Hi Manal,
I dont know how to explain in simple English what an 'allele' means,
but i will try.
Allele is a part of the gene in which it represents a trait in the humans or the living organism. For each trait, each person has got two alleles.. For example, for your eye colour, height, skin colour..etc.. you have two alleles on some part of the gene, and likewise for thalassaemia also you have two..
One of these come from the father while the other one comes from the mother.. two of the alleles together make the trait..
Some alleles can be co-dominant, dominant and some alleles can be resessive, for example in the case of thal minors, u have a normal allele and an affected allele. The normal allele is dominant over the affected one and thats why thal minors ( or carriers) are not transfusion dependent..
I dont know how to explain what IVS1-6 indicates genetically, but i can tell you this much. That it is the mutation that has occured on the allele, and since it says IVS1-6 heterozygous, it means that allele is different from the other... and thats why the doctor has recommended to check for the second allele if your son show thal traits...
I know this is not so clear,
but i hope it helped to get you an idea.. I think there would be many others on the forum who would be able to give you a more clear idea of what alleles are what IVS1-6 means...
tc and all the best
Maako