Independent Research Project

Conor Miles HONORS 220 B 9 December 2018

Foot length heritability is more limited than the heritability of other physiological traits, according to limited survey

Abstract This study set out to assess the heritability of shoe size and determine whether foot size is passed down from parents to offspring in a similar manner to height. To do this, I constructed a survey that would gather participants’ sex and foot size as well as the foot sizes of their biological parents. Based on what little data I gathered, the offspring’s sex did not seem to relate to their foot size compared to their parents. There was also only a slight positive correlation between parent and offspring foot size across all sex groups, indicating low heritability. However, the survey received 21 responses—not enough to develop a particularly reliable conclusion.

Introduction Physiological proportions tend to be heritable; though slight variation between offspring is possible through random mutations and environmental differences, with heritability accounting for 78% of height in adult men and 75% in women (Silventoinen et al. 2000). However, height can be influenced by environmental factors such as stress and diet. There’s no doubt, then, that other physiological traits might be similarly influenced by the environment— perhaps even more radically. In this study, I examine whether foot size is heritable in the same manner as height. I hypothesize that an individual’s foot size will display signs of heritability, with covariance between parents’ and offspring’s’ measurements, with slight variation between sexes. This is because similar studies have established the heritability of body proportions tend to be high and established sexual dimorphism in proportions.

Methods I conducted this study through a survey made in Google Forms. The survey contained four questions: participant sex, participant foot size, biological mother’s foot size, and biological father’s foot size. All questions required participant completion prior to submitting the survey; this ensured that complete data could be collected about each participant and their parents. The survey questions were designed to be easily understood, with participants selecting shoe size instead of actual foot length; while height is relatively easy to measure and, for many people, common knowledge, feet measurements are neither. Originally, I attempted to translate the shoe sizes into percentiles, but was unable to find any prior research or data that would help me do so. In light of this, once survey results were finalized, I instead translated them into approximate millimeter measurements using foot size conversion charts. This allows any sexual dimorphism

between respondents to be visible while also standardizing the foot sizes for more accurate comparison. I then compiled all of the data into an Excel document which allowed me to create scatterplots of the data more easily. If my hypothesis was correct, there would have been a clear covariance in the responses between parent and offspring foot size. Additionally, I would expect to see some sexual dimorphism in the form of one sex with larger average foot size than the other. If my hypothesis was incorrect, I would have seen little correlation between offspring and parents’ feet sizes, as well as little difference between sexes.

Results The survey yielded 21 responses—6 from male participants and 15 from female participants. The results are displayed in Figures 1-3. Fig 1 is a scatterplot of the results grouped into four categories: female participant vs mother, female participant vs father, male participant vs mother, and male participant vs father. A line of best fit was also assigned to each of the four groups using Excel. While there is slight covariance observable in all four groups, there is also noticeably not enough data to make any decisive conclusions. This is especially apparent in the group with the highest covariance—male participants vs fathers—as the line of best fit would never intersect with the origin. Despite the limited results, there is also a clear difference between male and female participants, with males having larger feet sizes. Fig 2 is another scatterplot that compares participants’ foot length to their parents’ foot length averaged—also the median between the two parents. The data displayed here also demonstrates slight positive correlation; the relationship here is admittedly more reliable because all responses were combined into one graph rather than separated out as they were in Fig 1. Fig 3 is a table of average foot length of participants and both parents across participant sexes. This table’s purpose is mainly to assess the sexual dimorphism of foot length and rule out the possibility of

differences due to parents—not participant sex. The table does indicate sexual dimorphism; while female participants’ parents had larger feet on average, the participants had smaller feet than their male counterparts.

Discussion Because the survey only garnered 21 responses, any conclusions drawn from its data must be tentative. The two main conclusions that come from the data are that foot length is a sexually dimorphic trait and that it is also slightly heritable. While this study was able to capture foot size data well, it was also too limited to be reliable. Ideally, the study can be replicated at some point in the future with a much larger number of responses. The main question that arises from the results is: what non-genetic factors influence feet size? To investigate this question, one would need to gather substantially more data and, moreover, put more controls in place. In such a study, one could assemble a group of participants with parents whose feet size are the same as each other—meaning every father would have the same foot size and every mother would have the same foot size. Once this is controlled for, one could examine differences in the environments the individuals grew up in, what their socioeconomic status was growing up, etc. in an exploratory study. DNA samples could also be taken from the subjects and analyzed to see if any genetic differences could have contributed to meaningful differences in foot size.

Figures/Tables Figure 1: Full Results

Foot Size Comparison Between groups Parent foot length (mm)

320 300 Female-Mother

280

Female-Father Male-Mother

260

Male-Father

240

Linear (Female-Mother) Linear (Female-Father)

220

Linear (Male-Mother) Linear (Male-Father)

200 200

220

240

260

280

300

Offspring foot length (mm)

Figure 2: Mean Comparison

Parent foot length averaged (mm)

Foot Size Comparison Between groups 290 280 270 260 250 240

All-average

230

Linear (Allaverage)

220

This graph displays a scatterplot of survey results, with the offspring’s foot size mapped on the x-axis and the parent’s foot size mapped on the y-axis. There is at least a slight positive correlation visible across all groups, with clear sexual dimorphism between respondents— indicated by the magenta/violet dots versus the blue dots.

210

This graph compares the average/median foot length of each participants’ parents to the participants’ foot length. There is a visible positive correlation between the participants’ actual foot size and their parents’ mean foot size, indicating some degree of inheritance.

200 200

250

300

Offspring foot length (mm)

Feet length averages (mm) Respondent

Parents

Male Respondents

274

259

Female Respondents

245

262

Figure 3: Foot Length Averages Across Groups This table displays the average foot length in mm for all participants and their parents grouped by participant sex. While the parents’ foot sizes across both sexes were not significantly different, male participants—on average—had much larger feet than female participants.

Research Cited Silventoinen, K, et al. “Genetic and Environmental Contributions to the Association between Body Height and Educational Attainment: a Study of Adult Finnish Twins.” Current Neurology and Neuroscience Reports., U.S. National Library of Medicine, Nov. 2000, www.ncbi.nlm.nih.gov/pubmed/11523706/.