You might recall the diagram to the left from last week's module. We confirmed then that each chromosome includes a molecule of DNA and those molecules each include numerous genes that code for protein.Since proteins are synonymous with traits in biology, we will need to figure out how Aaron's and Jared's daughters got their chromosomes to know whether or not they are indeed identical. Of course, each of their daughters gets her chromosomes from the egg and sperm cell that fused to create the first cell in her body.
That earlier form of cell division, mitosis, is done by many cells throughout our bodies in order to grow and replace damaged/old cells (and happens out of control in cancer).
Whatever the case, mitosis is meant to create new cells that are genetically identical to the old cells.
On the other hand, the cell division used to create sperm and eggs (meiosis) occurs only in reproductive organs and produces new cells that differ from the old ones.
Most notably, the nuclei of the new cells contain only half of the DNA found in the nuclei of the old cells. That's because, in sexual reproduction, each biological parent only passes along half of their DNA to a child.
To put some numbers to this, the nuclei in each of our regular body cells ("somatic cells") typically have 46 chromosomes.That includes 44 "regular" chromosomes and two sex chromosomes (X or Y chromosomes). Individuals with two X chromosomes are considered genetically female while individuals with one X and one Y chromosome.
Also, since eggs always carry an X chromosome, the sperm will determine the genetic sex of the child (by contributing either an X or a Y).
Altogether, this means that our reproductive organs need a strategy for sending only 23 chromosomes out of the original 46 to egg or sperm cells.That way, when egg and sperm fuse, we will be back to the original 46 typically found in human cells.
A Note On Imperfections in Our Biological Terminology... There are numerous alternate possibilities for genetic sex beyond XX and XY (e.g., X, XXY, XXX, etc.).
In addition, external/internal manifestations of sex are impacted by numerous environmental conditions (not just chromosomal make up). All that is to say that sex, even as a biological phenomenon, is naturally non-binary in humans! Simply talking about "male" vs. "female" is overly simplistic when considering human physiology.
And while gender is traditionally considered a separate, socially-constructed identity, sex and gender interact in complex ways. In the field of biology, we are still working with outdated language to describe all of this and we need to get better...both to be respectful of folks that don't have identities matching our terminology and to be more biologically accurate. If you have interests in this or opinions to share, I would most welcome your input and collaboration in working on my own use of language for this!
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