Homozygous means the "same zygotes"
Home = Same
Zygous = Zygote
Heterozygous means the "different zygotes"
A zygote is basically a fertilized egg, and I won't go too far in depth to that because it would be going even further from your question.
To tell if a gene is homo or hetero, you look for what is the same and what is different.
A homozygous gene would either be all capitals, or all lowercase lettering.
A heterozygous gene would have a mix.
Homo:
Extension: EE or ee
Agouti: AA or aa
Creme: CC or cc
Roan: RnRn or rnrn
Dun: DD or dd
Pearl: PrlPrl or prlprl
Silver: ZZ or zz
Champagne: ChCh or chch
Grey: GG or gg
White: WW or ww
Tobiano: ToTo or toto
Splash White: SplSpl or splspl
Sabino: SbSb or sbsb
Rabicano: RbRb or rbrb
Frame: OO or oo
Leopard Complex: LpLp or lplp
PATN-1: PATN-1PATN-1 or patn-1patn-1
PATN-2: PATN-2PATN-2 or patn-2patn-2
Hetero:
Extension: Ee
Agouti: Aa
Creme: Cc
Roan: Rnrn
Dun: Dd
Pearl: Prlprl
Silver: Zz
Champagne: Chch
Grey: Gg
White: Ww
Tobiano: Toto
Splash White: Splspl
Sabino: Sbsb
Rabicano: Rbrb
Frame: Oo
Leopard Complex: Lplp
PATN-1: PATN-1patn-1
PATN-2: PATN-2patn-2
**Brindle isn't accounted for because it's either X+ (Brindle) or X- (No Brindle)
Warming, with Sleet and Heavy Icing
