Chapter 6: Genetic Linkage and Mapping in Eukaryotes

Unlike Mendel's classical predictions that assume independent assortment of genes on different chromosomes, genes positioned close to one another on a shared chromosome tend to be transmitted as a coordinated unit rather than segregating freely during meiosis. The chapter contrasts these two inheritance patterns and establishes linkage as a key deviation from Mendelian expectations. Landmark experiments conducted with fruit flies revealed that certain trait combinations did not follow predicted independent ratios, providing crucial evidence that genes exist in defined linkage groups along chromosomes. The chapter introduces recombination frequency as a quantifiable measure of crossing over events during meiosis, explaining how the proportion of recombinant offspring directly reflects the physical separation between linked genes. Recombination frequency is expressed in map units, also called centiMorgans, which provide a relative scale for gene positioning along chromosomes. Students learn to perform and interpret testcrosses, extracting recombination data to calculate distances between gene pairs and construct linkage maps that show their relative order. The chapter extends these mapping techniques to three-point crosses, requiring students to identify parental phenotypes, single crossover classes, and double crossover classes in order to determine accurate gene sequence and refine distance estimates. The phenomenon of crossover interference is introduced as evidence that one crossing over event can suppress the occurrence of another in nearby chromosomal regions, adding complexity to crossover distribution patterns. Students calculate the coefficient of coincidence and interference values to quantify this regulatory mechanism. The chapter concludes by differentiating between genetic maps derived from recombination analysis and physical maps based on direct DNA sequence information, noting that recombination rates vary across different chromosomal regions. This framework enables prediction of inheritance patterns and demonstrates how recombination mechanisms both challenge and refine traditional Mendelian models.