Week 2 Notes
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This 5 page Class Notes was uploaded by Amanda Notetaker on Thursday January 7, 2016. The Class Notes belongs to 012 at University of Vermont taught by Dr. Hill in Fall 2016. Since its upload, it has received 45 views. For similar materials see Exploring Biology in Biomedical Engineering at University of Vermont.
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Date Created: 01/07/16
CH. 8 Inheritance, Genes, and Chromosomes 8.1: Genes are Particulate and Are Inherited According to Mendel’s Laws Original Hypotheses: Blending inheritance: gametes’ genes blended when fused Particulate inheritance: each gene is physically distinct and remains intact when fused Why garden peas? - Flowers have male/female sex organs (pistils/stamens) - Self-fertilize Characters: physical feature (color, shape) Traits: forms of character (purple, wrinkled) Phenotype: observable trait from genes (purple, white) Genotype: genetic code (RR, Rr, rr) Parental Generation (P) à First Filial Generation (F1)à Second Filial Generation (F2)… Hybrid: offspring of crosses between organisms with different characters Monohybrid Cross: (Mendel’s 1 experiment) Crossed plants differing in one character Hypothesis: When two strains of peas with differing traits are bred, their characteristics are irreversibly blended in future generations. REJECTED: F1 offspring exhibited only one trait; Trait reappeared in F2 offspring Conclusions: Trait in F1 had greater abundance à Dominant Trait in F2 in lesser abundance à Recessive Genes occur in pairs and are separated in gametes - Each plant receives one gene from each parent à 2 genes = one character - Different traits appear due to different forms of a gene for a character (alleles) - Homozygous Alleles: two copies of same allele (RR,rr) - Heterozygous Alleles: two different alleles (Rr) Law of Segregation: two copies of a gene separate during gamete formation (each gamete receives one copy of gene) Punnett Square: Connect back to BCOR 011… Genes produce proteins w/ specific function à dictates phenotype - Ex: lack of starch branching enzyme due to insertion in genes à wrinkled pea phenotype Test Crosses: Cross F1 w/ recessive genotpe homozygote (rr) - If F1 was homozygous (RR) à all offspring would be dominant - If F1 was heterozygous (Rr) à half of offspring would be recessive Example from class… Trial A,C: Parents must be homozygous dominant in order to produce offspring all showing the dominant trait. Trial B: Parents must be heterozygous, producing offspring showing both the dominant and recessive trait. Dihybrid Cross: Cross peas differing in two characters Hypothesis: Genes are distributed independently to the offspring. - Crossed RRYY (round yellow seeds) w/ rryy (wrinkled green seeds) o If linked: F1 offspring with RY and ry, F2 offspring similar to monohybrid cross o If independent: F2 offspring would be RY, ry, Ry, or rY. Results: four different phenotypes in F2 offspring Conclusion: Segregation of R from r is independent from segregation of Y from y Law of Independent Assortment: Alleles of different genes assort independently during gamete formation Probability: - Geneticists began using probability calculations to predict ratios of genotypes/phenotypes - Probability 1 = 100% certainty, 0 = 0% certainty - Multiplication Rule: Probability of 2 independent outcomes happening together, “and” - Addition Rule: Probability of an event that can occur in multiple different ways, “or” Pedigrees: family trees that show patterns of inherited phenotypes over several generations - Dominant Inheritance: Everyone w/ abnormal phenotype has affected parent; All (if homozygous parent) or half (if heterozygous parent) of offspring are affected - Recessive Inheritance: Affected people have two unaffected parents; Within affected family ¼ offspring of unaffected parents are affected à parents are heterozygous carriers 8.2: Alleles & Genes Interact to Produce Phenotypes Mutations: rare, inherited changes in genetic material - Wild Type: allele present in majority of population; other alleles are mutant alleles - Polymorphic: any trait that has more than two forms of that gene at a locus - One gene can have more than 2 alleles (Ex: rabbit’s coat) - The more alleles, the more possible phenotypes; may show a hierarchy of dominance in heterozygotes Incomplete Dominance: Alleles that appear together as a blended trait; Alleles are neither dominant/recessive, instead heterozygotes have an intermediate phenotype Ex: Red snapdragon flower + white à intermediate pink Codominance: two alleles of a gene express both phenotypes in heterozygote Ex: ABO Blood groups, Cows with brown/white patches Epistasis: phenotypic expression of a gene is affected by another gene Ex: Fur of Labs, Gene E (pigment deposition) determines expression of gene B (black) Hybrid Vigor (heterosis): hybrid offspring grow larger/more seeds than parents; decreases chance of recessive traits being passed on to offspring Vigor: complex trait, phenotype often determined by multiple genes Quantitative Traits: traits that must be measured, not assessed qualitatively Environment’s Affect on Phenotype Ex: Point restriction in rabbit/cat coat pattern; dark fur is temperature-dependent Penetrance: proportion of individuals in a group that actually show expected phenotype Ex: Inheritance of mutant allele causes many people, not all, to develop breast cancer à mutation is incompletely penetrant Expressivity: degree a genotype is expressed Ex: Individual w/ mutant allele may get breast and ovarian cancer while another may only get breast cancer à mutation has variable expressivity Heretiability: (similar to penetrance) contribution of genetic vs. environmental factors to variation in a character in a population - Varies from 0 to 1 - Ex: Human height (quantitative trait) varies from 0.65 to 0.8, meaning 65% variation due to genetics, 35% due to environment - Estimates apply only to variation in populations, not individuals 8.3: Genes are Carried on Chromosomes Gene: sequence of DNA residing on locus of chromosome Genetic Linkage: can alter pattern of inheritance described by Mendel’s law Hypothesis: Alleles for different characteristics always assort independently - Based on Mendel’s law of independent assortment F1 genotypes should appear in equivalent amounts - REJECTED: Parental phenotypes appeared in greater abundance than recombinant ones Conclusion: Genes for body color/wing size are linked, their alleles don’t assort independently; The two loci are linked on same chromosome Recombinant Phenotypes: two homologous chromosomes exchange corresponding segments during Prophase I of meiosis (crossing over) Recombination Frequency: calculated by dividing the recombinant offspring by total number of offspring - Higher for loci that are farther apart on chromosome - Used to infer locations of genes along chromosome to make genetic map Fruit flies have 1 pair of sex chromosomes, 3 pairs of autosomes Sex Chromosomes: involved in sex determination, two chromosomes of diff sizes Autosomes: all other chromosomes, same in size Sex Chromosomes: Males = XY, Females = XX - Many genes on X chromosome aren’t present on Y - Males only have one copy of some genes, will express all alleles of his X chromosome - Hemizygous: gene present as a single copy in a diploid organism - Ex: Gene for eye color carried on X chromosome, wild type red (R) vs mutant allele white (r) Sex-Linked Inheritance: inheritance of a gene that is carried on sex chromosome - Most commonly involves genes on X chromosome b/c usually larger than Y Patterns in X-Linked Recessive Phenotypes: - Appear more frequently in males since only one copy is needed for expression in males - Male w/ mutation can only pass it on to daughters (sons get his Y chromosome) - Heterozygous Carriers: daughters who receive one X-linked mutation; normal phenotype but can pass mutant allele on to offspring - Mutant phenotype can skip a generation (if male à daughter à son) Maternal (Cytoplasmic) Inheritance: Inheritance of organelles/their genes, non-Mendelian - Egg cells have lots of cytoplasm and organelles vs. sperm’s nucleus meaning mitochondria/plastids are inherited only from the mother - Mutations have large effects b/c organelle genes are important for organelle assembly/fuction (Ex: in plants a mutation affecting the protein that assembles chlorophyll molcules into photosystems à all white plants) 8.4: Prokaryotes Can Exchange Genetic Material Prokaryotes reproduce asexually (binary fission) à genetically identical offspring, BUT mutations can occur in prokaryotes Horizontal/Lateral Gene Transfer: gene transfer from one individual to another w/o sexual reproduction; generates diversity Bacterial Conjugation: process of bacteria exchanging genes - Sex Pilus: projection that initiates contact between cells - Genetic material passes from donor to recepient through conjugation tube (NOT recipient à donor) - In recipient cell, donor DNA lines up with recipient’s DNA à crossing over - Genes from donor incorporated into recipient’s genome to be passed on to offspring Plasmids: bacteria’s small, cirular DNA molecules that replicate independently - responsible for transferring genes between bacteria; - Often have genes for unusual metabolic tasks(hydrocarbon breakdown) / antibody resistance - Can move between cells during conjugation bringing new genes to recipient bacterium
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