Although it's a tremendous amount of fun to talk about "the birds and the bees," I will spare us from an awkward illustration and instead attempt to elucidate the phenomenal sex determination systems of animals, including those of birds and bees.

XX or XY: Mammals

When a couple is expecting, one of the first anticipated questions they're asked is, "Is it a boy or is it a girl?"

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Well, there is a 50% chance it will be a boy and 50% chance it will be a girl, because the sex of a baby is determined genetically via an XY sex determination system. Parents have sex chromosomes, the mom has XX and the father has XY, and each parent will pass down a chromosome to their baby. Since mom's egg cells can only have a chromosome makeup of XX, she can only pass down an X, while the father's sperm cells can pass down an X or a Y. Therefore, the chromosome inherited from the father will ultimately determine the sex of the baby.

ZZ or ZW: Birds and Some Reptiles

Birds and some reptiles have a similar sex determination system to mammals with a (somewhat) minor difference. Males have sex chromosomes ZZ (like the female mammals with XX) and the females have sex chromosomes ZW (like the male mammals XY). Therefore, in this case it is a mother's egg that determines the sex of the organism, not the sperm.

Haplodiploidy: Insects

In insects, such as bees, the number of chromosome sets is the sex determinant. Males will develop from unfertilized eggs (haploid) while females will develop from fertilized eggs (diploid). Therefore, it is impossible for a male to have a father or to have sons, but males can have daughters or grandsons (that is, if his sperm is stored and used by the queen to fertilize her eggs).

Sex-determination is immensely varied. For example, in animals like grasshoppers, there is no Y chromosome. Therefore, females are XX, whereas males are XO ("O" representing no chromosome). Also, sex-determination can be environmentally determined; in animals, such as alligators, the temperature of the environment in which the eggs are incubated will determine the sex of the individual. In higher temperatures the embryo will develop into a female and in lower temperatures into a male. Furthermore, some fish, such as clown fish all start their lives as males and live in a social group of mostly males with a hierarchy of one breeding male and female, but if the female was to die or somehow be removed from the group, the breeding male transforms into a female and a male from the group moves up the hierarchy to mate.

Although the idea of sexual transformation in tropical fish may seem bizarre, there are naturally occurring examples in humans. In the BBC series, Countdown to Life: The Extraordinary Making of You, Michael Mosley covers a rare case in Salinas, Dominican Republic in which one percent of the boys are biologically disguised as females until puberty. This condition was termed Guevedoces (meaning penis at age 12) because these individuals appear to be completely female when born and its not until puberty that their penis will grow and testicles descend. In the first few weeks of embryo development, the fetus has no anatomical or hormonal sex differences. It's a gene on the Y chromosome of males that codes for an enzyme called 5-alpha-reductase which helps create the male sex hormone dihydrotestosterone (DHT) that aids in the development of male sex organs. Guevedoces is the result of a gene variant on the Y chromosme that causes a deficiency in DHT, so it isn't until testosterone levels rise during puberty that these males start to develop their male parts.

*As if it wasn't complicated enough, try including this lesson in your future explanation of "the birds and the bees." Good luck!