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Biology 2250 Principles of Genetics Announcements Test I marks are posted outside SN-3021 and in the lab. Exams will be returned Tuesday Spring Biology Courses at Harlow: http://www.mun.ca/harlow/ Weekly Online Quizzes Oct. 14 Oct. 21 Oct. 28 Nov. 4 Nov. 10 Marks Example Quiz 2** for logging in Quiz 1 2 Quiz 2 2 Quiz 3 2 Quiz 4 2 WebCT Quizzes Log in: http://webct.mun.ca:8900/ Mendelian Genetics Topics: - Transmission of DNA during cell division Mitosis and Meiosis - Segregation - Sex linkage - Inheritance and probability - Independent Assortment - Mendelian genetics in humans - Linkage - Gene mapping - Tetrad Analysis (mapping in fungi) - Extensions to Mendelian Genetics - Gene mutation - Chromosome mutation - Quantitative and population genetics Basic Concepts of Genetics Cell/nuclear division Mitosis (somatic tissue): identical cells Meiosis (germ tissue): gametes (variation) Behaviour of chromosomes can explain the behaviour of genes (segregation and independent assortment) Chromosome and DNA Replication 1. DNA replication results in chromosome replication 2. Nuclear and cell division Mitosis 2n 2n Meiosis 2n n Mitosis 1. one parent cell--------> 2 identical daughter cells 2. same in all organisms 3. simple: (a) each chromosome doubles (identical) (b) identical halves separate Meiosis (overview) diploid (2N) ---------------> haploid (N) gametes Chromosome replication once --------> 2 nuclear divisions (meiosis I, II) one nucleus -------------------> 4 nuclei Mitosis Meiosis I (reductional division) Pairing of homologous chromosomes Continued Meiosis II (equational division) MEIOSIS ANIMATION (Textbook website) 4 products Mitosis Meiosis I (reductional division) Pairing of homologous chromosomes Meiosis II (equational division) 4 products Comparison (Fig. 4-24) Mitosis somatic cells one doubling Meiosis cells of sexual cycle one doubling 1 division 2 cells 2 divisions 4 cells same amount of DNA ½ amount of DNA Comparison (continued) Mitosis Meiosis # chrs doesn’t change #chrs. halved No pairing of chr. Synapsis of homologs Centromeres divide at anaphase Conservative Not at anaphase I, but at anaphase II Variation Genetic Terminology Genes: hereditary elements Alleles: forms of a gene: A , a b, b+ Genotypes Heterozygote: Homozygotes: Gene pairs Aa AA aa bb+ bb b+ b+ Genes on Chromosomes Expect behaviour of genes to correlate with the behaviour of chromosomes: genes chromosomes Diploid (2n) AA, Aa, aa pairs Haploid (n) A one set Genes Meiosis I A Correlation of genes and Chromosomes during meiosis a 1/2 1/2 A A A A a a a a A a B Meiosis II A b OR b a B Mendelian Genetics Genes - cannot be observed directly Phenotypes - observed directly ** inheritance of phenotypes used to infer the inheritance of genes Mendelian Genetics Requirements: 1. Attributes of the phenotype that vary among individuals 2. Phenotypic variation caused by genetic differences Mendel’s Experiments Seven Pea varieties “True Breeding Lines” Character Phenotypes 1. seed shape round, wrinkled 2. seed colour yellow, green 3. flowers (pods) axial, terminal 4. pods full, constricted 5. pods yellow, green 6. flowers violet, white 7. stem tall, dwarf Advantages: • • • • easy to grow matures in a season self-fertilizing easy to cross fertilize Cross Pollination Mendel’s Experimental Approach • • • • suitable experimental organism examine few traits in each experiment accurate quantitative records analyzed data------> formulated hypotheses Genetical Analysis (pea shape) Parental round F1 (filial) F2 3/4 round X wrinkled cross round self 1/4 wrinkled (3:1) Hypothesis to explain results 1. Hereditary determinants (genes) 2. Each adult plant has a gene pair (alleles) F1 plants: one allele dominant phenotype one allele recessive phenotype Hypothesis (continued) 3. Alleles of a gene pair segregate equally into the gametes 4. Each gamete has only one allele of a gene pair 5. Gametes combine at random to form zygote Hypothesis P AA X A F1 aa a gametes Aa Self Aa X F2 Aa Self F1 F2 Aa X Aa equal segregation 1/2 A 1/2 a 1/2 A 1/4 AA 1/4 Aa 1/2 a 1/4 Aa 1/4 aa F2 Genotypes 1/4 AA 2/4Aa 1/4 aa (1:2:1) F2 Phenotypes 3/4 A1/4 aa (3:1) Test of Equal Segregation Hypothesis (Test Cross) round Rr wrinkled X rr r 1/2 R 1/2 R r 1/2 r 1/2 r r round wrinkled 1:1 round:wrinkled Mendel’s First Law Equal segregation of two alleles of a gene pair during gamete formation Genetic Terminology Genes: hereditary elements Alleles: forms of a gene: A a Heterozygote: Aa Homozygotes: AA aa Genotypes Phenotypes Dominance AA, Aa same phenotype different genotypes Summary Experimental: 1. Two pure breeding lines 2. Cross --------> F1 hybrid 3. Self F1 ------> F2 Summary Results: 1. F1 one phenotype 2. F2 3:1 ratio of 2 phenotypes Summary Inference: 1. Single major gene 2. dominant phenotype 3. equal segregation 4. existence of genes inferred P AA x F1 aa ------ Aa ---- F2 ¾ AA, Aa ¼ aa observed Dominant Phenotype Expected 3:1 Fig. 5-2 Mendelian genetics applies to all organisms Correlation of genes and chromosomes Bb Bb Virtual Fly Parents F1 X F2 3 wildtype .. 1 vestigial Parental AA x F1 Hybrid aa Aa Aa x Aa A F2 F2 ratio AA Aa aa 1 : 2 : 1 (¼ ½ ¼) 3 : 1 a A AA Aa a Aa aa Principle of Segregation Implications 1. Answer questions on inheritance 2. Explore other questions Examples: 1. Shell Colour Variation in the molluscs: Scallop: Hermaphodites self-fertilization Parent Offspring dark orange dark 11 0 orange 12 45 Examples 2. Paternity Families mother offspring AA all AB AA 26AA, 24 AB AA 48 AA, 2 AB father ? BB AB AA + BB ? Examples: 3. Detecting Hybrids: Species 1 AA Hybrids Species 2 x BB Mytilus Hybrid tross. edulis AB AB AA BB BB Sex-linked Inheritance Correlation between inheritance of genes and sex Sex Linkage reciprocal crosses X Round wrinkled X wrinkled Round Drosophila melanogaster (T. H. Morgan) White eye (mutant) Red eye (wild) X Drosophila Red Eye White eye Cross A red female F1 F2 X white male all red red : white 3 : 1 white all male red 2 : 1 female : male Cross B white female F1 X females F2 red male males females 1 :1 males : 1 :1 Conclusions 1. Sex and eye-colour gene associated 2. Criss-cross inheritance - daughters inherit father’s phenotype - sons inherit mother’s phenotype Interpretation Sex chromosomes (X, Y) Females: 3 pairs of autosomes 1 pair of sex chromosomes (XX) Males: 3 pairs of autosomes 1 heteromorphic pair (XY) Chromosomes Male autosomes Female Explanation Eye colour gene on X chromosome Wildtype (red) dominant to white sex chromosomes Females Males XX XY eye-colour genotype WW Ww ww W w Red white Alternative Notation eye-colour genotype Females w + w+ Males w+ Y Eye colour w+ w ww wY Cross A Cross B FEMALE Fig. 5-8 MALE Cross A Cross B F1 X F2 X Cross B ww F1 X ww+ wY F2 w w+ 1 w+Y :1 w ww ww+ : 1 Y wY w+Y :1 Conclusion 1. Eye colour gene associated with sex chromosome (X) 2. Eye colour not related to sexual function. Genes on sex chromosomes not related to sexual function Sex Determination XX XY XXY XO * Drosophila * Human Homogametic * klinefelter Heterogametic turner * Sterile Sex Determination Humans: Y chr. -------> maleness Drosophila: sex -----> X/A ratio 2X/2A = 1.0 ------> X/2A = 0.5 ------> Gynandromorphs Nondisjunction during Early development (mitotic cell division) Normal mitosis Xw,m X+,+ Xw,m X+,+ Xw,m Wild female Xw,m X+,+ X+,+ Xw,m Xw,m Xw,m X+,+ Xw,m X+,+ Gynandromorphs Xw,m Xw,m X+,+ Nondisjunction during Early development Other sex determining systems Birds & Moths homogametic ZZ heterogametic ZW Sex Linkage (X Y) - Y chromosome lacks homology with X - alleles on X expressed in males - hemizygous: a gene present in one copy Xw Y Summary Traits controlled by a single gene: - precise Mendelian ratios: 3:1, 1:1, 1:2:1 - ratios due to chromosome segregation during meiosis - sex linked inheritance sex chromosomes