The myth of human races

19. Of Genes and Chromosomes: No One is Like You

Chapter 19 OF GENES AND CHROMOSOMES: No ONE Is LIKE You He does not realize that, instead of conceiving him, his parents might have conceived anyone of a hundred thousand other children, all unlike each other and unlike himself. Peter Medawar Nobel Prize Winner In chapter four, we mentioned that the thrilling movie Tainted Blood was based on three false assumptions, one being that there was something "special" about a boy and a girl who were twins. But the twins, in this instance, were of different sexes and were therefore fraternal twins. Fraternal twins, unlike identical twins, are no more similar than regular brothers and sisters. They originate from separate fertilization events: two eggs, ovulated in the same menstrual cycle, each fertilized by a different sperm. Different eggs and different sperm combine to produce complete and different individuals, which happen to be born the same day. Identical twins, on the other hand, originate from a single fertilization event, a single egg fertilized by a single sperm. This fertilized egg, a special cell called a zygote, accidentally divides at an early stage in its development to form two separate cells each of which grows into a separate embryo. In this case we have two individuals with the same heredity and obviously of the same sex. Before explaining further why different eggs or sperm have different hereditary makeup, we must say a few words about genetics, a science of heredity that appeared within the study of biology about 1900. Genetics revolutionized scientific thinking about how traits are transmitted from parents to offspring. According to this science, elementary units of inheritance, which are separate and independent of one another are passed with mathematical precision from one generation to 155 156 Alain Corcos the next through the sex cells. It is these units, called genes, that are inherited-not "traits," and certainly not "race," which is, in fact, a construction of our minds. It is in the course of human development that genes, in interaction with the environment, produce our physical characteristics. This view contrasts with the old idea that the characteristics of parents were blended in the offspring, as one might blend two liquids by mixing them together, as discussed in chapter four. Gene theory, on the contrary , asserts that there is no blending or dilution of individual characteristics . To press the analogy, genes produce either clear water or water of a particular hue. Moreover, genes will continue to create these distinct products generation after generation. Different genes have different functions. For example, one gene is responsible for the production of the growth hormone. Another is responsible for the production of the beta chain of hemoglobin, the protein that carries oxygen to all cells of our body. These two genes not only have different functions, but they are different in their internal structure. Though genes are exceptionally stable in composition and function, they can and do change through a process called mutation. Genes reproduce themselves and it is during the process of self-replication that mutations may occur. If a gene happens to mutate to a new form, subsequent generations of that gene reproduce themselves in the mutant, or altered, form. Such alternative forms of a gene are known as alleles. The extreme diversity that we observe among human beings is due to the fact that we have different alleles, not to the fact that we have different genes. And this is true for any species. Alleles lead to the different forms of various products that genes manufacture. For example, we all have wax in our ears. Some of us have wet wax, others have dry wax. Some of us have both kinds of wax. The different types of ear wax are due to different alleles of the gene responsible for producing that substance. Though geneticists themselves have defined the terms gene and allele, they have, in many cases, used the two words interchangeably. For example, Curt Stern wrote in his book, Human Genetics: 1 It is to be expected that American Caucasians possess African genes acquired by direct race mixture and by the process of blacks "passing" into the Caucasian population. Numerically, this African fraction among Caucasian alleles is small. It has been estimated, with 95 percent probability, that it is less than 1 percent. Chapter Nineteen 157 Stern should have used the word allele throughout the paragraph. There is no such thing as an African gene, but there might...