Chapter 9 



THE DISTRIBUTION OF TWO OR MORE PAIRS OF 

 ALLELES IN TWO OR MORE CHROMOSOMES 



Two Pairs of Genes in Two Pairs of Autosomes 



In Zea mays there are ten pairs of chromosomes, and they 

 can be distinguished from one another by their size and gen- 

 eral morphology. Their morphological characteristics include 

 the position of their centromeres, the presence and position of 

 secondary constrictions, and the presence of knobs of various 

 sizes on the ends of the chromosomes (Fig. 38). The ability to 

 distinguish the various chromosomes enables us to say that 

 when a maize plant undergoes meiosis, one member of each pair 

 of chromosomes enters each gamete. Furthermore, there is a 

 great amount of evidence to show that the segregation of any 

 one pair is entirely unaffected by the segregation of any other 

 pair, but most of this evidence is genetical rather than cyto- 

 logical. 



In chromosome VI of maize, a gene is present for purple plant 

 color, PI, which is dominant to its allele for green, pL A homozy- 

 gous purple plant crossed with a green gives a purple Fi and 

 three purple to one green in the F2. In chromosome III, the 

 gene for normal leaves, Cr, is dominant to crinkly leaves, cv, 

 and when a homozygous normal is crossed with a crinkly, the Fi 

 is normal and the F2 segregates into three normal and one 

 crinkly. If a plant which is homozygous for both purple color 

 and normal leaves is crossed with one that has green color and 

 crinkly leaves, the Fi is purple and normal and is heterozygous 

 for each pair of genes. This Fi plant receives the genes PI and 

 Cr from its female parent and the genes 'pl and cr from its male 

 parent, and when it undergoes meiosis the chromosome which 

 bears PI separates from the one with yl. In like manner the 

 chromosome bearing Cr separates from its homologue which 

 bears cr. However, the segregation of the PI and pl chromo- 



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