Sex Chromosomes

Copyright © Karl Dahlke, 2023

As we learned in high school, girls have two X chromosomes, denoted XX, while boys have an X and a Y, denoted XY. The father passes an X or a Y to his child at random, and the mother passes an X, because that's all she has. The child has an even chance of being a boy or a girl, which preserves the gender balance of the population.

Imagine a planet where the girl has two different sex chromosomes. I'll call them Z and W, not to be confused with X and Y. The boy is ZZ and the girl is ZW. The mother passes Z or W to the child, which determines whether that child is a boy or a girl. The ZW system seems just as viable as the XY system, so perhaps this is how gender is determined somewhere in the universe.

As it turns out, you don't have to go to another planet to find ZW, just look out your window. Birds use the ZW system, along with some reptiles and insects. Genetic evidence indicates that ZW evolved independently of XY. In fact the Z chromosome in some birds is similar to our chromosome 9, as though chromosome #9 was co-opted for the purpose of sex determination.

This has implications for parthenogenesis, wherein a female has a child without the benefit of a father. This is rare, but it happens in certain insects, amphibians, and reptiles, when males are not to be found. Recently, Charlotte the stingray became pregnant on her own, adding another species to the list.

Since a sperm is not forthcoming, the single strand of DNA in the egg is replicated, or joined with another strand from the mother, to produce a full complement of DNA. Instead of bringing in a sperm, the mother just makes another copy of her own. The egg is now a clone of the mother, almost. If the mother carries one copy of a recessive gene, this gene has a 50% chance of being part of the egg. If present, it is copied, so that the recessive gene sits on both sides of the DNA ladder, and the child will have the recessive trait. This would not happen in a proper clone of the mother, or in a two-parent child. thus sexual reproduction is preferred for its genetic variability; parthenogenesis is a last resort.

With this in mind, let's return to the sex chromosomes. If the animal uses the XY system, then the mother has XX, and the child has XX, and is always female. Parthenogenic reproduction always produces a daughter. If there are no males nearby, the green frog can produce daughter frogs, which produce more daughter frogs, and so on down the line. Similarly, Charlotte's baby will be a girl.

Parthenogenesis has never been observed in mammals, and certainly not in humans. Jesus was not born of a virgin, and if he was, if that was a one in 10 billion event, then he would be a female. Jesus was a man, hence his Y chromosome came from Joseph - end of story.

The komodo dragon shows us another aspect of reproduction. She is capable of parthenogenesis, and she uses the ZW system. Her haploid DNA contains Z or W at random, and the resulting egg contains ZZ or WW. The former is a male, and the latter is not viable, and does not develop to term. Thus all the parthenogenic children of a komodo dragon are boys.

Other sex determination systems exist, such as XO, wherein the female is XX, and the male is X alone, denoted X0, with no corresponding sex chromosome from the father. Alligators determine the sex of their offspring by the temperature of the nest. The clutch is entirely female at 30°c or less, and entirely male at 34°c or greater. These and other systems are beyond the scope of this book.