192 FUNDAMENTALS OF CYTOLOGY 



are formed is due to synapsis and disjunction of homologous chromosomes. 

 The independence of many character pairs in inheritance has its basis 

 in the random assortment of the various pairs of chromosomes in meiosis. 

 The linked inheritance of many characters is a result of the location of 

 their differential genes in the same chromosome pair. Recombinations 

 of the characters composing two hnked pairs are due to crossing over of 

 chromatids somewhere between the two differential gene pairs. 



Sex inheritance in dioecious plants and animals has its basis in the 

 behavior of a chromosome pair specially differentiated \\ith respect to 

 genes influencing sex and often in visible morphology. Nonsexual 

 characters are linked with sex, though not restricted to one sex, when 

 their controlling factors are located in the X chromosome. 



Which chromosome of a genome carries a given gene or linked group 

 of genes can be ascertained by using trisomic strains. The chromosome 

 observed to be present in triplicate is responsible for characters showing 

 special trisomic ratios in inheritance. Other methods involving further 

 chromosomal aberrations also are available. 



The serial order of the linked genes in a single chromosome pair is 

 found by comparing the recombination percentages obtained after crosses 

 involving various couples of character pairs. 



The actual positions of the genes in the chromosome are determined 

 by relating abnormahties of characters depending on these genes to 

 visible abnormalities in chromosome structure, such as deletions. 



As a result of the normal operation of the chromosomal mechanism, 

 members of a human family resemble each other and their ancestors 

 because they bear many similar genes in chromosomes derived from 

 common sources. They differ from one another in particular ways 

 because of the orderly reshuffling of the chromosomes at each meiosis 

 and the various combinations resulting from syngamy. The human 

 race is so highly heterozygous that complete similarity is to be expected 

 nowhere except in identical tmns, which arise from the same fertilized 

 egg and therefore carry identical genie outfits, barring new mutations. 

 Extensive studies on such twin pairs reared together and apart have 

 strongly emphasized the fundamental role of genetical constitution in 

 determining the results of development. Unlike environments may 

 influence the development of different capacities, but the experimental 

 evidence indicates that the initial potentialities in identical twins are 

 similar to a degree rarely approached in other pairs of individuals. The 

 naivete of attempts to explain the development of characters without 

 reference to the genetical nature of the material should be obvious. 



