by Renée Cohen
It is one of the most profound urges, perhaps even stronger than hunger. It comes so naturally, yet it is so complex. It has been a subject of curiosity and interest since creation. Sex, the human means of reproduction, the genes that make us individuals, and the infinite combinations of heredity and variance have all perplexed and entertained people throughout history.
Sex plays a prominent role in almost every aspect of modern life, from sociological, psychological, physiological and spiritual realms to even financial exchange. But sex has not always been so widely acknowledged across such a diverse spectrum of fields. Where sexuality used to be stifled and hidden, it is now actively discussed in public education, even flaunted throughout advertisements and other forms of popular culture as a means of capturing attention.
Clearly, this most profound of animal instincts plays a substantial role in every human's conception of themselves and their relationship with the world, their peers and their families. But how much does the average person actually understand about sexual reproduction? While we may have an in-depth understanding of the social side of sex, the physical biology of how each of us came to be the unique beings that we are goes relatively unquestioned. How is sex relevant to the fact that your eyes may be brown and others' are green? How is sexual reproduction related to the fact that some people have straight blond hair while another person's is red and curly?
In short, all of this physiological variation in humans comes from the unique combinations of our parents' genes and the activity of chromosomes, which form during the sexual lifecycle, or meiosis.
Not all living creatures reproduce using the sexual lifecycle. In fact, many plants and animals reproduce asexually, or without sex, through a cell-division process called mitosis. The hydra, a relative of the jellyfish, is a classic example of an animal that forms its young asexually by "budding." Asexually reproducing organisms create young which are usually genetically identical to their singular parent. In fact, the saying "chip off the old block" applies better to asexual organisms than it does to humans because young asexual organisms are precisely that: little pieces of their parents that will grow to be exact replicas, or clones, of their parents. Their only chance for variation from their ancestors lies in rare genetic mutations which may affect the parent's reproductive process before or while the offspring is formed.
And then there are humans. Humans, like all sexually reproducing organisms, can vary from their parents in terms of shape, size and coloration, among other traits. In fact, it would be impossible for a person, as a result of sexual reproduction, to be a perfect clone, or "chip off the old block," of their parents because there are necessarily two parents transferring genetic information in the process of sexual reproduction.
So, the source of our wide genetic variation as humans lies in the fact that each individual of our species is the result of two unique sets of genetic information, one set from each parent, combining to form yet another unique genetic code in the new human.
While sexual reproduction between two members of a species explains why we differ from our parents (some of us thankfully, perhaps others regretfully), it does not account for differences between siblings who share the same two parents and therefore the same two sets of original genetic information. To answer this question, we need to delve deeper into the workings of the sexual cell-division process, meiosis.
Meiosis begins at the gene and chromosome level. Each somatic cell (cell other than sperm or egg cell) contains 46 chromosomes: 23 from each parent. Chromosomes are tiny cylindrical structures, which contain genetic material, and can be distinguished using a light microscope because they vary in size, shape and bands of color which become visible with certain dyes. Chromosomes come in pairs, or homologues, with one member of each pair coming from each parent. Not surprisingly, each of the chromosomes in the homologous chromosome pair carries the same type of genetic information. For example, if one chromosome carries the gene for the mother's hair color at a certain location, or locus, then the other homologue will carry the father's genetic information for hair color at the same locus of the chromosome.
The new genes, which carry a brand-new combination of DNA derived from the two parent sets present in the chromosomes, then program the somatic cells to synthesize enzymes and other proteins, which go on to form the offspring's actual inherited traits, such as blond hair and brown eyes. Like the vast majority of rules, however, this rule of homologous chromosome pairs has a major exception: the sex of the offspring.
Human females have a homologous pair of X chromosomes, but males have one X and one Y chromosome as a pair. This occurs because one member of the sex chromosome pair comes from the mother and one from the father, as mentioned, and sex cells (ova and sperm) each contain a single set of 22 regular chromosomes (also called autosomes), plus a single sex chromosome, X or Y. The exception to the homologous pair rule lies in the fact that the whole of the parent information, and not selected bits from each sex chromosome, is used in determining the sex of the child.
When a sperm and an egg have met and fused together in a process called syngamy, the full 46 chromosomes are present in the space of only a few cells and, thanks to meiosis, the offspring's genetic template is complete, including the sex.
But the complex process of transferring and combining genetic materials through meiosis does not need to occur for each of the millions of cell divisions that makes up a human. Rather, mitosis, the same type of cell division used in asexual reproduction, takes over and forms the rest of the baby's cells with identical genetic information according to the blueprint established during meiosis. So, with each meiotic process, a new blueprint for a unique organism has been created. Somewhere in that incredible microscopic blueprint of DNA within genes and chromosomes lies the source of every single cell in your body.
While sex education may impose ethical and societal questions, the fact remains that our children are the inherent prizes in the gene frequency game, a human consequence stemming from flattery, charisma and seduction.
About the Author:
Renee Cohen, RPR, is a JCR Contributing Editor.
Test for Human Reproduction
1. Sexuality is commonly used as a means of capturing attention in advertisements.
2. Besides anatomy, males and females differ in:
d) all of the above
3. Mitosis is known as the sexual life cycle.
4. All living creatures reproduce sexually.
5. What is a zygote?
a) a fertilized egg
b) a body cavity
c) a mustache
d) a fossil
6. Sex plays a role in:
d) all of the above
7. Which organisms reproduce by budding?
a) most small reptiles
b) hydras, relatives of the jellyfish
c) fish from the percomorph family
8. The saying "chip off the old block" refers most literally to:
a) sexually reproducing organisms
c) meiotic cell division
d) asexual reproduction
9. Organisms which reproduce sexually cannot be clones of their parents because:
a) they only inherit one set of DNA
b) they inherit two full sets of DNA
c) their genes become mutated
d) their ovulation cycles vary
10. Human offspring of the same parents share:
a) identical genetic codes
b) many physical features
c) identical chromosome pairs
11. A human cell other than a sex cell is called:
a) a somatic cell
b) a sperm or ova
c) an autosome
d) a homologue
12. Each person receives how many chromosomes from their mother?
13. Chromosome pairs, under a microscope, look like:
a) two rigid straws next to each other
b) a double helix pattern
c) cloudy blobs of color
d) two worms in a rough X shape
14. DNA is:
a) dioxide nitrogen atom
b) dinucleotide autosome
c) diploid nucleic acid
d) deoxyribonucleic acid
15. Meiosis and mitosis are both cell division processes which form the genetic codes of offspring.
16. Human females receive from their mother:
a) two X chromosomes
b) one Y chromosome
c) one X chromosome
d) two Y chromosomes
17. A child's sex is determined using:
a) half of each the mother's and father's genetic information
b) all of genetic information from either only the mother or only the father
c) all of both parents' genetic information
d) scientists are still working on this question
18. Oswald Avery, a prominent scientist of the 1930s and 1940s, discovered:
a) the structure of DNA
b) the existence of DNA
c) the fact that DNA is physically passed from parent to child
d) the cause of Down syndrome
19. Meiosis is the basis for:
a) every cell division
b) every transfer of genetic material from parent to offspring
c) the enormous variety among sexually reproducing organisms
20. The meeting and fusing of sex cells is known as:
b) genetic mutation