Some diseases require mutations in 2 or more specific genes to cause the disease. Complex Inheritance Complex inheritance means that several genes may combine to influence a single trait or disease or non-genetic factors may also play a role. There are numerous projects underway to map out the genes in the human genome. The goal is then to use this information to study the human body and diseases . One type of map is a genetic linkage map which gives the relative chromosomal locations of genes and other DNA segments by their pattern of inheritance.
Markers act like mile posts along the DNA, giving approximate location of genes.
Two markers located near each other on the chromosome tend to be passed together from parent to child. Recombination can occur during egg and sperm production, where DNA strands break and rejoin in different places on the same chromosome or on the other copy of the same chromosome. The closer the markers, the less likely they are to be separated. An inherited disease can be located on the map by following the DNA marker present in affected individuals but not in unaffected individuals. This is possible even though the gene has not been identified.