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Genetic 102

Discussion in 'APBT History' started by misterdogman, Jun 21, 2005.

  1. misterdogman

    misterdogman CH Dog

    Gentics 102
    PATTERNS OF INHERITANCE: (purple flowers and red noses: or black is better to sniff white with) read this joke again at the end
    In 1865 Gregor Mendel (a monk) refuted the theory of blended inheritance (GRIN It was popular back, then not just among dog breeders, but the entire scientific community) and instead he proposed a theory of particulate inheritance. According to Mendel's theory, characters are determined by discrete paired units of information that are inherited intact down through the generations (read that sentence 10 times until you memorize it, trust me this will help cement the concepts). Mendel’s experiments were simple and elegant and now they serve as a framework or introduction to a more detailed understanding of the mechanisms of heredity.

    READER: OK that’s it! IF you talk about peas I am leaving!

    ANSWER: Well if you cannot grasp the concept of inheritance through the following simple and logical Mendelian experiments then you should go visit the spay/neuter clinic immediately.

    Gregor Mendel was at the monastery at Brünn where he began experimenting with plant hybridization (breeding) in a series of classical studies that posthumously (after he died) earned him distinction as the founder of the science of genetics. Mendel's studies constitute an outstanding example of good scientific technique, something from which your art-science of breeding will benefit. He chose research material with defined phenotypes (color, texture etc), he carefully constructed experiments, he documented his results carefully, and he used statistical analysis to validate his hypothesis.

    Yep as you already knew Mendel studied the garden pea. Mendel chose seven different characters to study (we will discuss at least one of these). The word character in this regard means a specific property of an organism (phenotype); geneticists use the term phenotype as a synonym for characteristic or trait.

    OK for those of you who have stuck around this far I will also temper this dry discussion of peas by throwing in a parallel virtual APBT experiment using black nose and red nose APBTs.
    DOMINANT AND RECESSIVE
    MENDEL: For each of the characters that he chose, Mendel obtained and bred stable lines of plants. To ensure these lines were stable he selectively bred them for two years to make sure that they were pure (he line bred different groups of peas for specific phenotypes). A pure line is a population that breeds true for (shows no variation in) the particular character or phenotypes being considered; Mendel had made a clever beginning: he had established a fixed line of plants for his future studies so that any changes observed subsequent to deliberate manipulation in his research would be scientifically meaningful.

    APBT BREEDER: In our experiment we have created two bloodlines of APBT. One of these bloodlines always produces black nose dogs, the second separate bloodline always produces red nose dogs. We have bred these bloodlines for 4 generations to ensure that this particular trait always breeds true.

    MENDEL: Two of the pea lines studied by Mendel bred true for the phenotype of flower color. One line of peas bred true for purple flowers; the other, for white flowers. Any plant in the purple-flowered line when crossed with other plants from the same line produced seeds that all grew into plants with purple flowers The white-flowered line similarly produced only white flowers through all generations . Each pair of Mendel's plant lines can be said to show a stable trait or phenotype. Thus, Mendel has good contrasting phenotypes to start his experiments on heredity.

    DEFINITION: The term phenotype (derived from Greek) literally means "the form that is shown"; it is the term used by geneticists today. Phenotype is the form that is show but the term Genotype is more important from a breeding standpoint. The genotype is the form that is shown combined with the form that is hidden.

    Mendel pollinated a purple-flowered plant with pollen from a white-flowered plant. We call the plants from the pure lines the parental generation (P). Strangely enough, all the plants resulting from this cross had purple flowers. This first generation is called the first filial generation (F1). (The subsequent generations produced by backcrossing the F1 are symbolized F2 , F3 , and so forth.)

    APBT BREEDER: The breeder crossed a black nose sire with the red nose dam. The sire and dam from the original pure lines are the parental generation (P). In this case all of the puppies (F1 generation) from this breeding had black noses. What happened to the red noses? Shouldn't we see

    MENDEL: If one pure-breeding parent is purple flowered and the other is white flowered, all plants in the F1 have purple flowers. The purple flower color in the F1 generation is identical with that in the purple-flowered parental plants. In this case, the inheritance was not a simple blending of purple and white colors to produce some intermediate color. To maintain a theory of blending inheritance, we would have to assume that the purple color is somehow "stronger" than the white color and completely overwhelms any trace of the white phenotype in the blend.

    Why was the red nose phenotype (or the white flowers) not expressed in the F1 pups? Mendel actually coined the terms dominant and recessive to describe this strange phenomenon. The purple flower/black nose is dominant to the white flower/red nose and the white flower/red nose phenotype is recessive to the purple flower/black nose phenotypes. Thus we can see that if we breed a true line of black nose dogs to a true line or red nose dogs the F1 (first filial generation) will be black nose because the black nose gene is dominant.



    As we already mentioned and continue to stress, it becomes very difficult to apply the theory of blending inheritance to devise an explanation of these results.



    GENES OCCUR IN PAIRS
    Mendel then selfed the F1 plants (purple flowers), allowing the pollen of each flower to fall on its own stigma. Because pea plants have both sex organs (flower and stigma) they can be bred directly to themselves. This is termed selfing. Here is where it got interesting for Mendel, because while most of the flowers were purple some of the resulting plants actually had white flowers. Like magic, the white phenotype had reappeared.

    Mendel did something that was amazing for the times and something which earned him the title of Father of Genetics: he counted the numbers of purple flowers and white flowers. Mendel counted 705 purple-flowered plants and 224 white-flowered plants. He noted that the ratio of 705:224 is almost exactly a 3:1 ratio (in fact, it is 3.1:1).

    APBT BREEDER: With dogs the closest we could come to selfing, would be inbreeding an F1 male to an F1 female. After doing this virtual inbreeding we find that out of 8 puppies two were red noses. The other 6 had black noses like their F1 parents. Like magic these two black nose F1 dogs produced two red nose (F2) puppies. Importantly, just like in Mendels experiment this is also a 1:3 ratio of red nose to black nose.

    WAIT: It is easy to understand if black is dominant and we breed a black nose P sire to a red nose P dam that all the F1 generation puppies would have black noses but how in the world did we get red nose puppies when we breed two black nose F1 dogs?

    YEP THERE IT IS: The overwhelming lesson from this is that even though the F1 dogs were black nosed, the dogs themselves carried thehidden ability to produce red nose F2 pups.

    The answer is simple and gives us our second concept: GENES (actually chromosomes) IN DOGS OCCUR IN PAIRS, EXCEPT IN THE EGG AND SPERM

    Lets say it again, Genes occur in pairs in the dog. The dog has 39 chromosome pairs (total of 78 chromosomes). This is what is meant by diploid (diploid = double). There is another term of importance which is haploid (haploid = half) that describes the genome of the egg and sperm. Those definitions are not precise but will make the terms easier to remember. The figure below, in addition to showing haploid and diploid will help us illustrate several more terms and concepts we need to understand.

    Genetic 102 to be continued
     

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