In addition to figuring out "Who Done It" on TV crime shows, DNA is also useful in figuring out "Who Begot Whom." It works like this. All humans have 23 pairs of chromosomes, making the total number of chromosomes equal to 46. One set of 23 chromosomes came from the mother and the other set of 23 chromosomes came from the father. Each of the father's 23 chromosomes is paired up with the corresponding chromosome from the mother. Each chromosome consists of a long string of DNA entwined with proteins called "histones." Histones unwind to permit the DNA to be read; the histones are inherited along with the chromosomes. (Segal, 2006).
DNA is a chain of chemical units called "nucleotides." It is like a computer code (...011000101...), but instead of using only zeroes and ones, each nucleotide uses one of four different chemical bases, which are known by their first letters, A, C, G, and T (...ATTGCATCCA...). A "gene" is a string of DNA that "codes for" a polypeptide, which is just a string of chemically linked amino acids. The order of those A, C, G, and T bases in the coding portion ("exon") of the DNA sequence of a gene determines which polypeptide is made, and stringing different polypeptides together produces different proteins. 1 (See Appendix). Proteins and other substances are assembled to give various traits, the "phenotype." Less than 2% of our genome is required to make all the proteins we need to live.
All humans have the same genes, 2 but not the same form of those genes. To clarify, we all have the EYC3 gene for eye color, but one A-C-G-T sequence of that gene makes eyes blue and another A-C-G-T sequence of that gene makes eyes brown. Each different A-C-G-T sequence of a gene is called an "allele." In some populations, a gene may come in only a single allele, so everyone in that population has the same A-C-G-T sequence for that gene and has the same trait, i.e., the allele is "fixed"; genes in other populations come in many alleles, some of which only very few people have. Some alleles are very beneficial and give an individual a highly desirable trait, such as greater intelligence, athletic ability, or good looks, and other alleles may be lethal or debilitating. There is an average of 14 different alleles for each gene.
In addition, regulators (the "epigenome") determine whether or not a string of DNA is read. 3 The epigenome also differs between people and is inherited with the chromosomes. Putting all this together, it is obvious that unless two people are identical twins, it is extremely unlikely that they will be genetically identical, and even "identical" twins, i.e., twins with the same DNA sequences, may differ slightly due to differences in their epigenomes. 4
And, hang on, it gets even more complicated. If two alleles have different A-C-G-T sequences they can nevertheless still code for the same polypeptide (i.e., the two alleles are "synonymous"), or they can code for different polypeptides ("non-synonymous"). 5 Each A-C-G-T difference, e.g., a "T" instead of an "A," is called a "single nucleotide polymorphism" (SNP). The difference between an "A" and a "T" may be only in how difficult it is for a cell to obtain and assemble an "A" instead of a "T," or the difference may be advantageous, disadvantageous, or even deadly.
| Very occasionally, there is a throwback ("atavism"), a person whose gene regulators have turned on genes that were turned off a long time ago in the rest of us. (LePage, 2007).|
Figure 3-1 is a picture of Azzo Bassou. Bassou was living in the Valley of Dades, near the town of Skoura in Morocco in 1936, where the original white population has mixed with blacks. If he is a throwback, he should express some primitive white and/or African traits, along with his mulatto traits. Some experts believe that Bassou was a microcephalic (e.g., had a genetic defect that left him with a small brain), but he was not a drwarf, as many microcephalics are. (The villagers would not permit an examination of his body when he died.) His behavior, aside from its primitiveness, was also not that of most microcephalics.
"With arms so long his fingers hang below his knees when he stands upright; with massive, bony ridges above his eyes and a sharply receding forehead; with jaws, teeth, chin, and cheekbones all showing pronounced ape-like characteristics. He sleeps in the trees there and subsists on dates, berries, and insects. He wears no clothes (although he was persuaded to don a burlap sack for the photograph which appears here), uses no tools, and speaks only in grunts." (National Vanguard, Issue No. 44, 1976).
Table of Contents
1. Because polypeptides can be assembled different ways, humans have over 500,000 proteins but only 20,488 genes, though more genes may be found. Exons are only 1.5% of the human genome. (Carroll, S.B., "Regulating Evolution," Scientific American, May, 2008). Back
2. There may be a few exceptions. (Miller, 2006; also see gene APOE). Back
3. Epigenetics is an exciting new science with much promise of important discoveries. (Watters, 2006, p. 33; Cropley, 2006). Back
4. (Fraga, 2005). The number of copies of an allele may differ in identical twins. (Bruder, 2008). Back
5. See the Appendix for an explanation. Until recently, it was assumed that synonymous alleles produced exactly the same biological product. Although they do produce the same string of polypeptides, it has been found that they can cause the resulting protein to have different shapes. (Soares, C. "Codon Spell Check," Scientific American, May, 2007). Back
6. Because reproductive success is a sine qua non for all life, with large numbers of individuals over long time periods, reproductive success determines even the finest details of a species' traits. (Miller, 2007). Back
7. DNA is also found in the chloroplasts of plants. Inherited RNA is found in centrosomes, which oversee cell division. (Alliegro, 2006; Wikipedia, Extranuclear Inheritance). Back
8. Some other parts of cells (e.g., cilia, flagella, and centrioles) are also believed to be the remnants of captured microbes. (Patterson, 1999, pp. 133-134). In addition to the incorporation of microbe DNA into our own DNA, we have 10 times as many microbial cells in our body as our own cells. Back
9. One parent may contribute more copies of a gene than the other, resulting in greater genetic differences between people, including racial differences. (Redon, 2006). Back
10. The last two sentences explain why it is much easier to find mtDNA than nuclear DNA in fossils. Bones and teeth are made of a hard, calcium-based mineral, hydroxyapatite, that helps preserve DNA by keeping out bacteria and fungi. Although strongly acidic soil can kill the microbes, acid also attacks both the calcium and DNA; heat and temperature fluctuations also destroy DNA. (Sykes, 2001, pp. 171-172). Back
11. That may seem like a huge number, but the single-celled amoeba, Amoeba dubia, has over 670 billion base pairs. (Wikipedia, "Gene"). Back
12. An example is the UDG ("uracil DNA glycosylase") enzyme, which latches on to DNA blocks that are the wrong size. (Parker, 2007). (Wikipedia "DNA Repair"). Back
13. Genes account for only 1.2% of our genome's three billion base pairs. (Birney, 2007). Junk DNA can regulate the expression of a gene, e.g., how exons are spliced and folded to make them active. Humans have more junk DNA than other vertebrates. Back
14. Also, the human egg has about 250,000 mitochondria, while the sperm has only a few, just enough to create the energy needed to swim the last few millimeters to the egg. (Sykes, 2001, p. 54). Back
15. Even more confusing, it has just been found that, at least in mice, RNA in the sperm can also enter the egg and affect traits. (Rassoulzadegan, 2006). A similar phenomenon may occur with crosses between wild Mallards and White Pekin ducks, where the color of the duckling is determined by which species lays the egg. Back
16. A female may occasionally have an XY (androgen insensitivity syndrome, "AIS") or three sex chromosomes, an XXY. Thus, if the female gives her male child a Y chromosome and the normal (XY) father gives the male child an X chromosome, then the assumption that the Y came from the father will be false. (A male could also have three sex chromosomes, an YYX, or extremely rarely, even an XX, but that is not important here.) Back