Linkage Maps, Linkage Groups
The organism that gene mapping was first performed in was fruit flies. Working with fruit flies gave the early gene mappers many advantages. First of all, the fruit fly geneticists observed a great of deal of genetic variation among fruit flies for body part traits (wing size, eye shape and color, leg bristle types etc.) that were easy to see if they had a low powered microscope. The phenotype variants arose naturally or could be induced by chemical or radiation mutagenesis. Additionally, short generation time, large offspring numbers and low rearing costs allowed them to complete informative linkage experiments in a short time. Finally, cytogeneticists knew that fruit flies had four pairs of chromosomes so all of the genes would map into four linkage groups. One chromosome was very small and few genes mapped to this chromosome. The X chromosome and the largest autosome have many genes and more than 100 map units separate genes on opposite ends of the chromosome.
Lets stop and think about how they determined this large map unit distance. If the A,a and W,w loci are 106 map units apart, how frequently will a AW / aw dihybrid make the gametes Aw and aW? Since these are the recombinant gametes and map distance is the frequency of recombinant gametes, we are tempted to say 106% but that is impossible. The parental gametes for any two loci will never be made less than 50% of the time, the percentage we observe when two loci are independently assorting. In fact, 50% is the correct answer here. Anytime genes that are on the same linkage group are 50 map units or more apart, they will behave as if they are independent of one another. That is because the genes are far enough apart to always allow at least one crossover to occur between them during prophase I. So how can we ever determine that A,a and W,w are 106 map units apart? Simply by determining the distances between these genes and other genes in between them that are less than 50 map units apart (Fig. 4). Fig. 4 shows that distances can be found between A,a and B,b, B,b and C,c , then C,c and W,w. Each of these distances is added to arrive at the 106 map units between A,a and W,w. Therefore if the geneticists combines the mapping information from many linkage experiments or performs a multiple point linkage analysis on a progeny group segregating for many linked genes, they can deduce the larger map unit distances.