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  • The Gene / Genealogy Forum III

    Edwin M. Knights Jr., M.D.

    Published Date : May 6, 2004

    The study of heredity can now be divided into three branches. Oldest among these is the branch dealing with changes from generation to generation; these folks are the classical geneticists who study transmission genetics. They analyze the transmission of traits within a family pedigree or in cross-breeding experiments on other organisms. Other researchers, who now are studying structures and functions at the molecular level practice molecular genetics. This is the area that has been shifted into high gear by the completion of the Human Genome Project and spawned the new industry of genomics. The third group deals with the distribution of genes in populations, or population genetics. Although animal populations have been studied considerably, most of the efforts have focused on searching the origins of human populations and tracing human migration throughout the world. Such studies depend considerably on the sciences of geography, ecology, archaeology, physical anthropology and linguistics.

    As we noted previously, plants and animals had been bred and crossbred many centuries before Mendel, but he selected a plant that was easy to breed and crossbreed, planned his experiments well and kept excellent records. By keeping his studies simple, he was able to demonstrate clearly hereditary traits which have become known as "Mendel's laws." Since his time, the experimental plants and animals have changed many times, being selected for their usefulness in studying scientific problems which were of current interest. After the importance of DNA became recognized, organisms used most commonly were those having nuclei in each cell where the genetic material DNA is located. These organisms are known as eukaryotes, meaning they have a true nuclei bordered by nuclear membranes.

    Six organisms have been most popular research subjects (we'll not try to impress you with all their Latin names): yeast, fruit flies, roundworms, a mustard-like weed, mice, and humans. This by no means completes the list, as many studies have also used orange bread mold, various protozoans, algae, corn and chickens. And what do we have in common with these strange bedfellows? Quite a lot! It's true, we don't fit in little bottles and we tire of a diet of bananas, but we do have similar traits in common with our diminutive cousins here on Planet Earth. But first please note that we refer to the genetic composition of an organism as a "genotype" and the observable traits as "phenotypes." And even though Mendel never used these terms, credit his work for inspiring the creation of a new lexicon for future scientists!

    Mendel's Laws
    Mendel's first law is the principle of segregation. In practice this means that the two members of a gene pair (known as alleles) segregate from each other when they form a mature reproductive cell (a gamete) which is ready and waiting to fuse with another similar cell of the opposite sex. Result: a diploid cell called a zygote. The alleles carry alternative forms of a gene -- in the pea seed, one would be S, which is the smooth form and the other is the s allele, or the wrinkled form.

    His second law is the principle of independent assortment. Members of different allele pairs are transmitted independently of one another during production of gametes.

    While it is quite simple to interbreed other organisms - in fact they seem to participate in the studies quite enthusiastically -- we dare not even think of interbreeding humans, lest we be banished from NEHGS and cyberspace and acquire an X-rating for their web site! The penalty for controlled crosses is crucifixion. But Genealogists to the Rescue!

    Human geneticists, in contrast to inhuman geneticists, analyze genetic traits by pedigree analysis, documenting recurrences of a trait in family trees. Using charts called genograms, the patterns of heredity become apparent. They are also useful in understanding complex family relationships and environmental effects. Later we'll examine them in more detail.

    Pink-Eyed Fruit Flies
    While progress in human genetics took a turn for the worse in the era of eugenics, Thomas Hunt Morgan and his associates were doing some fascinating studies with Drosophila fruit flies. Breeding these little creatures with variable eye colors, wing features and other mutations, they were able to confirm the laws of Mendel. But they went further than Mendel, describing small structures they called "chroma (color) soma (body)" or chromosomes in the nuclei of their cells. The chromosomes appeared in pairs, derived from each of the parents, and were found to be the location of the factors of inheritance, or genes. The locations of these genes could vary, i.e., "crossovers" could occur. Eventually over 3,000 genes were described in Drosophila. Science had progressed from picking peas to swatting flies. Morgan's group made enormous contributions to the science of genetics and fruit flies are still swarming around mashed bananas in many laboratories.

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