480 - Heredity and Evolution 



members of each homologous pair of chro- 

 mosomes become separated from each other, 

 and each mature gamete comes to possess just 

 a single member of each chromosome pair. It 

 is, however, entirely a matter of chance as to 

 which member of any chromosome pair goes 

 to a particular gamete, and the various pos- 

 sibilities occur with equal frequency. 



Take for example the sperm formed from 

 spermatogonia possessing two pairs of chro- 

 mosomes, namely: 



A a 

 Bb 



During spermatogenesis, this type of germ 

 cell can produce only four kinds of sperm, 

 namely: 



A 

 B 



A 

 b 



a 

 B 



These sperm will be produced in equal num- 

 bers, since each chromosome has the same 

 chance of finding its way into any of the 

 gametes. Similar!}, oogonia with two pairs of 

 chromosomes can produce only four kinds of 

 eggs, with corresponding combinations of 

 chromosomes. 



But if the spermatogonia (or oogonia) 

 possess three pairs of chromosomes, namely: 



A a 

 Bb 

 Cc 



the following eight kinds of gametes will be 

 produced in equal numbers: 



A a A ii A a A a 

 B b B b b B b B 

 C c c C C c c C 



When an individual animal produces eggs 

 or sperm, therefore, the several chromosomes 

 that were received from its maternal and 

 paternal parents are free to assort themselves 

 at random in the next generation of gametes; 

 however, one member of every pair of chro- 

 mosomes is always (normally) represented in 

 every gamete produced. 



In plants, ihe origin of the haploid gam- 



etes is less direct than in animals, but essen- 

 tially the same system of transmitting chro- 

 mosomes from parent to offspring is at work. 

 Among plants, fertilization does not take 

 place immediately after meiosis. Instead, a 

 whole haploid generation, the gametophyte 

 generation, intervenes — and the diploid con- 

 dition is not restored until the gametes unite, 

 forming a zygote and initiating the sporo- 

 phyte generation. This peculiarity, however, 

 does not fundamentally alter the processes of 

 heredity, which are essentially similar in all 

 sexual organisms. 



BREEDING EXPERIMENTS 



The first accurately controlled and thor- 

 oughlv documented breeding experiments 

 were published in 1866 by Gregor Mendel. 

 This Austrian monk worked with garden 

 peas: and by strictly controlling the pollina- 

 tion of his plants, Mendel discovered a well- 

 defined pattern that governed the transmis- 

 sion of a number of hereditary features, such 

 as color, height, hardiness, etc., throughout 

 man}' successive generations. The importance 

 of Mendel's experiments was not recognized, 

 however, until about 1900. By this time 

 much more had been learned about chro- 

 mosomes; and now biologists were ready to 

 recognize the crucial role of the chromo- 

 somes in the fulfillment of the Mendelian 

 laws. 



Genetic experiments presuppose a very ac- 

 curate knowledge of the stocks that are to be 

 crossed. The aim is to study the transmission 

 of single hereditary differences, either sepa- 

 rately or in combination, bv crossing two 

 stocks and determining the numerical dis- 

 tribution of the hereditary peculiarities 

 among the offspring. But unless the number 

 of differences is relatively small, an analysis 

 of the results becomes very complex. Conse- 

 quently ii was most fortunate that Mendel 

 began his work with a self-pollinating species 

 of plant, in which the original stocks pos- 

 sessed a high degree of genetic homogeneity. 



Modern genetics owes a great debt to a 



