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1. Mendelian Genetics.docx

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BIOL 239
Christine Dupont

Mendelian Genetics - Chapter 3 Genetic traits are passed on from one generation to the next Artificial selection – purposeful matings  Man-made selection: humans mating plants/animals to their benefit  Selecting certain traits (phenotypes; something you see like behaviors)  Domesticated plants and animals Dogs - domesticated wolves. Most compatible wolves were bred - selected for  Animals can be tamed and get accustomed to humans  Domestication occurs over long periods of time; animal loses its wild instincts, change their breeding cycles  Become genetically pre-programmed to be used to humans in beneficial way First non-wolf canine skull found dated at ~15 0000 years old (arctic)  Wolves hanging around human camps to acquire food  Humans realized that they kept other animals away  Started to encourage and select for more compatible wolves (positive attitudes)  Selectively bred these wolves for certain genes to eventually get desired trait  May look very different but have very similar genes  Genetically, chimps are way more diverse than humans Later pigs, goats, sheep, cattle ~ 10 0000 years ago  Largely for eating 1950s Belyaev  Stalin’s Russia; communist that disliked scientists (study of genetics)  Worked with silver fox: silver morph (colour variant) of normal red fox  Easier to work with foxes if they were more accustomed to humans  Selected for foxes that showed less aggressive behavior and foxes that showed VERY aggressive  Within four generations (less than ten years), ended up with line of foxes that were very happy to be with humans and a line that were very aggressive  Showed how the influence of selective breeding; nature vs. nurture  Scientific American 1  Man's new best friend? A forgotten Russian experiment in fox domestication After dogs, came domestication of many different plants Crops ~ 13,000 years ago (wheats): Middle east - Iraq, Iran, Syria  Conditions were right for finding these plants(grass type plants)  Breeding them and domesticating them  Wheat, barley, rye How do you domesticate a plant? Get rid of wheaty characteristics (few seeds that easily fall, thin stalks etc.)  Natural/ wild form of plants, like corn, from grass family had very few seeds and was not able to nourish  After domestication: numerous seeds, packed with carbohydrates, big heavy robust stalks to help it from falling over Humans have been playing around with artificial selection for a very long time, nobody really knew what was manipulated when you were doing these selections. Gregor Mendel (1822-1884)  Augustinian monk (growing and carrying out artificial selection) and expert  Plant breeder (Austria 1800s)  Experiments with garden peas Tried to answer what it was we were manipulating; can we predict what we’re going to get? Advantages to Mendel’s approach 1. Garden pea : Easy to cross-fertilize, large numbers of offspring, short growing season  Don’t need to force them to mate  Offspring can produce more offspring in short time  Compared to animals (dogs, pigs etc.) 2. Clear-cut alternative forms of particular traits : purple vs. white flowers, yellow vs. green peas, round vs. wrinkled peas = phenotype  Bred specific lines that would always produce a specific trait 3. Establishment of pure-breeding lines : offspring carry parental traits that remain constant from generation to generation  Allowed him to look at the pattern of these traits and how they were being passed along 4. Carefully controlled breeding : Use of reciprocal crosses and self-fertilization 2 Phenotype: Observable characteristic (largely determined by genotype)  Commonly referred to as a trait  Colour, shape, size, behavior (biochemical reactions) Genotype: Genetic make-up; description of the genetic information carried by an individual Mendel’s experiments with garden peas: Cross-fertilization Plants able to self-fertilize (have both male and female parts) -Mendel controlled fertilization by removing the male parts (pollen) on one flower -Transferred pollen into the female stigma (ovules) -True breeding lines, in this case, TALL -Whenever he bred these plants together he got TALL plants for many generations -Cross-fertilized to see what he got Each pea is an embryo; not clones of each other, but are individual They can have different genotype, despite similar phenotype (look the same)  Looking at corn (indian corn), each kernel is different and has different phenotypes (colors)  Different genetic make ups (fraternal twins) Mendel used selective breeding to produce true-breeding lines of peas  Offspring always showed SAME phenotype as the parent plants  As long as you are only breeding within that line Mating of parents with antagonistic traits produces hybrids Looked at various traits (green, yellow, round, wrinkled) If he took true breeding lines of YELLOW peas and crossed them with GREEN peas; he always got YELLOW 3  YELLOW/ROUND trait is a dominant trait: it’s always going to show up if you carry that allele Looked at the ratios of offspring:  Found patterns in these ratios  Developed laws based on his findings Found a consistent pattern of inheritance, from which he based his theories Prior to Mendel: 1. Theories that ONE parent contributes most to an offspring’s inherited features (disproved through reciprocal crosses)  Male was the important/dominant contributer of traits  Female was the carrier of offspring  In animals; males usually have larger and more dominant features  Reciprocal cross: Purple pollen (male) crossed with white (female), offspring is purple  If you take white pollen (male) with purple (female), same outcome 2. Parental traits become MIXED and forever changed in the offspring (disproved through reappearance of recessive traits)  Taking true breeding lines and making monohybrids Mendel took true breeding line of SHORT plants and true breeding line of TALL plants and bred them together  Are these traits going to mix together and disappear?  Offsprings TALL, no SHORT characteristics  When you breed these offspring together; SHORT comes back  It never disappeared, but it was being masked Generational line  Parental line are those you begin with (P generation)  First filial are the second generation (F1): here it is the TALL plants, the kids of the parents  Second filial are produced by first filial 4 Monohybrid crosses: Characteristics of these two plants are IDENTICAL to each other except for this ONE trait (size)  Looking at one gene that was two different flavors: TALL or SHORT Monohybrids: hybrid of tall or short, everything else being the same  They all look tall, but they all carry that dwarf allele (masked by tall)  Breed monohybrids together; you will get back a portion of that recessive trait Discrete units of inheritance are alleles of genes. Alleles are alternative forms of a SINGLE gene. Hypothetical example  Let’s say each of the following features is determined by a SINGLE gene  Usually controlled by multiple genes  Everyone has this gene; but the gene isn’t working 5 Most human traits (including the above) are determined by multiple genes with multiple alleles. Most eukaryotes have two copies of each chromosomes; TWO copies of each gene  One set from one parent and another from the other parent: diploid  Not true for ALL eukaryotes  Looking at specific chromosomes; hundreds of genes on chromosome 1 in humans  Two different versions of the gene: HOMOLOGOUS chromosomes  Difference between different alleles in same gene, different DNA sequence  We can’t say whether it will produce a phenotypic outcome or not Italicized to tell us we are dealing with the GENE/ALLELE and NOT the protein product  The protein product could be designated as non-italics  Need to know whether we’re talking about the gene/allele or the product of that gene or allele Two chromosomes (diploid), looking at gene that tells us specific texture, looking at TWO alleles 6  Smooth allele as RED (call it w) Two copies of dominant allele, describe genotype as HOMOZYGOUS dominant  The phenotype is SMOOTH Heterozygous means one of each; homozygous means only one whether it’s dominant or recessive 1) A gene may have several alleles that normally occur in a population= polymorphic  Many different versions of this gene; how frequent do you see those versions in a population  Hair color determined by single gene and has multiple alleles for that gene  Black, brown, blonde, red allele  Find phenotype examples of this allele in everyone within a class  Red heads at least frequent allele But a maximum of TWO for one gene can exist in a diploid individual e.g., humans 2) Some genes have only one allele that is normally present in a population= monomorphic  OPPOSITE of polymorphic  Vary those genes in metabolic processes will have large impacts (eg. Sickle cell anemia)  Other versions (alleles) are at a really LOW frequency 7 Mendel’s law of segregation: The two alleles for each trait separate (segregate) during gamete formation, then unite at random, one from each parent, at fertilization.  Describes how alleles of ONE gene behave Each parent
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