Genetics and Inheritance

Each a-life has 10 traits that it can inherit randomly from either of its parents. 4 of these are body part genes, and another 4 are genes that are linked to each body part. The remaining 2 are flight pattern and genetic behavior.

Body part and linked genes

These are 4 genes that determine what each of a nightopian or mepian's body parts look like. In order, they determine the torso, head, arm, and leg parts. Each one has three possible alleles, numbered 0, 1, 2, and 3 in the game's code. 0 codes for a normal nightopian body part, while 1, 2, and 3 correspond to one of the three types of nightmarens that can be found in each dream. These genes are inherited randomly and separately with a varying chance to get either parent's allele. A weighting system is used causing some parts to be more likely to be inherited from one parent than the other. 

Linked genes are inherited along with body parts. If an mepian inherited its torso part from a shleep, it will always inherit the shleep's lifespan value as well. The following table shows the values of linked genes speed, vision, and lifespan for each nightmaren type, along with that nightmaren's body part id in the stage.

(more to be added)

Body part

Determines the appearance of the a-life's torso along with its wings and tail, if present. Lifespan is linked to body. 

Head part

Determines the appearance of the a-life's head. Vision is linked to head. The starting population of nightopians has a variety of vision values.

Arm part

Determines the appearance of the a-life's arms. Flight stamina is linked to arms. The starting population of nightopians includes all values of flight stamina.

Leg part

Determines the appearance of the a-life's legs. Speed is linked to legs. Nightopian legs have 4 speed linked to them, with two exceptions that are probably unintentional. One nightopian in splash garden has 5 speed and one in soft museum has 6.

Weighting system

Body parts are usually not inherited equally from both parents. This is because a weighting or dominance system is used making it more likely to inherit some alleles than others. The dominance system is extraordinarily complicated as well as impossible to notice without prior knowledge of the game's code. There is no way to determine an a-life's dominance values in game, so this information is not useful for gameplay and is purely a programming curiosity.

Each body part gene has a dominance property attached to it. This property can take one of 4 possible values, designated A, B, C, and D in the game's a-life patent. The dominance values have a rock paper scissors arrangement where each type is weak or strong against at least one other type. However since there are 4 values rather than 3 or 5, the weighting is imbalanced with C and D being weak against 2 types and A and B being strong against two types.

This chart from the patent (on the left) describes the intended weighting of each value when paired with another one.

However, this weighting is implemented incorrectly in the game, resulting in the actual probability being one quarter less and producing different results depending on the order of the parents. (See second table) More on this bug here.

Basically, when each body part is passed down, one of the parents is treated as the column and one as the row in the dominance table. The numbers in the table show the likelihood of the row's allele passing on instead of the column's. So with a pairing of C as the row and D as the column, the allele with C dominance is intended to pass on 75% of the time, and will actually pass on 50% of the time in-game. It's important to note that each of the body part alleles has its own dominance value, and an individual a-life can have body parts with different dominance values. 

A confusing aspect of the dominance value is that it is permanently tied to the allele it's attached to (aside from one exception below), but at the same time, dominance is not tied to a certain body part type. Dominance values are always passed down with their associated body part allele. So if you have a shleep nightmaren with D dominance on all of its parts and it has a mepian offspring, all of that mepian's shleep parts will have D dominance, and any shleep parts it passes on to its descendants will have D dominance. However, not all shleeps have D dominance, and not all parts with D dominance are shleep parts. When any nightmaren are generated in the level, a random dominance value is chosen and applied to all 4 of that nightmaren's parts. 

The starting population of nightopians all have A dominance on all of their parts. However, when breeding a-life, if both parents' genes had A dominance, the offspring's dominance value will be converted to C. This conversion is also supposed to change C to A, but because of a bug it doesn't work.This is the only way that a gene's dominance can change, and it also means that full blooded nightopians and king pians can only have A or C dominance. B and D are limited to nightmarens and their mepian descendants.

Genetic behavior

Unlike all of the other a-life genes, this one is inherited as a diploid trait. That means each individual a-life has two alleles and one is randomly chosen from each parent to pass to the offspring. However, the inheritance has one unique condition: if both inherited behaviors are the same, increment one by 5. Which one is incremented is random with a 50% chance for either the first or second allele. This mechanic is hinted at on the official Japanese NiGHTS into Dreams website, which says that the player should bring pians with the same behaviors together to see new ones in the offspring. 

This gene determines what activities a nightopian or mepian will engage either when it moves off-screen, moves near another a-life, or touches another a-life. Out of 24 total, 4 behaviors (6, 12, 18, and 24) are specific to each level and include actions such as throwing snowballs (frozen bell), building a statue (soft museum) and playing with a remote toy NiGHTS (stick canyon). The remaining 20 are shared between each level. A-life can engage in either of the two behaviors they have the alleles for. Most of the behaviors are the same for nightopians and mepians, but behaviors 15, 20, 21, 22, and 23 are different between them.  

Flight pattern

This gene is inherited in a unique way that is similar to diploid but removes the random choice between each parent's two alleles. It will always pick the first allele from one parent and the second allele from the other parent, and set them as the first and second allele, respectively, in the offspring's DNA. This means that alleles in the first and second places can never swap with each other. This gene determines the way a nightopian typically flies when it is not performing a special action. See here for more details on flight patterns.