= Solution
The left- and right-chiral components of a charged <Standard Model fermion> transform in different electroweak <group representations>. A bare <Dirac mass term> $-m\bar f_Lf_R+\mathrm{h.c.}$ is therefore not invariant under the full electroweak <gauge group>. The <Higgs doublet> supplies the missing transformation properties. With $\widetilde H=i\sigma_2H^*$, the gauge-invariant <Yukawa interactions> are
$$
\mathcal L_Y=-\bar Q_LY_dHd_R-\bar Q_LY_u\widetilde H u_R
-\bar L_LY_eHe_R+\mathrm{h.c.},
$$
where each $Y_f$ is a flavor-space <Yukawa matrix>. For example, hypercharges in the down-quark term add to $-1/6+1/2-1/3=0$, while those in the up-quark term add to $-1/6-1/2+2/3=0$. Thus the Higgs and conjugate Higgs are required in different terms.
After <electroweak symmetry breaking>, choose <unitary gauge> with $H=(0,(v+h)/\sqrt2)^T$. The <fermion mass matrices> become
$$
\boxed{M_d=\frac{v}{\sqrt2}Y_d,\qquad M_u=\frac{v}{\sqrt2}Y_u,\qquad M_e=\frac{v}{\sqrt2}Y_e.}
$$
A <singular value decomposition> $U_{fL}^\dagger M_fU_{fR}=\operatorname{diag}(m_{f,1},m_{f,2},m_{f,3})$ defines nonnegative physical tree-level masses. The mismatch $U_{uL}^\dagger U_{dL}$ is the <Cabibbo-Kobayashi-Maskawa matrix> in the <weak charged current>. The corresponding physical <Higgs boson> couples diagonally as $-(m_f/v)h\bar ff$ in the minimal one-doublet theory. This relates its coupling strength to each <mass>, but the measured <Yukawa couplings> remain independent input parameters: the minimal theory does not predict their hierarchy.
The minimal renormalizable <Standard Model> contains no <right-handed neutrino>, so it has neither a neutrino Dirac Yukawa term nor a gauge-invariant renormalizable left-handed <Majorana mass term>. Its <neutrinos> are massless. Observed <neutrino oscillations> require going beyond that minimal field content or operator set: adding right-handed singlets permits <Dirac masses>, while the dimension-five <Weinberg operator> gives a <Majorana neutrino> <mass> of order $v^2/\Lambda$ after symmetry breaking. These alternatives should be distinguished from the charged-fermion Yukawa mechanism.
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