Chapter 10 



THE GENETIC DISTRIBUTION OF TWO PAIRS OF 

 GENES ON ONE PAIR OF CHROMOSOMES 



The extensive work of the late T. H. ^Morgan and his students 

 and of others who have worked on Drosophila melanogaster has 

 resulted in the discovery of over five hundred different genes 

 in that small organism. Since there are only four pairs of chro- 

 mosomes in this species, there must be many more than one 

 gene on each chromosome. If a fly is heterozygous for two 

 pairs of genes on two different chromosomes, the two alleles 

 on one pair of chromosomes will segregate independently of those 

 on the other pair, thus fulfilling Mendel's second law. If the 

 genes are on the same chromosome, should they also show in- 

 dependent assortment? On chromosome II are found the genes 

 c, curved, and sp, speck, causing a dark spot in the axil of the 

 wing. A fly that is homozygous for C and Sp will have one C 

 and one *Sp gene on each of its second chromosomes, w^hereas a 

 double recessive fly will have one c and one sp gene on each. 

 The Fi fly wdll have one chromosome with C and Sp and an 

 homologous chromosome with c and sj). 



When meiosis takes place in the Fi flies, the two chromosomes 

 will separate from one another. Some germ cells will have a 

 chromosome with both C and <Sp and others will have one with 

 both c and sp. If these genes had been on separate chromosomes, 

 four types of gametes, C Sp, C sp, c Sp, and c sp, would have 

 been produced by the Fi in equal 7iumbers. Since they are on 

 the same chromosome, however, the two genes, such as C and Sp 

 or c and sp, that entered a fly from the same parent must always 

 enter the same gamete, unless, of course, the chromosomes should 

 break. Barring any breakage of chromosomes or chromatids, 

 these two genes always remain together as they pass from one 

 generation to another. This phenomenon is called linkage. 



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