The Organism as a Whole, from a Physicochemical Viewpoint

by Jacques Loeb

As has been stated, it not infrequently happens that all the hybrids of the first genera­tion are alike. In such cases the one character is “recessive,” i. e., overshadowed or covered by the other the “dominant” character, which alone appears in the hybrids. Thus when Mendel crossed peas having round seeds with peas having angular seeds all the hybrids had round seeds. The round form is dominant, the angular recessive, i. e., all the hybrids have round seeds. When these hybrids were bred among themselves the next genera­tion produced round and angular seeds in the ratio of 3:1 (5474 round to 1850 angular). The explana­tion is as follows. Let R denote round, A angular character; the pure breeds of parents have the gametic constitu­tion RR and AA respectively. When crossed, all the offsprings have the constitu­tion RA and since A is recessive this hybrid genera­tion resembles the pure RR parents. The F₁ genera­tion produces two kinds of eggs R and A and two kinds of pollen R and A in equal numbers, and these if inbred give the following four combina­tions in equal numbers:

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]

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.

According to this theory, all the cells of an individual (inclusive of the egg cells and sperm cells) have two sets of homologous chromo­somes, one from the father, the other from the mother. Before the egg and sperm are ready for the produc­tion of a new individual, each loses one set of homologous chromo­somes in the so-called reduc­tion division, but the lost set is made up indiscriminately of maternal as well as paternal chromo­somes, so that while one egg retains the maternal chromo­some A the other will retain the paternal one, and so on. If before the reduc­tion division all the eggs had the chromo­some constitu­tion AA₁, BB₁, CC₁, DD₁ (where A B C D are the paternal and A₁ B₁ C₁ D₁ the maternal chromo­somes), after the reduc­tion division each daughter cell has a full set of four chromo­somes, but maternal and paternal mixed. Thus the one cell may have AB₁CD₁, the other A₁B₁C₁D₁, etc. This, according to Sutton, is the basis of the Mendelian heredity. Suppose the determiner of a certain character (violet colour of flower in the bean) is located in a chromo­some A of this species. The homologous chromo­some in beans with white colour may be designated as a. According to the chromo­some theory of Mendelian heredity a differs from A in one point, though this difference is probably only of a chemical character and not visible.

If an egg with A is fertilized by a pollen with a (or vice versa), after fertiliza­tion the chromo­some constitu­tion of the fertilized egg is Aa. All the other homologous chromo­somes are identical and therefore need not be considered. All the nuclei of the F₁ genera­tion have the chromo­some constitu­tion Aa. All will form eggs and pollen with nuclei of the same chromo­some constitu­tion Aa, but all these sex cells will go through the matura­tion division before they are fertilized; and this reduc­tion division leads to the existence of two kinds of eggs in equal numbers, one containing only the A, the other only the a chromo­some; and the same happens in the pollen. When therefore the hybrids F₁ are mated among themselves, the following four chromo­some combina­tions will be produced:

Possible combina­tions in fertilized eggs AA, Aa, aa, in the ratio 1:2:1.