You are interested in performing some marker mapping on a small population of your favorite organism. You perform a mating between parents P1 and P2...

Analytical genetics question! The question is the following: You are interested in performing some marker mapping on a small population of your favorite organism. You perform a mating between parents P1 and P2 and obtain 12 progeny. Allele specific PCR primers are used for 4 different marker loci, M1-M4 (these markers are not alleles, they are different unlinked loci). Each marker has 2 alleles, plus and minus. The gels are attached in the document with a table to categorize each one as homozygous +, heterozygous and homozygous -. How can I distinguish the homozygous from the heterozygous?