Het is short for Heterozygous which means it carries both the normal gene and a mutated gene. Each parent passes on 2 genes.
ex: A snake with one normal gene and one mutated gene that can produce melanin will produce offspring that will look normal. Since both copies of that gene are different, this animal is called “heterozygous for amel.”, or het for amel.
An animal het for something will appear normal but carry the recessive gene. Crossed with mate that is amelistic will produce half of the offspring that is amellistic and the other half of the clutch that is het for amellistic.
I think I got that right. This is a good site that explains dna
http://www.cranwill.com/