Chapter 6 



THE GENETIC DISTRIBUTION OF A PAIR OF 

 ALLELES LOCATED IN AUTOSOMES 



It has been shown that plants and animals may be homozygous 

 for a dominant or for a recessive gene or that they may be hetero- 

 zygous. For example, an evening primrose plant may be homo- 

 zygous for the recessive gene, bullata, and have short, crinkled 

 leaves, or it may contain the dominant allele, in which case it 

 will have noncrinkled, or normal, leaves. In the four-o'clock, 

 plants which are homozygous for the gene for red will have the 

 character, red flowers, and those homozygous for white, the allele 

 of red, will have the character, white flowers. As these genes do 

 not exhibit dominance, the character shown by the heterozygote 

 is pink flowers. Obviously, it is the gene and not the character 

 that is transmitted from generation to generation since germ 

 cells do not have such structures as leaves and flowers. The 

 method by which the genes are distributed is one of the most 

 important and best-understood problems of the science of 

 genetics. 



Since the genes are located in the chromosomes, the problem of 

 the distribution of genes is inseparable from the problem of the 

 distribution of the chromosomes. The behavior of the chromo- 

 somes in the formation of spores, gametophytes, gametes, and 

 zygotes has already been pointed out. The next step is to study 

 the behavior of a pair of chromosomes which contain a certain 

 known pair of alleles. In the second chromosome of Drosophila 

 melanogaster the locus of c, the gene for curved wings, is found 

 about three-fifths of the distance (genetically) from the end of 

 one arm. The other five hundred odd genes in this species can be 

 ignored for the present and observation can be limited to gene c 

 and its allele, C. In cooperation with a large number of non- 

 allelic wild-type genes, C produces normal wings. When only 

 one pair of genes is under consideration, the situation is simple. 

 Organisms which are heterozygous for only one pair of alleles or 



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