The concept of the second to fourth digit ratio (2D:4D) may not be familiar to everyone. But it is the distance between the second finger and the fourth finger. The 2D:4D is calculated by dividing the length of the index finger of a given hand by the length of the ring finger of the same hand. Ecker first identified this measurement in 1875 and then rediscovered by Wilson in the early 1980s. This is relevant because this measurement is often used as one indicator of how much testosterone is present and is helpful to tell the biological sex.
Many researchers have adopted 2D:4D as a noninvasive way to check sex hormones while in utero. Over 300 papers using this measure have been published in the last 10 years, and 2D:4D has been shown to correlate with a wide range of diseases and physiological and psychological traits, such as autism, attention deficit disorder, visuo-spatial ability, athletic performance and age at menarche. However, little progress has been made in identifying the individual variants underlying genetic variation related to this trait.
Thus, researchers performed a genome-wide association study in 1,507 11-year-old children from the Avon Longitudinal Study of Parents and Children (ALSPAC) and a sample of 1,382 12- to 16-year-olds from the Brisbane Adolescent Twin Study. In both samples, participants’ hands were photocopied and measurements of second and fourth finger were taken during a clinical visit A meta-analysis of the two scans identified a single variant (rs314277) in the LIN28B gene that was significantly associated with 2D:4D. Furthermore, each copy of the minor allele (A allele) of the rs314277 was associated with a 0.6 increase in mean 2D:4D for the right and the left hands. The effect was subsequently independently replicated in an additional 3,659 children from the ALSPAC cohort. In addition, rs11264329 in the EFNA1 gene, rs340600 in the C2orf43 gene and rs7837520 showed a suggestive relationship with mean 2D:4D.
The association between rs314277 in LIN28B and 2D:4D is particularly interesting because the minor allele was previously associated with increased height and a delayed initial period in females.
The direction of effect in these studies is consistent with the overall correlation between the variables. Girls who experience periods later tend to be taller as adults than girls who reach puberty earlier. In this study, however, the minor allele at rs314277 was associated with an increased 2D:4D ratio, which is opposite the direction predicted by these earlier reports. It is unclear why this might be the case, but researchers suggested that the relationship between 2D:4D, age at menarche, and height was complex. Read more about the study here: https://pubmed.ncbi.nlm.nih.gov/20303062/
Are you interested in learning more about your genetic tendency for your ratio of second to fourth digit lengths? You can login to your Genomelink YOUR TRAITS to see this new genetic trait.