When a cell is getting ready to divide creating two daughter cells, it packs its DNA into bundles called chromosomes. Chromosomes are just bundles of DNA. For humans, there are consistently 23 pairs of chromosomes, each with a consistent size and shape. Chromosomes are numbered. Chromosome number 1 is the largest chromosome; chromosome number 2 a little smaller and so on. Among the 23 pairs of chromosomes there is a pair called the sex chromosomes. This is something of a misnomer, since there are many functions on the "sex" chromosomes that have nothing to do with sex. In females, the sex-chromosome pair consists of two similar size chromosomes called X chromosomes. Males have one X and one small Y chromosome.
Unless it has been purified, our DNA is actually not a loosely tangled string as illustrated but rather is well organized and packaged into what are called chromosomes. A chromosome is a tightly folded bundle of DNA. Chromosomes are most visible when cells divide. In a microscope, chromosomes look something like this without the numbers and letters:
The illustration shows a pair of chromosomes named chromosome number 4, one pair among 23 pairs of chromosomes. The illustration also shows the position of a locus that happens to be called "GYPA." In this example, the chromosome on the left has the variation called the B allele while the chromosome on the right has the variation called the A allele.
What are alleles?
Alleles (ALL-EELS') are just variations at a particular site on a chromosome. Since each chromosome has a similar chromosome partner (except for males with their X and Y chromosomes) each locus is duplicated. Loci can vary a bit. If a person has two identical versions of the locus, they are said to be homozygous (HOMO-Z-EYE'-GUS). If there is a difference, they are said to be heterozygous (HETERO-Z-EYE'-GUS).