
However, with a little effort, it can also be read correctly.
What they apparently aim to say is that when a particular bit of the DNA
codes for a hereditary property (is a gene), then when you look at the same
locations on other specimens of this particular chromosome (the same locus),
[they forget to mention that these bits should differ enough in the sequence
of the bases for it to be reflected in the expression of this hereditary
property, when they get the chance] then these bits are called alleles. I
guess you should look at the accompanying drawing, or look in a book that
has such a drawing.

The use of "genes" here is slightly idiosyncratic. Also, they should perhaps
have been a little more emphatic that alleles are form of a gene. Also, the
"pair of chromosomes" is limiting this to a too restricted, and not
necessarily relevant, subclass of this set. I suppose they do this to keep
thing simple, but they fail. I would almost think that schoolbooks in the US
are as bad as people say, but of course the sample is much too small to draw
conclusions.

Again, find a book with better pictures.
PvR

They use the expression "not allelic" otherwise. I understand "allele"
as "sister", except for genes.