 Minimal anomaly-free chiral family in SU(6)

The SU(6) representations $15={\Lambda }^{2}6$ and $\overline{6}$ branch under SU(5) as $10\oplus 5$ and $\overline{5}\oplus 1$. Their cubic gauge anomaly coefficients are $2$ and $-1$ in fundamental normalization. Thus $15\oplus 2\overline{6}$ is anomaly-free and gives one Standard Model family $10\oplus \overline{5}$, a vectorlike gauge spectrum $5\oplus \overline{5}$, and two singlets. With these representations and their conjugates, precisely one net chiral family requires $m_{15}-m_{\overline{15}}=1$ and $m_{\bar{6}}-m_6=2$, so this is the smallest such choice.