It is one thing to look like your parents, but it is quite another to be as tall as them. Have you ever noticed that in spite of obvious similarities in body shape, facial expressions and complexion, you might not necessarily reach the same height as your parents? This article aims to deal with this unexpected outcome.
As you might have guessed, height is only partially determined by genetic heritage. It is true to say that 80% of an individual’s height is determined by his or her DNA, but the same DNA does not purely result from parents’ genetics. Firstly, this is because it is a 50-50 chromosomic deal. Secondly, it is due to that fact that we can transmit genes that were passive in our genetic makeup. Finally, it is because genetic combinations are comparable to winning the lottery.
Most importantly, there are also environmental factors. They affect height development in children according to their socio-economic conditions, with nutrition being a crucial factor among others.
I was interested to find that you can calculate an estimate of your height based on how tall your parents are and your given gender. Dr. Swati Sharma from Denville, a New Jersey based endocrinologist, suggests summing your parents’ heights, then adding 5 units, for males, or subtracting 5 units for females, and then dividing the total in half.
In my case, that only applies to one of my siblings. The rest of us are way smaller than we should be according to this method. Who, or rather what, should we blame?
Well, because height is determined by a series of gene variants, different combinations lead to siblings having distinct heights. Most importantly, other biological mechanisms come into play. Specifically, hormones entail pretty unpredictable mechanisms.
In fact, environmental factors really seem to weight in the balance. Factors such as the nutritional status of your mother during pregnancy, whether she smoked, and her exposure to hazardous substances might affect how, as an embryo and so later a child, you are to grow and evolve.
As an embryo, there is a noticeable phenomenon of ‘crowding within the uterus’. Unexpectedly, this might contribute to height discrepancy between parents and children. This shows that environmental factors actually play a role before birth itself.
In the same way, the physical conditions of those who carry the baby matter. Major factors such as illness, malnutrition or therapeutic drug treatment might inhibit the fetus’ growth. So, there is a cumulative aspect to environmental factors both coming from the mother and the baby. Both a mother’s inappropriate diet during pregnancy, and a deficiency of protein or calories in a child’s diet can lead to growth inhibition.
Indeed, lack of dietary protein during childhood will be the most important environmental factor affecting final height. Minerals, in particular calcium, and vitamins A and D intake will also influence height. Similarly, later on during adolescence, eating disorders affecting caloric intake will reduce or delay pubertal growth spurt.
It is important to note that what is generally referred to as ‘growth’ does not only affect the child’s height but also brain development.
Maternal malnutrition is linked to a reduction in brain development of a fetus. In particular, severe consequences are to be feared when the mother consumes alcohol, smokes cigarettes, or in general suffers from addiction and substance abuse. Such habits may affect the weight and height of babies, just as the maternal age and order of birth would. Smoking in particular will increase the risk of prematurity.
Unfortunately, an exemplary pregnancy won’t seal the deal. The growth of children and adolescents’ nutrition will also depend on illness, socio-economic status, urbanization, physical exercise, climate, and psychosocial deprivation.
Therefore, education is crucial in ensuring a steady and healthy development in children. Teaching children to exercise, lead an active lifestyle, and to eat properly, is certainly determinant of height development.
That being said, not all get to benefit from good nutrition or get to experience healthy habits. As a consequence, those from less developed countries see a lesser equivalence between their parents’ height and theirs, primarily because of malnutrition.
Although environmental factors such as nutrition constitute a smaller contribution to growth compared to gene variants, they can be at the root of height disparity between ethnicities. Factors such as hunger, insalubrity and failing to address health issues will be decisive. These might be linked to climate, dietary habits and lifestyle of different ethnic populations.
This suggests that non-genetic factors will also create a difference in height between ethnicities. However, out of the 700 gene variants that have been discovered, some more concretely define growth by directly or indirectly affecting cartilage in growth plates.
Indeed, excessive exercise may impair growth by damaging growth plates. Whereas moderate exercise will allow an increase in growth through exercise-induced growth hormone response (EIGR). Specifically, long-interval exercise, or several exercise sessions a day may be beneficial.
In contrast, there is only a weak correlation between sleep duration and height. That being said, lack of sleep will easily lead to stress, potentially suppressing the secretion of the human growth hormone.
It is important to note that most of these studies are based on Western populations, and measurement errors come into play. For this reason, a Western tradition of twin studies elucidating environmental and genetic influences, might need to be set aside here. They might not be the most reliable to estimate heritability of height in infancy. The twin-to-twin transfusion syndrome is a good example of this.
During these rare pregnancies, identical twins share one placenta. A donor twin gives away more blood than it receives in return and runs the risk of malnourishment and organ failure. Meanwhile, the recipient twin receives too much blood and his or her heart is overworked, risking cardiac complications.
In the same way, data reporting that there are greater heritability estimates for males than for females in childhood and adulthood, and a greater mean height in Western populations (80%) compared with East-Asian and African populations (65%) should be carefully cross-referenced.