The inheritance of a parental or paternal instinct is yet another myth used to explain why humans and many other animals give up their life for young. It is just as easy and honorable to learn this instinct under natural conditions where the processes of attunement provide instruction. The bonds needed to make a parent care for its young will be learnt through the daily effort, curiosity and interaction experienced during birth and infant rearing. Those bonds and memories become all the motivation needed by parents to care for their young.
It is worth understanding instincts and how they form. Then you can avoid wasting time blaming 'primal instincts' and 'evolutionary instincts' if things go wrong, in favor of greater understanding of how to learn your instincts so they better reflect the inner strength you want to feel.
Under normal conditions the parental instinct becomes a strong motivating force in people. However, it can take time to learn. When a baby is born it does not always elicit deep feelings of love, but may be a curiosity instead. Some have even said that they are like an ugly little Buddha. This lack of feeling may itself be distressing to the parent, if they were expecting that an inherited instinct should suddenly appear and fill them with strong sensations. The guilt is one of the symptoms of post natal depression. Sometimes the instinct may never develop strongly, especially if past experience has shown that family life is not fulfilling. Some might feel anger or revulsion for the baby, especially if there is not much community or family support, or prospects for personal ambition are disrupted. If a child is taken away from its parent early for adoption, the bond and degree of concern from the natural parent again may not develop. For the adoptive parents, the parental instinct can grow just as strongly, while they enjoy experiencing and interacting with the baby.
Incest further demonstrates that the parental instinct is not inherited. In a natural and aware society, the parental instinct would be learnt without twisting into the depths of incest. But in a false society, instinct learning can go awry. Incest is widespread in human society, and robs children of the equality they need to create in their own way (their childhood). Without the guiding hand of wildness and natural interaction, the role and place of human desires and emotions will be confused, and sound judgement on how to learn instincts lost. For some, this confusion will lead to exploitation and 'dominance' through incest.
Bonding at birth or while young can be very important on deciding if and how a parental instinct will develop. Expectations and a readiness to find a new soul mate provide all the drive needed to make the relationship instinctive. During birth there is often a period of intense bonding or imprinting, where the golden rule of who is family is learnt. A famous example was the goslings that followed Konrad Lorenz around his grounds, because he allowed them to imprint on him as their parent during hatching. The cuckoo has evolved a niche that relies on a similar need by parent birds to feed whoever is in the nest. The cuckoo can invade a host bird's nest, throw out the real parent's eggs, and rely on the instinct learning processes that will soon take place to ensure the cuckoo's young will be reared, despite the obvious difference in appearance (Some biologists will tell you that cuckoos are selfish and are exploiting the parental instinct. However, cuckoos are not exploiting selfishly like a human. They remain linked to the wildness, so can only 'exploit' to the degree allowed by the wildness. Cuckoos have actually evolved a harmonious relationship with their hosts, where both have time to adapt.)
In nature, some animals leave their young to die during conditions of stress or food shortage. For example, seal pups might be abandoned after a severe shortage of fish. Neanderthals also apparently practiced infanticide during food shortages, when the alternative was starvation. They demonstrate that the parental instinct is not an inbuilt rule, because it can be broken when environmental conditions bring them no choice.
There are of course many ways to bring up young in the animal kingdom. Often, some helpless feature triggers parental care. For example, a chick cries and attracts its parent. One animal might care for its young for months because it is helpless, while many fish abandon their young upon birth because they are adapted to fend for themselves and are born in high numbers. If you want to understand why an animal does or does not defend its young, you need to look more closely at its adaptations and the wildness of its ecology, rather than for an 'inherited parental instinct'.
Recently, several genes have been identified that have a role in maternal care. They are called the Peg1 or Mest gene, and the Peg3 gene. They have been called genes for good motherhood. Mice that lack expression of these genes were poor mothers, as only 8% of their pups survived compared to 83% for normal mice. Death was due to a lack of gathering the pups together to keep them warm, failure to feed their babies (due to an inability to produce milk), or carry out maternal duties. Similar genes occur in humans on chromosome 19.
This work sounds to me like a demonstration of an inhibited ability to learn the maternal instinct, rather than proof that a maternal instinct is inherited in normal mice or humans. The mice with the defective genes appeared to be genetically sick in a number of ways, with failure to nurture being one outcome. The mother mice were small, and some would suffer from slowness to learn and perhaps autism (which would inhibit the development of a wide range of relationships). Peg3 appears to affect the production of oxytocin, a compound that helps start milk production. If milk does not flow, I think parental bonding would fail to flourish (mice cannot bottle feed) because of an absence of pleasure and reward confirmations. The genetic defect may cause insensitivity to the young, because the mother does not receive or recognise the stimulation from the young, or she is unable to provide the normal and rewarding responses that would help to bind the relationship. (Article posted 2005)