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What are the Two Laws of Mendelian (Classical) Genetics? What are Alleles?. Developed by Gregor Mendel (1822-1884): studied heredity in pea plants (mainly texture and color of seeds); based solely on observations (no knowledge of DNA or meiosis) – see cartoon
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What are the Two Laws of Mendelian (Classical) Genetics? What are Alleles? • Developed by Gregor Mendel (1822-1884): studied heredity in pea plants (mainly texture and color of seeds); based solely on observations (no knowledge of DNA or meiosis) – see cartoon • Law of Segregation: there are two sets of genes for a particular trait (one from each parent), but only one gets into gamete during gametogenesis • Law of Independent Assortment: during gametogenesis, a gene that enters a gamete does so independently of those for other traits (ex. if red hair expressed, blue eyes not necessarily expressed) • Alleles: different forms of same gene (found at same locus) • Dominant allele: the form expressed in offspring (if present) • Recessive allele: masked by dominant allele (not expressed if dominant allele present), but can still be passed on to next generation (by a carrier)
What are Genotypes and Phenotypes?How do we Solve Genetics Problems? • Phenotype: description of form of physical trait an individual exhibits (ex. trait of hair color, “red hair” is a phenotype) • Genotype: description of individual’s condition at the genetic level; three possible genotypes: • Homozygous dominant (AA): both genes for trait instruct to produce dominant phenotype • Homozygous recessive (aa): both genes for trait instruct to produce recessive phenotype • Heterozygous (Aa): genetic instructions conflict; for Mendelian traits, dominant phenotype results (recessive masked) • Solving Genetics Problems: Mendel described traits in P (parental) generation and F1, F2 (filial) generations • Monohybrid Cross: single trait; parents’ genotypes crossed using Punnett Square • Dihybrid Cross: two traits; find results for each single trait with Punnett Square, then multiply probabilities (ex. ¼ X ¼ = 1/16)
What are Some Modern Additions to Mendelian Genetics? • Polygenic traits: traits caused by multiple genes • Variation in population often follows bell curve when frequency is plotted against measurement of phenotype (ex. height) • Multiple alleles: ex. blood types • Only two alleles in any cell, but more than two in population • Linked genes: loci typically in close proximity • Incomplete Dominance (and Co-dominance) • Phenotype for heterozygous genotype is a mixture (blend) of those caused by homozygous genotypes • Problem-solving is the same as Mendelian traits; need to evaluate genotypes differently; any mention of three pheno- types in problem? • Examples: color of petals in roses; Sickle-cell anemia
How is Sex Determined? What are Some Examples of Sex-linked Traits? • Sex Determination: 23rd pair of human chromo- somes are the sex chromosomes, others are autosomes - see cartoon • Females: XX; Males: XY • Sex-linked traits: X-sex chromosome has many genes other than those for sex determination, but Y-sex chromosome does not no male carriers for sex-linked traits • Examples: color blindness, hemophilia • Same methods to solve problems, but must account for sex of parents and offspring [ex. XHXh x XHY XHXH, XHY, XhXH (= XHXh), XhY]
