Chromosome maps 



In a chromosome map, genes or DNA fragments are assigned to their respective 

 chromosomes (Fig. 10). With in situ hybridization, the DNA (tagged with either a 

 fluorescent or radioactive label) is used to seek out and bind to its complementary 

 strand in an intact metaphase chromosome. 



Until recently, the best chromosome maps could be used to locate a DNA fragment only 

 to within a region of about 10 million base pairs — the size of a typical chromosome 

 band. However, new methods promise to improve the resolution of the maps and allow 

 narrowing of the range to around 1 million base pairs. 



Genetic maps 



Any molecular or physical characteristic that differs between individuals and is 

 inherited is a potential genetic marker. The human genetic map is constructed by 

 observing the pattern of inheritance of pairs of genetic markers. DNA sequence 

 differences are especially useful markers because they are plentiful and easy to 

 characterize precisely. Two markers located near each other on the same chromosome 

 will tend to be passed together from parent to child (Fig. 1 1 ). 



B 



father 

 blue eyes 

 HD H 



mother 



H 



children 



distance between genes for blue eyes and HD 



Fig. 11. How genetic maps are 

 made. Each parent contributes one 

 of each type of chromosome 

 through the haploid egg and sperm 

 so that the offspring will have a 

 diploid chromosome set. In this 

 illustration, the vertical lines show 

 just one pair of chromosomes for 

 each individual in a family. The 

 father has, in this example, two 

 traits that will be visible in any child 

 that inherits them: blue eyes (B) 

 and Huntington's disease (HD). The 

 fact that one child received only a 

 single trait, B, indicates that one of 

 the father's chromosomes 

 rearranged in the process of 

 producing the child. (Note: In this 

 example the mother's eyes were 

 also blue.) 



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