Blood traits, such as complete blood count and fetal hemoglobin, are implicated in various physiological and pathological processes. The value of these traits is variable from one individual to another and a significant part of this variation that is due to genetic factors that have been identified. It has been shown in adults that these genetic factors impact patients’ management. However, their impact is poorly known in children. Per example, whether lower white blood count have an impact on infection risk and work up and whether a mildly lower hemoglobin level impairs growth is unclear. Moreover, whether the impact has a causal relationship is unknown. Finally, fetal hemoglobin is a type of hemoglobin specific to fetus that has been selected throughout evolution to favor oxygen delivery to the newborn as it ties more strongly to oxygen than the adult hemoglobin from the mother. As some fetal hemoglobin may persist in adults, the difference between mother and child fetal hemoglobin varies across pregnancies. However, the impact on weight at birth and later in life of this difference remains to be studied. Here, we propose to leverage state-of-the-art genetic approaches to address these two aspects blood traits. We will used the genetic variations known to be associated to blood traits to study their effect throughout childhood without the need of direct measurement. Deciphering the impact of blood traits in children will improve our understanding of their effect and may provide powerful tools to tailor patients’ management toward a personalized, precision medicine.