BI 206/216 Notes (1/19 - 1/28)
BI 206/216 Notes (1/19 - 1/28) BI 216
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This 4 page Class Notes was uploaded by JordanK on Friday January 29, 2016. The Class Notes belongs to BI 216 at Boston University taught by Dr. Celenza in Spring 2016. Since its upload, it has received 45 views. For similar materials see Intensive Genetics in Biology at Boston University.
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Date Created: 01/29/16
BI 206/216 Notes (Spring ‘16) Class Notes 1/19 through 1/28 Intro phenotype: sum of an individual’s inherited traits and the influence of the environment; overall appearance genetics is the study of inheritable traits and the analytical methods used to study it “central dogma” is that DNA encodes proteins preMendelian genetics include lended inheritance ex: light green parent + dark green parent = medium green parent) → disproved theory Mendelian Genetics Mendel used peas to control mating, used cleancut traits in purebreeding lines, did reciprocal crosses and used large populations observed 7 characteristics that had only 2 phenotypes (no “blending”) which showed one trait wasdominant over the other in a monohybrid cross, a consistent ratio of dominarecessive phenotypes (:1) was observed led to the idea that each parent hallele of the phenotype AA, Aa, or aa Law of Segregation: 2 alleles for each trait separate during gamete formation at random; 2 gametes (one from each parent) unite at random during fertilization can getomozygous genotypes (AA, aa) orheterozygous genotype (Aa) increases genetic diversity in a population testcross: crossing an unknown individual with a known homozygous recessive individual to predict the genotype of the unknown individual based on what the offspring are Current Genetics units of inheritance described by Mendel are calenes pairs of antagonistic traits (A, a) are alleles of the same gene meiosis and segregation of alleles cause random inheritance → genetic diversity dominant allele wild type (sometimes indicated with a superscript “+”) common allele in a population, serves as a benchmark for comparison many alleles of the same trait may all be wildtype Mendel’s 2nd Law Mendel’s aw of Independent Assortment alleles separate independently of each other during gamete formation ex) AaBb parent could form AB, Ab, aB, or ab gametes two AaBb parents produce offspring that show a 9:3:3:1 phenotypic ratio (9 showing both dominant, 3 showing only A dominant, 3 showing only B dominant, 1 showing both recessive) multiply expected ratios of each trait to predict ratios of phenotypes for a multitrait cross (multiplication rule) use addition rule to predict probability of one of several mutually exclusive events cystic fibrosis: recessive mutant allele fails to function as a chloride transporter which leads to excess mucus build up Interactions between Alleles of a Gene AA is chemically distinct from Aa, even if the phenotype is the same ex) amylopectin in peas (causes round shape) → Rr peas produce half as much as RR peas, even though both types appear round complete dominance : dominant alleles cause hybrids to only show dominant phenotype (ex: purple is dominant to white flowers) incomplete dominance : hybrid shows phenotype that looks like a blend of the two alleles (ex: purple x white parents = light blue offspring) codominance : hybrid shows discrete areas of both alleles in phenotype (ex: distinct patches of purple and white in same flower) seen in blood type → I Aor I encode for slightly different glycosyltransferases while i has a single deletion of a nucleotide leading to a nonfunctional protein; blood type can be A, B, AB (*codominance), or O pleiotropic: one gene can affect more than one phenotype/characteristic ex) HbS allele that causes sicklecell anemia also provides resistance to malaria organisms who are heterozygous for a recessive lethal allele will always have offspring in a2:1 ratio in a monohybrid cross (recessive offspring will die) some genes may be dominant and recessive lethal at the same time ex) AYA = dead offspring; AA = AY coat color, AA = A coat color Epistasis the effects of one gene on another gene (masking, amplification, etc.) ex) hair genetics in humans → B/b determines color, H/h determines presence of hair in the first place B_H_ = brown hair bbH_ = blonde hair B_hh = alopecia (hairless) bbhh = alopecia affects Mendel’s expected 9:3:3:1 ratio in a dihybrid cross ex) lentil color: A = tan, B = gray, AB = brown, ab = green (no pigment, just chlorophyll) complementary gene interaction: both genes phenotypes are required to produce overall phenotype ex) both enzymes A and B in flowers are required to be purple, otherwise flowers are colorless (white) creates a 9:7 ratio in a dihybrid cross for phenotype because the mutant allele for each gives the same phenotype recessive epistasis : the recessive allele of one gene will mask the other gene, no matter what the allele is ex) labrador retriever coat color → BB/Bb is black fur, bb is brown fur, but both need EE or Ee to work because ee will produce yellow fur no matter what creates a 9:3:4 ratio for phenotype dominant epistasis : dominant allele of one gene completely masks the effect of the other gene no matter what produces a 12:3:1 ratio or a13:3 ratio ex) 12:3:1 in squash: enzyme B will not produce color, even if enzyme A (yellow color) or enzyme a (green color) is working (need bb to produce color) ex) 13:3 in chickens: AA/Aa makes color, aa does not make color; BB/Bb will not deposit color but bb will deposit color redundancy : produces 15:1 ratio, different genes encode same function (only homozygous recessive aabb will produce different function) allele interactions and gene interactions can occur at the same time → leads to different phenotypic variations Mutant Genotype the same mutant genotype does not always result in the same phenotype incomplete penetrance: mutant or wild type phenotype will occur even though mutant genotype is predicted ex) Siamese cats have mutant allele for dark fur but it only works in cold temperatures (feet, ears, tail) variable expressivity : mutant genotype has varying levels of being expressed phenotypically Chromosome/Inheritance sex determination is superficially similar between species but mechanistically different humans: XX = female, XY = male fruit flies: 1:2 ratio of X chromosomes to pairs of autosomes = male, 1:1 ratio of X chromosomes to pairs of autosomes = female (0.51 = intersex) homogametic sex in some reptiles and birds is male (ZZ), while heterogametic sex is female (WZ) *opposite of humans different systems exist for different species Mitosis & Meiosis chromosomes contain both DNA and proteins karyotype : image of all the chromosomes of an organism cell cycle contains G1, S, G2, and M phases M phase: prophase, prometaphase, metaphase, anaphase, and telophase cytokinesis involves cleavage furrow in animals and also the splitting of the cell plate in plants checkpoints must be passed between phases: pass based on nutrient availability, amount of damage, and/or need for new cells meiosis: 2n → n crossing over occurs in this process → increases genetic diversity one sister chromatid from each pair in each new gamete (4 gametes) *know terms from figure 4.15 in textbook about different stages in meiosis Drosophilia short life cycles (~1 week) good for genetics experiments involved in discovery of sexdependent phenotypes ex) red or white eye gene located on X chromosome
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