Summon Draconic Spirit-resistance and Breath Weapon

[Confused Dad]

My 10th level Wizard casts Summon Draconic Spirit and makes a number of choices:

You call forth a draconic spirit. It manifests in an unoccupied space that you can see within range. This corporeal form uses the Draconic Spirit stat block. When you cast this spell, choose a family of dragon: chromatic, gem, or metallic. The creature resembles a dragon of the chosen family, which determines certain traits in its stat block. The creature disappears when it drops to 0 hit points or when the spell ends.

The creature is an ally to you and your companions. In combat, the creature shares your initiative count, but it takes its turn immediately after yours. It obeys your verbal commands (no action required by you). If you don’t issue any, it takes the Dodge action and uses its move to avoid danger.

DRACONIC SPIRIT
Large Dragon, Neutral

Armor Class 14 + the level of the spell (natural armor)

Hit Points 50 + 10 for each spell level above 5th (the dragon has a number of Hit Dice [d10s] equal to the level of the spell)

Speed 30 ft., fly 60 ft., swim 30 ft.

STR
19 (+4)
DEX
14 (+2)
CON
17 (+3)
INT
10 (+0)
WIS
14 (+2)
CHA
14 (+2)
Damage Resistances (Chromatic and Metallic Only) acid, cold, fire, lightning, poison

Damage Resistances (Gem Only) force, necrotic, psychic, radiant, thunder

Condition Immunities charmed, frightened, poisoned

Senses blindsight 30 ft., darkvision 60 ft., passive Perception 12

Languages Draconic, understands the languages you speak

Challenge — Proficiency Bonus equals your bonus

Shared Resistances. When you summon the dragon, choose one of its damage resistances. You have resistance to the chosen damage type until the spell ends.

Actions

Multiattack. The dragon makes a number of Rend attacks equal to half the spell’s level (rounded down), and it uses Breath Weapon.

Rend. Melee Weapon Attack: your spell attack modifier to hit, reach 10 ft., one target. Hit: 1d6 + 4 + the spell’s level piercing damage.

Breath Weapon. The dragon exhales destructive energy in a 30-foot cone. Each creature in that area must make a Dexterity saving throw against your spell save DC. A creature takes 2d6 damage of a type this dragon has resistance to (your choice) on a failed save, or half as much damage on a successful one.

1: family: chromatic, metallic, gem
2: Draconic resistances: pick one of the resistances to share with me
3: type of breath weapon damage

I see nothing that links 2&3.

We are facing a Mummy doing necrotic damage and vulnerable to fire. IMHO, I can choose Gem, resistance to necrotic, and force damage breath weapon. My DM read that as “pick a resistance, you gain that AND the dragon has that as its breath weapon.

I couldn’t find a Sage Advice for this question and it may be moot in DNDOne, but does anyone know of an official (or WOTC published unofficial) ruling on this? Opinions?

[Amnestic]

You are correct. While personally I think it's more thematic to have the same resistance+breath weapon damage types, there's no need to do so in the spell text. The dragon can also change its breath weapon damage type each turn freely - there's nothing to indicate it's set in stone from when you summon it. The dragon's family and caster's resistance are 'set' on summoning, the breath weapon is not.