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Chapter 14: Mendelian Genetics Early Inheritance Ideas Preformationism – homunculus (little, fully formed human) inside either sperm or egg, fertilization stimulates growth to full size o Spermists vs Ovists Pangenesis – gemmules made by cells, accumulate info for traits through lifetime, passed on to offspring via sex cells Blend Inheritance – kids blended genetic traits of parents, like paint colors mixing Gregor Mendel Austrian (now Czech) Monk Father of Modern Genetics Used pea plants to systematically test blending hypothesis o Chose 7 traits, each with two distinct varieties o Removed immature stamens (no selfing), hand pollinated, covered w/ bags Monohybrid Cross o A hybrid is a cross b/w two varieties of organisms o Mendel started w/ true-breeding organisms to create ‘hybrid’ for one trait P (parent) generation crossed F1 (first filial) is offspring of P gen. F2 (second) filial is offspring of cross b/w F1 individuals F1 always 100% purple Purple males & females used to eliminate sex as a factor F2 a 3:1 ratio of purple to white o Mendel’s Conclusions Unit Characters (Particulate Hypothesis) Traits are inherited in discrete units (genes) that come in pairs (one from each parent) F1 must have a unit for both purple & white flowers since white flowers re-appear in F2 Law of Dominance When two contrasting units are present in an individual, one will always dominate the other o Unit that dominates is the Dominant trait (capital letter, P for purple) o Unit that is dominated is Recessive trait (lower case, p for white) Law of Segregation During gamete formation, pairs of units are segregated from each other in such a way that each gamete contains only one unit o Meiosis separates homologous chromosomes Genetics Terminology o Gene – unit of inheritance o Allele – contrasting forms of a gene P & p for flower color (purple & white) o Homozygote – individual w/ same alleles (PP/pp) True or pure breeding since always produce same type of offspring when self-pollinated o Heterozygote - individual w/ different alleles (Pp) o Genotype – actual combination of alleles (PP, Pp, pp) o Phenotype – physical manifestation of allelic combination (purple or white flowers) Predictable Results o For Monohybrid cross of homozygotes o F1 generation 100% heterozygous o F2 generation Phenotype – 3:1 dominant Genotype – 1:2:1 Due to Law of Segregation Test Cross o Used to figure out genotype for a dominant phenotype o Cross individual with a recessive o Purple (PP/Pp) w/ white (pp) PP = 100% purple Pp = 50:50 purple/white Dihybrid Cross o Cross b/w individuals that are heterozygous for two traits o Mendel wanted to see if traits where inherited independently of others Seed Color (Y=yellow, y=green) Seed Shape (R=round, r=smooth) P generation = YYRR x YYRR F1 = YyRr x YyRr o Law Of Independent Assortment When alleles of two different genes segregate, they do so independently Genotype: YyRr Possible Gametes: YR, Yr, yR, yr Use ‘FOIL’ if needed Predictable 9:3:3:1 ratio in F2 Genes close to each other tend to be inherited in pairs Law of Probabilities Probability of two or more independent events occurring together is product of their independent probabilities o What is probability of tossing a coin three times & getting three heads? o ½ x ½ x ½ = 1/8 Dihybrid Cross & Higher o Each trait can be treated separate Do a monohybrid cross for each Multiply probabilities for each desired combination o Calculate phenotypic probabilities of dihybrid cross (YyRr: yellow/round) o Parents: AaBBCc x AABbCc Probability of AABbCC? Beyond Mendel Sex-Linked Traits o Traits that are found on the X chromosome o Usually disorders are recessive Red-green color blindness Hemophilia o XX / XX+ & XY = Normal o X+X+ & X+Y = Affected o If dominant (very rare), one copy will affect female Incomplete Dominance o Heterozygote is a ‘blend’ of the dominant & recessive alleles o Snapdragons & Carnations Red (dominant), pink (heterozygous), white (recessive) o Human hair Curly/wavy/straight Codominance o Both alleles are expressed equally o AB blood type (A & B alleles) o Sickle Cell Anemia o Tay-Sachs Disease o Many flowers (2 different colors) Multiple Allelism o Trait with 3+ alleles o ABO Blood Group Alleles IA (A antigen), IB (B antigen), i (no antigens) IA & IB are codominant, but completely dominant over i IA IA / IAi = type A IB IB / IBi = type B IAIB = type AB & ii = type O Pleiotropy o Most genes will affect multiple phenotypic traits (one gene = multiple traits) o Plants – flower color gene can affects seed coat color o PKU o Cystic Fibrosis o Sickle-Cell Epistasis o Two or more genes interact in a way that can hide or mask traits o Labrador Retrievers Black, brown, & yellow Gene (E) determines if pigment for fur color (B) will be deposited o Sweet Peas Purple vs. White flowers Need dominant 2nd gene for purple Polygenic Inheritance o Quantitative characters are traits that fall on a continuum o Caused by multiple genes interacting on one trait (phenotype) Human skin – 3+, more dominants = darker skin (plus environment) Human eye color – 2 main, ~16 total Human Hair – unknown, shades caused by varying levels of two chemicals Nature vs. Nurture o Plants Sunlight, soil nutrients, wind can affect color, height, weight o Humans Diet, sun, altitude chemicals can affect all human systems Skin color, blood cell counts, hormone levels o Expressivity & Penetrance Disorders & Pedigrees o Human disorders can be tracked by looking at pedigrees o Family tree where squares are males, circles are females o Shaded shape = affected o ½ shade (when available) = carrier o Use process of elimination for 4 options o Autosomal dominant or recessive o Sex-linked dominant or recessive