Exam 1 Study Guide Genetics
Exam 1 Study Guide Genetics Biol 2004
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This 7 page Study Guide was uploaded by Monica Ricci on Friday February 12, 2016. The Study Guide belongs to Biol 2004 at Virginia Polytechnic Institute and State University taught by Dr. Voshell in Winter 2016. Since its upload, it has received 77 views. For similar materials see Genetics in Biology at Virginia Polytechnic Institute and State University.
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Date Created: 02/12/16
Lesson 1: Introduction to Genetics, Chromosomes, and Cellular Repro Genes in everyday life Appearance, personality, intelligence Susceptibility to disease Direct development of organisms Ag and animal breeding Biotechnology and medical fields Ancient genetics People domesticated plants and animals 10,000-12,000 years ago Ancient Jewish writings show understanding of inheritance of hemophilia Ancient Greeks had theories on inherited traits—wrong, but impressive still Passed similar organs through repro organs Traits acquired during lifetime could be passed down How we study genetics Model organisms- short generation, numerous offspring per generation, reared in lab for controlled setting, control genetic crosses, a lot of info available Scientists Gregor Mendel- principles of heredity Schleiden and Schwann- cell theory Flemming- chromosomes Darwin- evolution Weismann- germ-plasm theory Sutton- genes are located on chromosomes Fundamental concepts Genes are the fundamental unit of heredity Alternate forms of a gene are alleles Genes are located on chromosomes An organism’s genotype contributes to its genotype (environmental factors too) DNA and RNA both carry info, DNA to RNA to protein Mutations are changes to genetic info, hereditary Evolution is change in genetic info over time/generations Prokaryotes vs. eukaryotes Prokaryotes- no nucleus, one circle of DNA, smaller, no organelles Eukaryotes- present nucleus and organelles, large amount of DNA, complexed with histones, multiple linear DNA Mechanisms of cellular reproduction Prokaryotic- single chromosome replicated and goes to new cell, not highly ordered, high rate of replication Eukaryotic- more complex b/c DNA is in nucleus, chromosomes are highly ordered and specific, histones tightly pack DNA Homologous Pairs Diploid organisms—two sets of chromosomes, one allele on each contribute to one trait Human genome—23 pairs of chromosomes After rep, two sister chromatids, and sometimes one chromatid, but both referred to as “one chromosome” Telomeres at ends to protect against degradation during replication Centromere—at center constriction, where kinetochores form, place for spindle fibers to attach Interphase G1—primary growth phase, getting larger May enter G0—non-growing, non-dividing phase G1/S Checkpoint—can only continue once all enzymes are present for replication (commits to dividing) S—DNA synthesis DNA duplicates G2—prepares for Mitosis G2/M Checkpoint—only proceeds if DNA was replicated properly, if issue to DNA, DOES NOT PROCEED Ready for Mitosis Spindle assembly checkpoint—checks to make sure all are attached properly M Phase Mitosis—separation of sister chromatid Interphase Prophase—chromosomes condense with 2 chromatids, spindles begin to form Prometaphase—nuclear envelope degrades, spindles attach to chromosomes Metaphase—chromosomes line up on metaphase plate, spindles form “pole” Anaphase—spindles contract and move them to opposite poles Telophase—chromosomes reach poles, nuclear membrane starts to reform Cytokinesis—separation of cytoplasm, two cells result Genetic consequences of the cell cycle/mitosis New cells have half the amount of cytoplasm and organelle content of original New cells have full complement of chromosomes Genetically identical Sexual reproduction through meiosis/reductive division Two separations, one replication—4 haploid gametes Only one copy of each chromosomes Interphase—chromosome replication Meiosis I—homologous pairs line up and separate Prophase I Synapsis—close pairing of homologous chromosomes Tetrad—closely associated four sister chromatids of two homologous chromosomes Crossing over at chiasmata—crossing over of chromosome segments from the sister chromatid of one chromosome to the sister chromatid of the other synapsed chromosome **First mechanism of generating genetic variation in newly formed gametes Additional Phases Zygotene—chromosomes begin to pair Pachytene—chromosomes shorten, synaptonemal complex forms between pairs Diplotene—chromosomes remain connected at chiasmata Diakinesis—chromosomes separate from each other Metaphase I—Random alignment—mix of paternal and maternal homologous chromosomes line up Anaphase I Separation of homologous pairs Random distribution of chromosomes into two newly divided cells Telophase I—reach poles Interkinesis Period between meiosis 1 and 2 Nuclear membrane reforms, chromosomes relax Meiosis II—sister chromatids separate Prophase II—chromosomes re-condense Metaphase II—individual chromosomes line up Anaphase II—chromatids separate Telophase II—chromosomes arrive at poles and cytoplasm begins to divide Genetic consequences of meiosis Four cells are produced from each parent cell Chromosome number is half of original cell Diploid parent, haploid gametes GENETICALLY DIFFERENT than parent cell Holding chromatids together Cohesin—protein that holds homologs and chromatids together, key to behavior in mitosis and meiosis Seperase—enzyme during anaphase (anaphase I) breaks bind at chiasmata Shugoshin—protects cohesion at centromere of chromatids in anaphase I **Failure of Cohesin and Shugoshin results in irregular amount of copies in daughter cell Animal life meiosis Spermatogenesis Spermatogonium—mitosis keeps making more of itself (2n) Spermatocyte—when a spermatogonium enters prophase I of meiosis I, creates 2 (1n) Spermatids—meiosis II, creates 4 (mature into sperm) Oogenesis Oogonium (2n) Primary oocyte, at birth (2n) Secondary oocyte and polar body, ovulation, meiosis I (1n) Ovum and polar body, only when sperm comes, after meiosis 2 Lesson 2: Basic Principles of Heredity Important terms Gene Allele Locus Characteristic Genotype Phenotype/trait Heterozygote Homozygote—two of the same allele at loci Gregor Mendel Heredity Pea plant (pisum sativum): seed color, seed shape, seed coat color, pod color, pod shape, flower position, stem length Grew all his plants for two generations to ensure they were true breeding (homozygous) Monohybrid cross Cross between two parents that differ in a single characteristic F1 or “filial 1” generation (AA with aa) all round F2 (F1 self fertilization) 3:1 ratio TRAITS DO NOT BLEND—THEY REMAIN DISCRETE Conclusions One character is encoded from two genetic factors Two genetic factors separate when gametes are formed Some gametes are dominant and others are recessive Two alleles separate with equal probability into gametes Formal Conclusions (Tie in with Sutton’s Chromosomal Theory of Heredity) Principle of segregation (Mendel’s first law): each diploid organism has two alleles for any particular characteristic, two alleles segregate when gametes are formed, each gamete ends up with one allele Concept of dominance: dominant can mask, shows up in phenotype Crossing over does not affect ratios of allele types Test crosses Cross an unknown individual with a homozygous recessive (figures out unknown genotype) Rules of probability Multiplication—probability of two independent events taking place together (use the word AND) Addition—probability of one of two mutually exclusive events happening (use the word EITHER/OR) **gametes from two different can combine in four different ways (probability of getting each gamete is ½) Dihybrid crosses and independent assortment 1 homozygous dominant and 1 homozygous recessive parent= all heterozygous offspring When gametes are located on different chromosomes, they will sort independently **Applying probability to a dihybrid cross Observed ratios may deviate from expected ratio Flipping a coin for example Chi Square goodness of fit—observed values column and expected (Punnett square) Expected number n-1 Probability .1<P<.5 (indicates by chance, no significance) Lesson 3: Sex determination and sex-linked characteristics Sex determination—the way sex is established during development Can be determined by genes, chromosomes, or environmental “Sex” refers to phenotype—assigned by size of gametes (females have larger gametes) Phenotype does not always match genotype Terminology Dioecious- either male OR female Monecious- both found in same individual Both phenotypes- Hermaphroditism Chromosomal sex determination Sex chromosomes are not truly homologous Pseudo autosomal regions—have enough similarity on male and female chromosomes to pair during meiosis Heterogametic—contain different forms of chromosomes (human males, female birds) Homogametic—contain same form of chromosomes (human female, male birds) XX, XO system Insects—females are homogametic, males have single chromosome (heterogametic) XX, XY system X:A ratio *predicts Example—fruit flies have 3 pairs of autosomes and 1 pair of sex chromosomes # of sex chromosomes /# of haploid sets of autosomes Males have 0.5 ratio (one X/two sets of autosomes) # of autosomes determines on how long the sex is displayed during development Genic sex determination No specific sex chromosome Genes that determine sex are found on autosomal chromosomes (do not look physically different) Plants, fungi, fish Bees, wasps, and ants—males are haploid (unfertilized egg) and females are diploid (fertilized egg) Haploid females are not possible b/c TWO CSD alleles must be present to develop a female Diploid males CAN occur if they have two copies of the same allele Environmental sex determination Temperature Relative location in colony of organisms Limpet sex determination—position relative to others on stack, “sequential hermaphroditism” Temperature dependent sex determination—can overrule chromosomal Sea turtles—warmer temperatures produce more females Crocodiles—warmer temperature, more males Human sex determination Y chromosome causes differentiation to occur at 6 weeks after fertilization Y chromosome has SRY gene—differentiation of testes then hormones for other characteristics Testosterone—stimulates male characteristics Mullerian-inhibiting- causes default female repro degrades Phenotype= result of complex series of events during early embryonic development Lack of crossing-over—led to degradation of X chromosome to Y YO does not exist Nondisjunction Chromosomes fail to separate leading to gametes with more or less chromosomes (trisomic or monosoimic) Nondisjunction to autosome—not likely to survive Nondisjunction to sex chromosomes—effect phenotype Turner Syndrome XO genotype 1/3,000 females Short stature, low hairline, broad chest, sterility Normal intelligence Klinefelter Syndrome XXY, XXYY genotype, more than one X 1/1,000 males Tall, small testes, reduced hair, sterility Normal intelligence Jacob’s Syndrome XYY genotype Nondisjunction in male gametes 1/1,000 males Can still be fertile Triplo-X XXX genotype (more than 3 results in disabilities) 1/1,000 Fertile and normal intelligence Some taller or thinner than average X-linked inheritance Hemizygous—males that are neither heterozygous or homozygous Wildtype—has plus symbol, normal allele Take note of ZZ and ZW (female only needs one allele) Female Z linked trait can only be inherited from the father Male can inherit the Z linked trait from mother or father Man will always inherit X from maternal line Dosage Compensation X-inactivation—men and women have same amount of expression, females have Barr body, random in early development Random X-inactivation- can lead to patchy fur Having one X chromosome is important—not all X-linked genes are inactivated Lesson 4: Extensions and Modifications of Basic Principles Complete dominance Dominant phenotype as long one form of dominant allele Allelic series M^R>M>M^d in ducks Number of genotypes N (n+1)/2….n=number of alleles Incomplete dominance Phenotype is intermediate Red, white, pink Codominance Both phenotypes displayed together, do not blend (blood type i^Ai^B) Penetrance Percentage of individuals having a genotype that display the phenotype Ex. incomplete penetrance when person has genes for polydactly but does not have physical trait # of individuals with condition/# of individuals with the genotype Chance X penetrance= % observed Expressivity The degree the trait is expressed Ex. variable expressivity of polydactly **incomplete penetrance and variable expressivity are usually result of environmental factors or other genes Lethal alleles Skewed ratio of phenotypes and genotypes Death before birth usually Sex-influenced characteristics Determined by autosomal genes Pattern of inheritance is the same, but PATTERN OF EXPRESSION differs between males and females Ex. beard in goats (male only needs one copy but female needs two), so more males usually have them Sex-limited Zero penetrance in sex that does not have the gene Ex. cock feathering in males is recessive Cytoplasmic inheritance Genes located outside of nucleus Usually inherited from mother Ex. mitochondria and chloroplast RECIPROCAL CROSSES WILL YIELD DIFFERENT RESULTS Extensive phenotypic variation b/c various proportions of alleles Ex. four o’clock plants, determined by maternal parent Seeds from green branches give rise to green progeny, despite pollen Seeds from variegated branches give rise to green, white and green **whether or not chloroplasts are active or deactivated Genetic maternal effect Phenotype of the offspring is determined by the genotype of the mother Ex. direction of shell curling in snails (even if it is ss, it is still dextral s+s b/c mother is) Genomic Imprinting Methylation of DNA has silencing effect Whatever genotype father has will be expressed Females are silenced Anticipation Trait becomes more strongly expressed or is expressed earlier as it is passed down Environmental effects influencing gene expression Fur color in cats or rabbits, temperature sensitive PKU Phenocopy—environmental factors cause a phenotype that is identical to a different genotype Discontinuous vs. continuous characteristics Discontinuous—blood types, pea color, fruit fly eye color Continuous or quantitative—human height, multiple genes at multiple loci Polygenic characteristics vs. pleiotropy Polygenic characteristics-encoded by genes at many loci (multifactorial), ex. human height Pleotropic-one gene effects multiple characteristics Gene interaction Genes at multiple loci determine single phenotype Ex. pepper color Y+_C+ Recessive epistasis (9:3:4 ratio….usually 9:3:3:1) One gene masks effect of another at a different loci (epistatic gene masks, hypostatic is masked) Ex. ee in dogs prevents pigment from forming Compound H—Bombay phenotype, compound H must be produced for antigen synthesis (hh recessive does not make H….makes it O) Dominant epistasis (12:3:1 ratio) Ww, W prevents all pigments Ex. cats or squash Duplicate recessive epistasis (9:7 ratio) Two recessive alleles at either of the two loci suppress phenotype Ex. in order to have pigmented shell, snail must have enzymes 1 and 2 (one dominant allele at both loci) Lesson 5: Analysis and Applications of Pedigrees Shapes Square-male Circle-female Line through middle-may carry it later Proband-person from whom the pedigree started with Oldest at the left Adopted parents connected with dotted line Double line between related individuals—consanguinity Identical twins Autosomal recessive Equally in males and females, skips generations, affected offspring receive allele from each parent Frequent in progeny of closely related parents (cousins?) Autosomal dominant Equally in males and females, every generation, receive allele from either parent Affected must have at least one parent with trait, typically heterozygous X-linked recessive More frequently in males, skips generation, NOT PASSED FROM FATHER TO SON All daughters of affected father will be carriers, affected males get it from their mother X-linked dominant Appear equally in males and females, every generation Affected males pass to ALL THEIR DAUGHTERS, NO SONS Can be heterozygous and pass to half offspring Affected males have affected mother Y-linked Only affects males, every generation, affected male will pass trait to all male offspring Twin studies Dizygotic- two separate eggs Monozygotic- same egg Concordant trait-trait shared by both twins Concordance- percentage of twin pairs concordant for a specific trait Genetic testing Ultrasonography th th Amniocentesis, amniotic fluid- 15 -18 week th th Chorionic villus sampling, outer placenta- 10 -11 week of pregnancy Maternal blood sampling Newborn screening
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