contracted relative to the rest of the chromosome. Whether such regions 

 are heterochromatic in the genetic sense is not known. There is some 

 evidence that they can be deleted without producing any obvious effect 

 on viability. 



In a number of plants and animals individuals in a population ma> 

 contain a number of chromosomes in addition to the normal comple- 

 ment. These extra chromosomes are known as supernumeraries. As a 

 rule, they are smaller than normal, appear to be genetically inert, since 

 variation in their numbers is not accompanied by any marked phenotypic 



B 



(a) 



Figure 4-14. (a) Photomicrograph of the Five Pairs of Chromosomes 

 from a Single Cell of Trillium grandiflorum, showing the pattern of differ- 

 entially reactive segments produced by prolonged cold treatment. Note dif- 

 ferences between homologues with respect to degree of expression, (b) 



{Contin. on opposite page.) 



92 / CHAPTER 4 



