Glossary { 321 



replicated and transmitted equationally to daughter nuclei; see nucleic 

 acid. 



Dicentric having two centromeres. 



Differential segments portions of chromosomes that do not pair in 

 meiosis. 



Differentiation the changes observed in development as a zygote be- 

 comes a multicellular entity in which many diverse kinds of cells, 

 tissues, and organs are found. 



Dikaryosis the condition of possessing two nuclei in each cell. 



Diploid having the zvgotic number of chromosomes, or having two 

 genomes, in reproductive cells other than gametes. 



Diplophase that part of the life cycle in which the zygotic chromosome 

 number is found in reproductive cells other than gametes. 



Diplosis establishment of the zygotic chromosome number, usually by 

 syngamy and karyogamy. 



Diplospory a mode of apomixis in plants in which a diploid gameto- 

 phyte is formed after mitotic or partly meiotic divisions of the spore- 

 forming cells. 



Directional selection selection resulting in a shift in the population mean 

 for the character considered. 



Directed alternate disjunction regular movement of alternate centro- 

 meres to the same pole of the spindle in nuclei heterozygous for one 

 or more reciprocal translocations. 



Disjunction movement of the centromeres of a bivalent to opposite poles 

 of the spindle during the first meiotic anaphase. 



Disruptive selection selection in which two or more different geno- 

 types are at an advantage and intermediate types are at a disadvan- 

 tage. 



Disulfide linkage a covalent bonding of cysteine residues (S-S) in or 

 between protein molecules; thought to be responsible for spindle 

 formation. 



DNA see deoxyribonucleic acid. 



Dominance the effect of the phenotype of a heterozygote for a given 

 locus being more similar to one homozygote than it is to another; the 

 allele involved in the proximate homozygote is referred to as the 

 dominant allele; in complete dominance the heterozygote phenotype 

 is identical to that of the dominant homozygote. 



Duplication the presence of a section of chromosome more than once in 

 a genome. 



Ecosystem the thermodynamically interrelated set of organisms in a par- 

 ticular environment. 



Effective breeding size the breeding size mathematically adjusted so 

 that populations with varying sex ratios, degrees of inbreeding, etc., 

 may be compared. 



Effectors small molecules (metabolites) that combine with repressor 

 molecules, activating or inactivating them with respect to their ability 

 to combine with an operator. 



