Purdue BIOL 10200 - Genotype and Phenotype Notes

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Genotypes and Phenotypes:Not all phenotypic variation is due to genotypes. (The tallest man in the world was tall becauseof a tumor pushing against his pituitary gland, for example.)To understand genetics we must consider: genes are on chromosomes, so when we analyzethe movement of chromosomes via meiosis, we are analyzing the movement of genes.● Gene: specific region of DNA that codes for a specific protein● Locus: location of a gene on a chromosome● Allele: different forms of a gene● Genotype: alleles carried by a person● Phenotype: appearance of a person● True-breeding: when we make experiments and predictions, we start with parents thatare showing a particular trait due to being homozygous (one dominant, one recessive)● Homozygous: having only one type of allele at a locus● Heterozygous: having 2 different alleles at a locusImportant experimental terminology● Crossing: specific mating between parents● Parental generation (P0): parents in the original cross● First filial generation (F1): offspring of the parental generation● Second filial generation (F2): offspring of the first filial generationRecessive alleles are only expressed in homozygous conditions.The principle of segregation says that: maternal and paternal alleles segregate during gameteformation. Each gamete contains only 1 allele for a particular genetic locus. Why? Homologuesseparated during Anaphase I, and they took the alleles with them!We use punnett squares to make predictions, but this model assumes that gametic types areequally likely, that the combinations are random and that the outcomes in each square areequally probable.Dihybrid cross (aka 2-factor cross) is where we cross over two factors, starting with truebreeding parents. If we did a dihybrid cross with a true-breeding parental generation, the F1would be heterozygous, and the F2 would have the iconic 9:3:3:1 ratio. This happens no matterwhat traits are on what parts of the chromosomes, because of independent assortment whichis the random assortment of traits.We can use probability to make our lives a bit easier. If independent assortment is occurring(and ONLY if its occurring), use laws of probability to predict genetic outcomes.● The “product rule” says that the probability of 2 independent events occurring = theproduct of each individual probability.● The “addition rule” says that if there are two different ways an event can occur, theprobability of the event = the probability of one way + probability of the other way(¾) * (¾) = (9/16)(¾) * (¼) = (3/16)(¼) * (¾) = (3/16)(¼) * (¼) = (1/16)Something to keep in mind. There are only two phenotypes for each allele (dominant orrecessive) but three genotypes (homozygous dominant, homozygous recessive andheterozygous). This means● For 1 loci: 2 phenotypes and 3 genotypes● For 2 loci: 4 phenotypes and 9 genotypes● For n loci: 2nphenotypes and 3ngenotypesTest cross: where you cross an unknown genotype with a homozygous recessive individual forall loci of interest. If the unknown is homozygous dominant it should produce no recessive, but ifthe unknown is heterozygous it should produce some recessives.Hybridization and Crosses:The first person to achieve and document hybridizations was Josef Kolreuter. His experiments,and many more, led to the discovery of segregation: when children have traits from one parentor the other, but not bothThis is when Mendel came along. Mendel’s experiments were unique not just for being sothorough, new and detailed, but also for using reciprocal crosses: reversing the sexes of theplants to make sure sex isn’t a factor.Mendel discovered the 3:1 ratio, which is actually a 1:2:1 ratio, when testing for one allele. Wenow know that there is also a 9:3:3:1 ratio when testing for two alleles.Mendel’s findings can be summarized as follows1. Parents do not transmit traits directly, but they transmit information (genes) containingthose traits2. Each individual receives one copy of each gene from each parent3. Not all copies are identical, and different forms/copies of a gene are called alleles.Combos of alleles can be homozygous (same) or heterozygous (different)4. The alleles neither blend nor alter each other5. The presence of an allele does not guarantee the trait will manifestTo find which traits are dominant, we use a pedigree: a chart of family history detailing genesThe Principle of Independent Assortment says that segregation of different allele pairs isindependentThere are some extensions we can make to Mendel’s findings1. Polygenic inheritance: more than one gene defines a trait2. Continuous variation: when a trait occurs continuously and in a gradium (slope)3. Quantitative variation: the name for a polygenic trait4. Pleiotropic: an allele that affects more than one trait5. Some genes have more than two alleles (like blood type)6. Dominance is not always completea. Incomplete dominance: when heterozygous genotypes are an intermediatebetween the two homozygotes (red + white = pink)b. Codominance: when heterozygous genotypes alternate between two traits (red+ white = red and white striped)Incomplete dominance displays a 1:2:1 phenotypic ratio, the same as the genotypic ratio. Thisproves it is not an example of Mendelian inheritance.Incomplete dominance is the result of biochemistry caused by gene expression. The gene CRcodes for a functional enzyme involved in the production of the plant pigment anthocyanin(found in flowers, leaves, corn, and berries). Homozygous CR produces enough enzymes tosynthesize sufficient anthocyanin and produce red. But heterozygous CR produces half thatenzyme, resulting in pink.A famous example of codominance is blood type. There are 4 phenotypes (A, B, O and AB) butonly 3 alleles (A, B and O). A and B are dominant to O but codominant to each other● Blood Type A exhibits Galactosamine● Blood Type B exhibits Galactose● Blood Type AB has both● Blood Type O has neitherChromosomes:Chromosomal theory of inheritance = traits carried on by chromosomesA trait that is on the X or Y chromosome is said to be sex-linked. Some disorders aresex-linked, like hemophiliaThe sex chromosomes in humans are the 23rd pair of chromosomes (45th and 46th) thatdetermine sex. They are X and Y.The autosomes in humans are the 1st-22nd pairs of chromosomes (1st through 44th) that donot determine sex.Heterogametic Sex: having 2 kinds of sex chromosomes (eg. X and Y)● Will be hemizygous (having only one copy of an

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