According to Msgr. Peter J. Elliot in Ceremonies of the Modern Roman Rite (Ignatius Press), genuflection on one knee is reserved for: 1) Our Lord present in the Eucharist on the altar, in the tabernacle, monstrance or pyx; 2) the cross during its veneration on Good Friday until the beginning of the Easter Vigil; 3) a relic of the true cross exposed for veneration.
A bow of the body (deep bow from the waist) is made to the altar if the Blessed Sacrament is not on it or behind it. A bow of the head is to be made at the mention of the three Divine Persons, at the names of Jesus and Mary and the saint in whose honor the liturgy is being celebrated.
1. All genuflect (bending both knees) when adoring the Blessed Sacrament unveiled, as at Expositions.
2. All genuflect (bending the right knee only) when doing reverence to the Blessed Sacrament, enclosed in the Tabernacle, or lying upon the corporal during the Mass. Mass-servers are not to genuflect, save when the Blessed Sacrament is at the altar where Mass is being said (cf. Wapelhorst, infra). The same honour is paid to a relic of the True Cross when exposed for public veneration.
3. The clergy in liturgical functions genuflect on one knee to the cross over the high altar, and likewise in passing before the bishop of the diocese when he presides at a ceremony. From these genuflexions, however, an officiating priest, as also all prelates, canons, etc., are dispensed, bowing of the head and shoulders being substituted for the genuflexion.
4. On Good Friday, after the ceremony of the Adoration of the Cross, and until Holy Saturday, all, clergy and laity alike, genuflect in passing before the unveiled cross upon the high altar.