556 GENERAL CONSIDERATIONS 



594. Behavior of Chromosomes in Maturation and Fertilization. — In 

 Chap. XXIII it was stated that synapsis occurred in the growth period 

 of both oogenesis and spermatogenesis. In the next chapter synapsis 

 was explained as being the temporary union of like chromosomes from 

 each of the two parents Fig. 391. It was followed by reduction. 



In fertilization (Chap. XXIV) the entrance of the sperm nucleus into 

 the egg cell was described and the statement was made that even after 

 the two pronuclei united the maternal and paternal chromosomes retained 

 their individuality. In each succeeding cell generation in the individual 

 produced from the fertilized egg these maternal and paternal chromosomes 

 appear separate and distinct (Fig. 391). In the maturation of the sex 

 cells of this individual, however, synapsis and reduction again take place. 



595. Effect of Chromosome Reduction. — In chromosome reduction 

 the two of each pair of chromosomes separate and go to opposite poles of 

 the meiotic spindle of either the spermatocyte or oocyte, as the case may 

 be. Since it is a matter of chance as to which of the two chromosomes 

 will go to either of the two poles, the resulting sperm cells or egg cells 

 may differ from each other in the assortment of maternal and paternal 

 chromosomes which they receive. During synapsis the two chromosomes 

 of each pair may twist about one another and fuse more or less so that 

 when separation occurs the two chromosomes which result may each 

 represent portions of both of the chromosomes which were united. Thus 

 it is apparent that different offspring from the same parents may inherit 

 different combinations of parental characteristics. 



596. Allelomorphs. — In fertilization the zygote receives chromo- 

 somes from both parents; when the sex cells mature in the individual 

 which develops from this zygote the corresponding chromosomes from 

 the two parents unite in synapsis and hence are called synaptic mates. 

 Corresponding genes exist in such synaptic mates (Fig. 391). If genes 

 for any pair of characters are alike, the individual is said to be homo- 

 zygous for that pair of characters; if not, it, is said to be heterozygous. The 

 unlike genes of a heterozygous individual are known as allelomorphs. Of 

 course the presence of both cannot be shown fully in visible characters and 

 one will be evident and the other repressed or concealed. The one which 

 is shown is called dominant, the other recessive. In some cases, however, 

 there is an incomplete dominance as shown in the blue Andahisian fowl 

 (Fig. 387). The term genotype refers to the whole combination of 

 inherited genes which any individual possesses ; phenotype, to the assem- 

 blage of characters which manifest themselves. If, for example, both 

 genes for hair color in a mammal were ahke, the animal would be homo- 

 zygous and have the common color; if one gene was for red and the other 

 for black, the animal would be heterozygous for this pair of characters 



