About 10% of people are left-handed. The genetic component is real but small — handedness is also influenced by the baby’s position in the womb and prenatal hormones.
About 10% of people are left-handed, a remarkably stable figure across most populations studied (Papadatou-Pastou et al. 2020 meta-analysis of nearly 2.4 million people). The two-parents-righty / one-parent-lefty / two-parents-lefty pattern of ~9.5% / ~19.5% / ~26% comes from large twin and family studies — heritability sits around 25%, lower than most people guess.
GWAS work has identified specific contributing variants — PCSK6 and LRRTM1 are the best-characterized — but no single "handedness gene." Prenatal factors (intrauterine position, birth weight, twin status) and developmental noise account for the rest.
Practical part: handedness doesn't reliably emerge until age 3–4. Don't try to push a baby toward right-handedness with feeding or play — they'll figure it out.
Sources: Papadatou-Pastou M et al. Psychol Bull 146:481–524 (2020), "Human handedness: a meta-analysis"; Brandler WM & Paracchini S. Trends Mol Med 20:83–90 (2014), "The genetic relationship between handedness and neurodevelopmental disorders."