Heredity — Genetics 679 



tro mer) (Gr. kentron, center; meros, part) or kinetochore — the point 

 for attachment to the spindle fiber when chromosomes migrate along the 

 latter. As prophase progresses, the chromonemata thicken, uncoil, and 

 acquire an accumulation of the matrix which surrounds them. The two 

 threadlike chromonemata and their matrix in each prophase chromo- 

 some are called chromatids. In later prophase the two chromatids of 

 each chromosome appear to be identical and lie next to each other. The 

 two centromeres lie in close contact. After the chromosomes are ar- 

 ranged on the equator, the two chromatids of each chromosome repel 

 each other, possibly through some electrical process between the two 

 centromeres. This action results in the migration of the daughter chro- 

 mosomes toward opposite poles. Attached to each chromonema in a 

 linear series are numerous, beadlike granular chromomeres (kro' mo mer) 

 (Gr. chroma, color; meros, part) of various sizes and different distances 

 apart. The chromomeres occur in different sizes and arrangements 

 which are constant and characteristic for each chromosome. In other 

 words, each chromosome has its unique arrangement of chromomeres of 

 specific sizes which characterize it. It may have a series of large, small, 

 or medium-sized chromomeres arranged along the chromonema, and 

 this arrangement is specific for that particular chromosome. In studies 

 of the giant chromosomes of the salivary glands of fly larvae of various 

 species the chromosomes appear like a cylinder with larger numbers of 

 characteristic crosshands or disks. Comparable to the differences in sizes 

 of chromomeres and distances between them, these bands may be thin 

 or thick, far apart or close together, all of this so characteristic and con- 

 stant for each chromosome of a set that each band can be indentified 

 and numbered. Are these bands the hereditary entities known as genes 

 (Gr. genos, descent) or are these bands associated with genes? Evidence 

 is inconclusive, although it is known that at least certain bands are asso- 

 ciated with more than one gene. 



3. Chromosomal Aberrations. — Normally the number of chromosomes 

 in the somatic (body) cells of animals and the sporophyte generation of 

 plants is double or diploid (2N), while it is single or haploid (N) in 

 gametes, spores, and the gametophyte generation of plants, but there are 

 exceptions. For example, the endosperm (stored food) of angiosperm 

 seeds normally and regularly contains a triple (triploid) number (3N). 

 In addition to the ploidy condition previously described, there are nu- 

 merous chromosomal aberrations such as those which involve pieces of 

 chromosomes, entire chromosomes, or entire sets of chromosomes 

 (genomes) . 



