GENETICS AND EVOLUTION | 647 
numper of chromosomes to one-half that of the somatic cells. 
In the primary spermatocyte and primary odcyte of animals at 
synapsis the chromosomes unite in pairs. One member of each 
pair was derived from the mother, the other from the father. 
When division occurs, the chromosomes do not divide, but entire 
chromosomes separate and are distributed in pairs to two daugh- 
ter cells. At the division resulting in spermatids and mature 
egg, the pairs separate and each daughter cell receives univalent 
chromosomes. A similar condition prevails during the division 
of the mother cells of pollen grains and embryo sacs in plants. 
Each spermatozoon and ovum is thus provided with one-half the 
number of chromosomes found in the somatic cells or primordial 
germ cells. 
DIsTRIBUTION OF GENES DURING MATURATION AND FERTILIZA- 
TION. 
of homologous chromomeres. Since the homologous chromo- 
somes carry the genes and the genes are also paired, the genes will 
be distributed as were the chromosomes. 
ALLELOMORPHS.—Genes that occupy corresponding positions 
_in homologous chromosomes are called allelomorphs. ‘The mem- 
bers of a pair of allelomorphs may be the same or they may differ; 
if they are the same, the individual bearing them is said to be 
homozygous as far as the character is concerned that is controlled 
by the particular genes; if they are different, the individual is said 
to be heterozygous. _ 
DomiNANTs AND ReceEsstvEs.—In a heterozygote the genes 
are of two kinds, dominants and_recessives. A gene is said to be 
dominant when the character. it represents appears in the hetero-_. 
zygote; its allelomorph in such a case is said to be recessive. 
In _ peas, tallness is dominant, dwarfness recessive; in the sun- | 
flower, the branched habit is dominant and the unbranched habit 
is recessive. The result of dominance and recéessiveness in 
allelomorphic genes is that heterozygotic individuals do not 
exhibit all that they inherit from their parents, since some of the 
genes are recessive. 
GENOTYPE AND PHENOTYPE.—The term genotype is used to 
express the entire genetic make-up of an organism. Individuals 
having a similar eine constitution are said to be of the same 
