Page 159 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

5.3 Mapping: Locating Genes Along a Chromosome 151

analyses reveal that most chromosomes in human sperm been assigned to a particular chromosome, the terms chro-
have undergone only a single crossover. mosome and linkage group become synonymous. If you can
Drosophila provides an extreme example: No recom- demonstrate that gene A is linked to gene B, B to C, C to D,
bination occurs during meiosis in males. If you review the and D to E, you can conclude that all these genes are syn-
examples already discussed in this chapter, you will dis- tenic. When the genetic map of a genome becomes so dense
cover that they all measure recombination among the that it is possible to show that any gene on a chromosome is
progeny of doubly heterozygous Drosophila females. Prob- linked to another gene on the same chromosome, the number
lem 19 at the end of this chapter shows how geneticists can of linkage groups equals the number of pairs of homologous
exploit the absence of recombination in Drosophila males chromosomes in the species. Humans have 23 linkage
to establish rapidly that genes far apart on the same groups, mice have 20, and fruit flies have 4 (Fig. 5.18).
chromosome are indeed syntenic. The total genetic distance along a chromosome, which
is obtained by adding many short distances between genes,
Multiple-Factor Crosses Help Establish may be much more than 50 m.u. For example, the two long
Linkage Groups Drosophila autosomes are both slightly more than 100 m.u.
in length (Fig. 5.18), while the longest human chromosome
Genes chained together by linkage relationships are known is approximately 270 m.u. Recall, however, that even with
collectively as a linkage group. When enough genes have the longest chromosomes, pairwise crosses between genes

Figure 5.18 Drosophila melanogaster has four linkage groups. A genetic map of the fruit fly, showing the position of many
genes affecting body morphology, including those used as examples in this chapter (highlighted in bold). Because so many Drosophila genes
have been mapped, each of the four chromosomes can be represented as a single linkage group.

X chromosome

bent wing
net veins cubitus veins
yellow body 0.0 artistaless antenna 0.0 roughoid eyes 0.0
0.0 scute bristles 1.3 star eyes 0.2 veinlet veins shaven hairs
grooveless scutellum
1.5 white eyes eyeless
3.0 facet eyes 4.0 held-out wings
5.5 echinus eyes
7.5 ruby eyes
13.7 13.0 dumpy wings
crossveinless wings
16.5 thick veins
20.0 cut wings 19.2 javelin bristles
21.0 singed bristles
26.0 sepia eyes
27.7 lozenge eyes 26.5 hairy body
33.0 vermilion eyes
36.1 miniature wings
41.0 dichaete bristles
43.0 sable body 43.2 thread arista
44.0 garnet eyes 48.5 black body 44.0 scarlet eyes
51.0 reduced bristles 48.0 pink eyes
55.0 rudimentary 54.5 purple eyes 52.0 rosy eyes
56.7 forked bristles 54.8 short bristles 58.2 stubble bristles
57.0 bar eyes 55.0 light eyes 58.5 spineless bristles
59.5 fused veins 57.5 cinnabar eyes 58.7 bithorax body
62.5 carnation eyes 62.0 stripe body
66.0 bobbed hairs 66.7 scabrous eyes 63.0 glass eyes
67.0 vestigial wings 66.2 delta veins
72.0 lobe eyes 69.5 hairless bristles
70.7 ebony body
75.5 curved wings
74.7 cardinal eyes

91.1 rough eyes

100.5 plexus wings 100.7 claret eyes
Wild type
104.5 brown eyes
107.0 blistered wings 106.2 minute bristles