528 GENERAL CONSIDERATIONS 



factor. By some writers all are used synonymously; by others the word 

 character is used in the sense just given. 



592. 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. 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 individuahty. In each succeeding cell generation in the individual 

 produced from the fertilized egg these maternal and paternal chromosomes 

 appear separate and distinct. In the maturation of the sex cells of this 

 individual, however, synapsis and reduction again take place. 



593. 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. 



594. Allelomorphs.— In fertihzation 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. If the genes for any 

 pair of characters are alike, the individual is said to be homozygous for 

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

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

 the presence of both cannot be shown 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. The term genotype refers 

 to the whole combination of inherited genes which any individual pos- 

 sesses; vhenoty-pe, to the assemblage of characters which manifest them- 

 selves. If, for example, both genes for hair color in a mammal were alike, 

 the animal would be homozygous 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 and might appear either black or red, depending 



