RR, RA, AR, AA. Since RA, AR, and RR all give round seeds the F₂ generation produces round seeds to angular seeds in the ratio of 3:1. The two organisms with the gametic constitution RR and RA look alike, yet they are different in regard to heredity. The gametically pure form RR is called homozygous, the impure form RA heterozygous. 2. W. S. Sutton[203] was the first to show that the behaviour of the chromosomes furnishes an adequate basis on which to account for Mendel’s law of the segregation of the characters in the sex cells of the hybrids. If we disregard the cases of parthenogenesis and the X chromosomes, we may state that each species is characterized by a definite number of chromosomes, _e. g._[204] [203] Sutton, W. S., “The Chromosomes in Heredity,” _Biol. Bull._, 1904, iv., 231. [204] Morgan, T. H., Sturtevant, A. H., Muller, H. J., and Bridges, C. B., _Mechanism of Mendelian Heredity_. New York, 1915, p. 26. man (probably) 24 corn 20 mouse 20 evening primrose 7 snail (_Helix hortensis_) 22 nightshade 36 potato beetle 18 tobacco 24 cotton 28 tomato 12 four o’clock 16 wheat 8 garden pea 7 In the fertilization of the egg the number of chromosomes is doubled (if we disregard for the moment the complication caused by the X and Y chromosomes which was considered in the previous chapter). It was noticed by Montgomery that each chromosome had a definite size and individuality, and he suggested that homologous chromosomes existed in sperm and egg and that in fertilization the homologous chromosomes of egg and sperm always joined and fused in the special stage designated as synapsis, which will interest us later. On the basis of this suggestion Sutton developed the chromosome theory of the mechanism of Mendelian heredity or segregation.