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Genotypes and Phenotypes Not all phenotypic variation is due to genotypes The tallest man in the world was tall because of a tumor pushing against his pituitary gland for example To understand genetics we must consider genes are on chromosomes so when we analyze the 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 that are 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 locus Important 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 generation Recessive alleles are only expressed in homozygous conditions The principle of segregation says that maternal and paternal alleles segregate during gamete formation Each gamete contains only 1 allele for a particular genetic locus Why Homologues separated during Anaphase I and they took the alleles with them We use punnett squares to make predictions but this model assumes that gametic types are equally likely that the combinations are random and that the outcomes in each square are equally probable Dihybrid cross aka 2 factor cross is where we cross over two factors starting with true breeding parents If we did a dihybrid cross with a true breeding parental generation the F1 would be heterozygous and the F2 would have the iconic 9 3 3 1 ratio This happens no matter what traits are on what parts of the chromosomes because of independent assortment which is 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 the product of each individual probability The addition rule says that if there are two different ways an event can occur the probability of the event the probability of one way probability of the other way 9 16 3 16 3 16 1 16 Something to keep in mind There are only two phenotypes for each allele dominant or recessive but three genotypes homozygous dominant homozygous recessive and heterozygous This means For 1 loci For 2 loci For n loci 2 phenotypes and 3 genotypes 4 phenotypes and 9 genotypes 2n phenotypes and 3n genotypes Test cross where you cross an unknown genotype with a homozygous recessive individual for all loci of interest If the unknown is homozygous dominant it should produce no recessive but if the 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 parent or the other but not both This is when Mendel came along Mendel s experiments were unique not just for being so thorough new and detailed but also for using reciprocal crosses reversing the sexes of the plants 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 We now know that there is also a 9 3 3 1 ratio when testing for two alleles Mendel s findings can be summarized as follows 1 Parents do not transmit traits directly but they transmit information genes containing those traits 2 Each individual receives one copy of each gene from each parent 3 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 other 5 The presence of an allele does not guarantee the trait will manifest To find which traits are dominant we use a pedigree a chart of family history detailing genes The Principle of Independent Assortment says that segregation of different allele pairs is independent There are some extensions we can make to Mendel s findings 1 Polygenic inheritance more than one gene defines a trait 2 Continuous variation when a trait occurs continuously and in a gradium slope 3 Quantitative variation the name for a polygenic trait 4 Pleiotropic an allele that affects more than one trait 5 Some genes have more than two alleles like blood type 6 Dominance is not always complete a Incomplete dominance when heterozygous genotypes are an intermediate between 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 This proves it is not an example of Mendelian inheritance Incomplete dominance is the result of biochemistry caused by gene expression The gene CR codes 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 to synthesize sufficient anthocyanin and produce red But heterozygous CR produces half that enzyme resulting in pink A famous example of codominance is blood type There are 4 phenotypes A B O and AB but only 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 neither Chromosomes Chromosomal theory of inheritance traits carried on by chromosomes A trait that is on the X or Y chromosome is said to be sex linked Some disorders are sex linked like hemophilia The sex chromosomes in humans are the 23rd pair of chromosomes 45th and 46th that determine sex They are X and Y The autosomes in humans are the 1st 22nd pairs of chromosomes 1st through 44th that do not determine sex Heterogametic Sex having 2 kinds of sex chromosomes eg X and Y Will be hemizygous having only one copy of an allele in an otherwise diploid organism In humans the Y chromosome usually has no loci corresponding to X chromosomes This means if an allele is present on the X chromosome in a guy it will be expressed no matter what because there is nothing to

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Purdue BIOL 10200 - Genotype and Phenotype Notes

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