Representing two planets, iron, copper and a couple of Olympian gods, the classical symbols for male and female pack a lot of meaning into a few squiggly lines.
The symbols themselves are ancient, and the associations they make date back to the dawn of civilization. The ancients, after observing how the movements of heavenly bodies like the Sun and planets heralded a corresponding change in events on our planet, eventually came to believe that there was a causal relationship. Logically, then, ancient scholars began to study the heavens in order to better predict, and prepare for, the future. They also came to associate different heavenly bodies with their powerful gods- Mercury, Venus, Mars, Zeus (Jupiter) and Cronus (Saturn).
Each heavenly body, along with its god, was also associated with a particular metal. So, for example, the Sun (Helios) was associated with gold (note: in truth, the Sun is white in the human visual spectrum, not yellow); Mars (in Greek, Thouros) was associated with the hard, red metal used to make weapons, iron; and Venus (in Greek, Phosphorus) with the softer metal that can turn green, copper.
Writing about these metals, the Greeks would refer to them by their respective gods’ names, and then as now, these were spelled with a combination of letters; after awhile, a type of shorthand arose; for example, relevant to Mars (Thouros) and Venus (Phosphorus):
In medieval times, European alchemists relied on these shorthand symbols, which were retained through the Enlightenment and used by such notables as Carolus Linnaeus (the father of modern taxonomy who made binomial nomenclature popular), to refer to such metals in his 1735 work Systema Naturae.
Linnaeus was also the first to use these signs in a biological context in his dissertation Plantae hybridae (1751), where he used the symbol for Venus to denote a female parent of a hybrid plant and the symbol for Mars to denote a male parent.
Linnaeus continued to use the symbols for the purpose of distinguishing male and female, and by 1753’s Species Plantarum, he was using the symbols freely 
Following in Linnaeus’ footsteps, other botanists incorporated the symbolism, as did scientists from other fields including zoology, human biology and, eventually, genetics.
Modern geneticists no longer use these familiar symbols and instead rely on a square (for male) and circle (for female):
This symbolism was developed by Pliny Earle, a doctor with the Bloomingdale Asylum for the Insane in New York in 1845 while explaining the inheritance of color blindness:
For the purpose of clearly illustrating the prevalence of this physiological peculiarity in the family, I have prepared the subjoined genealogical chart. Males are represented by squares and females by circles.
While it is not entirely clear why Earle deviated from the classical symbols, one explanation was later given by Royal Society Member, Edward Nettleship, who claimed that Earle had been “unable to get any printer’s symbols capable of use . . . except those employed in printing music.”
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- Two popular associations sometimes made with the symbols today, that the symbol for male also represents Mars’ shield, and the symbol for female represents Venus’ mirror, have been rejected by most scholars.
- In 1970, the Gay Activists Alliance (GAA) of New York adopted the Greek lambda (λ) as its symbol, and although the precise reason why isn’t well-known, there are several popular theories. One is associated with the great warriors of Sparta, who thought that the lambda represented unity; another holds that the Romans felt the lambda symbolized “the light of knowledge shining into the darkness of ignorance.” Another is that, in chemistry and physics, the lambda represents energy. Interestingly, some claim that the GAA chose the lambda because it had adorned the shields of the Sacred Band of Thebes, a 300-man highly accomplished army of elite soldiers made up entirely of lovers; there is little evidence to support this association, although there was a movie, The 300 Spartans (1962) (not Thebians), in which this particular 300 each had a lambda on his shield.