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Biol_139_Exam_Review_Sheets - Biology 139(Genetics Final...

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Biology 139 (Genetics) Final Exam Review Biology Undergraduate Society Mendelian Genetics Why garden peas? Easy to cross fertilize Large numbers of offspring Short growing season Terms Phenotype - observable characteristic (largely determined by genotype). Commonly referred to as a trait Genotype - genetic make-up Monohybrid Cross – matings between individuals that only differ in one trait Gene – unit of inheritance Allele – alternative form of a single gene Polymorphic – More then one wildtype allele; allele frequency greater than 1% Monomorphic – Only one wildtype allele; only one allele above 1% frequency 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. At the DNA level, alleles vary in nucleotide sequence. This can result in: o New amino acid sequence o Change in the amount of protein o E.g. Garden peas – R gives the pea a round shape because this version of the Starch Branching Enzyme is normal. The r allele as a different amino acid sequence and the enzyme is non-functional (i.e. no branched starch). Genetic Crosses Test Cross – Breeding an unknown genotype with the homozygous recessive will reveal its genotype. If it was homozygous dominant, all progeny are
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Biology 139 (Genetics) Final Exam Review Biology Undergraduate Society heterozygous and have the same phenotype. If it was heterozygous, we would see a 1:1 phenotypic ratio. The law of product: The probability of two or more independent events occurring together is the product of the probabilities that each event will occur by itself. The law of sum: The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities Dihybrid Cross – Matings of individuals that differ in 2 traits. More phenotypic classes are seen because there are more allele combinations for independent assortment. Mating between dihybrids produces a 9:3:3:1 phenotypic ratio. Multihybrid Cross – Matings that differ in 3 or more traits o Huge punnet square. Use a simpler method. E.g., Aa Bb Cc Dd x Aa Bb Cc Dd o Probability of an AA bb Cc Dd offspring: ¼ x ¼ x ½ x ½ =1/64 o Modifications of Mendellian Ratios Crossing – When a gene has more than 2 alleles, reciprocal crosses help determine dominance relationships by crossing pure breeding lines. A dominance series shows most dominant to least dominant. Types of Dominance Incomplete Dominance – The F 1 Hybrid doesn’t resemble either parent (intermediate phenotype) o E.g. Flower pigments. A red flower and a white flower produce all pink F 1 ’s. The F 2 ’s will have a 1:2:1 phenotypic ratio.
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