Table 4.

Intraclass correlation coefficients (ICC*), classic heritability estimates (h^{2 †}), and common environmental variance (CEV^‡) for spindle characteristics in monozygotic (MZ) and dizygotic (DZ) twin pairs, adjusted^§

Characteristic	Detector	ICC_MZ	ICC_DZ	h2	CEV
Duration	a2	0.73	0.10	1.25	–0.52
	a3	0.79	0.12	1.34	–0.55
	a4	0.64	0.17	0.94	–0.30
	a5	0.74	0.18	1.13	–0.39
	a7	0.63	0.41	0.45	0.19
	a8	0.79	0.13	1.30	–0.52
	a9	0.73	0.41	0.64	0.09
	a10	0.66	0.64	0.05	0.61
Amplitude	a2	0.74	0.58	0.34	0.41
	a3	0.70	0.51	0.39	0.31
	a4	0.67	0.48	0.38	0.30
	a5	0.69	0.50	0.39	0.30
	a7	0.64	0.53	0.24	0.41
	a8	0.73	0.45	0.56	0.17
	a9	0.60	0.51	0.19	0.41
	a10	0.66	0.51	0.30	0.35
Frequency	a2	0.70	0.50	0.41	0.29
	a3	0.80	0.58	0.43	0.37
	a4	0.75	0.57	0.36	0.39
	a5	0.78	0.64	0.29	0.49
	a7	0.80	0.69	0.23	0.57
	a8	0.79	0.51	0.57	0.22
	a9	0.85	0.68	0.35	0.50
	a10	0.75	0.53	0.44	0.31

Heritability is calculated using Falconer’s method; values closer to one indicate a pattern expected where variance in a trait is genetically controlled. Negative CEV values suggest that common environmental effects are likely not very important and that genetic dominance effects may be present.

*ICC = σ ²B/[σ ²W + σ ²B].

^†  h  ², heritability = 2(ICCMZ – ICCDZ).

^‡ CEV = (2 × ICCDZ) – ICCMZ.

^§ Adjusted for age, sex, and age by sex interaction.