The question in this supposed conundrum informs of gender bias in our language. The mere fact that the question is even asked makes the prejudice apparent.

The nature of the question assumes that the subject of all sentences is male and, therefore, proposes that the offspring referred to is, ipso facto, male. As an example, the statement "Each student chose his tools for the exercise," makes the reader assume that all the students were male, in spite of the possibility that half or even more of them were female. The answer to our question, though, should be self-evident: B is the daughter of A.

Gender bias can occur consciously or unconsciously. When such bias is unconscious, it can be considered to be the product of society: those by whom an individual is surrounded may use sexist language, and repetition normalizes the language. The speaker may then unconsciously produce his or her own sexist language where men are the norm and women the "other." 

Such bias in language encourages discrimination and can discourage people from pursuing their dreams. If engineers are, for example, always spoken of as male, a girl who wishes to be an engineer may feel that she has no future since "all" engineers are men. Partisan language also offends people when they find themselves excluded. One can, however, avoid using such discriminatory language by following a few simple rules. 

The goal is not to avoid referring to individual people as male or female but to be inclusionary when speaking in hypothetical statements or of mixed-gender groups.

• One should use humanity or the human race instead of man or mankind when referring to all people.
• When speaking of a single hypothetical individual, use person instead of man.
• Whenever possible, one should employ genderless titles, such as flight attendant instead of stewardess, firefighter instead of fireman, and homemaker instead of housewife. One should also refrain from adding gender markers to genderless titles, such as male nurse and use the genderless title alone (for example, nurse).
• When writing or speaking in the plural, one should strive to avoid gendered pronouns and possessive adjectives. This technique creates smoother and more grammatically correct prose than using a plural pronoun with a singular subject. (This sentence is sexist: Each soldier makes his own bed. This sentence is grammatically incorrect: Each soldier makes their own bed. This sentence is gender-neutral and grammatically correct: Soldiers make their own beds.)
• When sentences cannot be restructured in the plural, one should use "he or she" or "his or her" to be inclusive. (The trophy winner must claim his or her award by tomorrow.) However, using this structure too often can break up the flow of one's writing, so use it sparingly.

One should be careful not to take gender-inclusivity to extremes; each specific individual has a gender and can be referred to in that way. Similarly, some biological facts are applicable only to women or only to men. An example would be when one is writing an essay on giving birth. One should not refer to the subject as "a pregnant woman or man."

The statement offers a reasoning challenge by presenting arguments that may easily lead to the development of a fallacy. Focus on the letters A and B may divert one's concentration from the terminology used. In the second sentence, the term son and not child has been used. Thus, gender has been introduced into the reasoning challenge, meaning that gender considerations should be employed towards answering the question. In this case, A is the father of B, making B his child. Additionally, B can either be a boy or a girl. Thus, if the child is not a son then it is definitely a girl, meaning B is A’s daughter. A fallacy in this instance would be to assume that B or the child has to be a boy.

Hello!

This is the standard riddle, probably Bilbo and Gollum asked it one to another :)

It exploits our languages' historical sexism: somebody is by default he. Especially if "he" father has been mentioned already. Also a human being is called "a man" in English.

But "me, your father" may have not only sons, but also daughters. So the answer for this situation is "B is the daughter of A".

Let's consider this situation from the point of view of mathematics (functions, mappings). Denote the set of all people (live or not) as P and define the function F from P to P, F(human) = the farther of this human. This function is defined for every human from P, but it has no inverse function, because not every human is somebody's father.