Page:Popular Science Monthly Volume 62.djvu/277

Nothing has been said so far concerning the external appearances of the hybrid, whether it is intermediate between the parents, or resembles one or the other of them. The efforts of investigators since the time of Kölreuter have been directed to the futile attempt to find some law which would enable the breeder to predict the appearance of this hybrid. In general, this can not be done, with our present knowledge. There are two cases to consider. In certain instances hybrids are strictly intermediate between the parents. In others they are unlike either parent. These cases will be noticed later. In the more common case the hybrid either shows a parent character fully developed or shows it not at all. A parent character which is fully developed in the hybrid is said to be 'dominant'; if it is apparently absent it is said to be 'recessive.' In my work with hybrid wheats, beards have always been recessive, hence the designation of the hybrid as Sb.

It will now be seen that, externally, the progeny of the hybrid will consist of only two types, one type (B), constituting one fourth of the progeny, being like one parent, and the other, constituting three fourths of the progeny, resembling the other parent. Of this three fourths, one part (S) is actually like one parent, and will produce progeny like itself. The other two parts (Sb) are hybrids, and will produce progeny of all the types, exactly as the original hybrid did. Plants of the type S may easily be separated from those of the type Sb by planting the seed of each plant separately, and noting the character of the progeny.

The above diagram may easily be extended to any desired number of generations. Extended to the third generation it is:

B	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right\}\,}$			$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	B		$\scriptstyle {\left\{{\begin{matrix}\ \end{matrix}}\right.}$	B
							$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	B
		Sb			Sb			Sb
								S
S					S		$\scriptstyle {\left\{{\begin{matrix}\ \end{matrix}}\right.}$	S

Assuming that each of the types S, Sb and B are equally productive, and mixing and sowing all the seed of each generation, the following table shows the percentage of each type in each generation to the sixth:

Generations.		S			Sb			B
1					100
2		25			50			25
3		37	.5		25			37	.5
4		43	.75		12	.5		43	.75
5		46	.87		6	.25		46	.87
6		48	.44		3	12		48	.44

Here it is seen that the hybrid, based on a single pair of antagonistic characters, tends to split up into the two parent types. A hybrid