Inheritance Issue with Json Serializable

/////////////////////////////////////////////////////////////////////////////
"This is the implementation of my classes"

@JsonSerializable()
class BodyGroup{

final Map parts;

BodyGroup(this.parts){
debugPrint("Constructor called and initialized!:"+parts.toString());
}

//Abstract function
Map tabMap(){
return {};
}

void incrementTabMap(Map tabMap){
tabMap.forEach((String part, double hits) {
debugPrint("TabMap:"+tabMap.toString());
debugPrint("Part:"+part);
debugPrint("Parts:"+parts.toString());
if(parts[part])
tabMap[part] = tabMap[part] + 1;
});
}

factory BodyGroup.fromJson(Map data) => _$BodyGroupFromJson(data);

Map toJson() => _$BodyGroupToJson(this);
}

@JsonSerializable()
class Shoulders extends BodyGroup{

Shoulders({bool front=false,bool rear=false}) : super({"front":front,"rear":rear});

factory Shoulders.fromJson(Map data) => _$ShouldersFromJson(data);

Map toJson() => _$ShouldersToJson(this);

@override
Map tabMap() {
return {
"front":0,
"rear":0
};
}

}

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Now when I make an object of Shoulder class and serialize it......it doesn't include the "parts" map which is initialized always in the BodyGroup Parent class....

Can you please help me out here