Degree of Dominance.

An estimate of 1.64 for tho degree of dominance of genes affecting yield of grain in prolific corn was obtained by Robinson, et al. (Agron. Jour. 41: 353‑359, 1949). C. O. Gardner (unpublished), using a different procedure, has recently obtained an estimate of 1.51 for the genes conditioning the same character. These estimates involved the ratio of the variance due to dominance deviations,

s	2	,
	d

and the additive genetic variance

s	2	.
	g

The estimates were computed as

2s	2	.
	d
s	2
	g

With gene frequencies (q) of 1/2 (as obtained in the unselected populations from a cross of homozygous lines used in the work cited above) and no linkage effects

2s	2	= 1.0
	d
s	2
	g

if dominance is complete at all loci and greater than 1.0 if there is a preponderance of overdominance. Recent work at this institution has indicated that certain linkage effects in the material used in these investigations may result in values of the magnitude obtained with no more than complete dominance.

If dominance is complete but

q > ½,	2s	2	>1.0.
		d
	s	2
		g

Hence assuming q's over ½ in open‑pollinated varieties (Crow, J. F., Genetics 33:477‑487, 1948) one would anticipate

2s	2
	d
s	2
	g

considerably above 1.0 even with complete dominance. Any linkage out of phase equilibrium would further increase the ratio.

Additive genetic variance and dominance variance have been estimated for yield of grain from an extensive investigation in 1950 utilizing biparental crosses in two open‑pollinated varieties of corn. The ratio

2s	2	.
	d
s	2
	g

was 1.01 in Jarvis and 1.15 in Weekley. These estimates would indicate canplete dominance if one assumed gene frequencies were ½, and no more than partial dominance if the intensive selection to which these varieties have been subjected has resulted in the accumulation of more favorable genes for yield.

H.F. Robinson, R.E. Comstock, and P.H. Harvey