What is the significance of a double band for a male and single band for a female in a PCR reaction?

Answer 1

Generally, finding two bands means the individual is heterozygous while a single band indicates homozygosity.

Now it sort of depends on the particular gene and the kind of gene you're amplifying. If you're looking at a variable-number tandem repeat gene, the that's the case. For most genes, you wouldn't detect a difference in size between two different alleles in most cases...

If you are looking at a gene on the X chromosome, then you might expect to find heterozygous females, but all males would be homozygous.

Answer 2

A male will have a double band because of his sex chromosomes (XY). Since a female has the XX chromosomes, those two X chromosomes will form one band together--they're the same size. The Y chromosome, though, is much smaller than the X chromosome, so it will form a separate band from the X chromosome's band.