Study 1: This paper is published in the Journal Twin Research
The relative importance of genetic influences (heritability) on five general textural quality characteristics of the human iris was assessed using sex and age limitation models. Colour photographs of irises were available from 100 monozygotic twin pairs, 99 dizygotic twin pairs, and 99 unrelated randomly paired age-matched German subjects. Comparative scales were constructed and two judges who were blind to zygosity independently rated five characteristic of the subjects’ left iris. Inter-rater reliabilities were larger than .90 for all five scales. The heritabilities for the five iris characteristics ranged from .51–.90. No sex-specific genetic factors were found for the iris characteristics. Age-group differences in heritability were found for one of the five iris characteristics — “distinction of white dot rings”. Heritability was greater for the older cohort (90%) than the younger (73%). The iris characteristics that showed the next highest additive-genetic effect were “contractional furrows” (78%) and “frequency of crypts” in the main stroma leaf (66%).
Study 2: This paper will soon be published in the Journal Molecular Vision
Purpose: TO estimate the magnitude of genetic correlation among five general textural characteristics of the human iris.
Method: Color photographs of iris were available from 100 monozygotic and 99 dizygotic twin pairs. Comparative scales were constructed based on ratings of the subjects’ left iris. To explore the genetic and environmental covariation among frequency of Fuchs’ crypts, frequency of pigment dots, iris color, the extension and distinction of Wolfflin nodules and contraction furrows, a structural equation model with Cholesky decomposition was applied to variance-covariance matrices for MZ and DZ pairs.
Result: Significant genetic correlations fell between -.22 and .44 and accounted almost entirely for the phenotypic correlations among the iris characteristics. No evidence for individual specific environmental effects in common to the characteristics was found.
Conclusions: The modest genetic correlations indicate that there is little overlap in the genetic influence for these characteristics. Candidate genes with embryological and histological expression patterns in the eye could potentially influence the iris characteristics’ variability.
Study 2: This paper will be published in the Journal Personality and Individual Differences during in the spring 2005
Genes expressed in the human iris influence the division of the brain into two hemispheres (Wallis & Muenke, 2000), number of neurons in cerebral cortex (Heins et al., 2002), and the production of dopaminergic (Stoykova & Gruss, 1994) and serotonergic neurons (Ding et al., 2003), indicating that characteristics of the iris could be associated with behavior. This study explored the associations between three previously untested general iris characteristics and personality. Personality data, as measured by the Five Factor Model and ratings of iris characteristics from 428 undergraduate students were collected. All domains of personality except Neuroticism were significantly related to frequency of Fuchs’ crypts, either as a effect alone or as an effect cause by an interaction between frequency of Fuchs’ crypts and another iris characteristics. Neuroticism had a interaction between pigment dots and Contraction furrows. Most of the interactions demonstrated cross-over effects with the frequency of Fuchs’ crypts and frequency of pigment dots. Future association studies may benefit from considering candidate genes for crypt frequency (Larsson & Pedersen, 2004) and pigment dots.