292 



INTRODUCTION TO CYTOLOGY 



As a result a given gamete (or spore) in an organism with four pairs of 

 chromosomes may have any one of 16 possible monoploid combinations 

 (Fig. 168)." 



It will be observed at once that the distribution of the chromosomes 

 through the life cycle and successive generations is precisely like that of 

 the Mendelian factors. Two groups of factors are brought together in 

 syngamy and are associated in the body of the offspring. In the germ 

 cells the factors of each allelomorphic pair segregate and pass to different 



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Aa Bb 



Cc Dd 







Fig. 168. — Diagram showing the 16 gametic combinations formed by four independent 

 heterozygous pairs of factors. {After E. B. Wilson, 1913.) 



gametes (or spores). The factors and the chromosomes alike form a 

 duplex group in each somatic nucleus and a simplex group in each gamete 

 (or spore) : both chromosomes and factors are aggregated in syngamy and 

 segregated in meiosis. The precise nature of the parallelism is seen not 

 only in the factor and chromosome groups as wholes but also in the indi- 

 vidual pairs. When a single pair of chromosomes, for example Aa in 

 Fig. 167, is followed through successive cycles, it is seen that there is an 

 exact parallelism between the distribution of a given homologous pair of 

 chromosomes and that of a single allelomorphic pair of Mendelian factors. 

 This is just the condition that should result if the two factors are present 



* The possible number of different monoploid gametic combinations and of diploid 

 zygotic combinations after self-fertilization (or crossing with an exactly similar 

 individual), when each chromosome pair shows some degree of heterozygosity, may 

 be readily calculated. Where n = the number of pairs, the number of gametic 

 combinations is 2" and the number of zygotic combinations is 3". If two individuals 

 differing in all their chromosomes are crossed, there will be 4" possible kinds of com- 

 binations in Fi and 10" in F., (see Winge, 19196). 



