On the mode of Inheritance of certain Characters etc. 303 
assumption of partial coupling between the two pairs of allelomorphs 
of such a nature, that the combinations inherited (PW and pw) occur 
among the gametes with a frequency greater than the recombinations 
representing a mixture of characters derived partly from one parent 
and partly from the other (pw and Pw). When the mating is between 
individuals differing not only in plastid colour and type of sap-colour, 
but also in the double-throwing character, then it is seen that the 
factors for all three characters exhibit this kind of coupling. For 
example, when a non-double-throwing form with cream plastids which 
did not contain P was crossed with a double-throwing form with 
white plastids containing P (XYpw x xyPW), then it was found that 
the F, doubles were most (if not all) plants with white plastids, and 
almost always with pale sap-colours, when coloured; while the plants 
with cream plastids were most (if not all) singles and nearly always 
with full sap-colours when coloured. In a mating of this type where 
nearly 1000 plants were raised in F, the numbers obtained for sap- 
coloured individuals were as follows: 
Form of Mating. 
Non-double-throwing single with cream plastids not containing P x 
Double-throwing-single with white plastids containing P 
Proportion of sap-coloured forms obtained in F, | Singles | Doubles 
Balersan-eolours, with white plastids’: 2... « <<. «ss « « 354 234 
Full , 5 n Ae) oe ol sa. Ars. Rare 15 7 
Pale „ “ cream Hel OS ome ery hoe oy ete 16 = 
Full „ 5 A yo So onan mono 93 | = 
There appears then to be clear evidence that coupling of the nature 
of that suggested as occurring between X and Y exists in the case 
of other factors in Stocks. 
The conclusion given under (3) (inability of the pollen of ever- 
sporting types to carry X and Y) is capable of direct proof. We can 
test it by crossing together non-double-throwing single © and double- 
throwing single 3. If every F, individual yields doubles in F, then we 
may conclude that every pollen grain used to produce F, was carrying 
doubleness. The evidence on this point has already been given(?) 
and all later obtained facts are in complete agreement with this view. 
It may then be claimed that the scheme I have put forward 
provides an explanation of the behaviour of double-throwing Stocks, 
(1) See Journ. of Genetics, I9II, pp. 306—311. 
