Biology 2250 Principles of Genetics. Weekly Online Quizzes. WebCT Quizzes. Mendelian Genetics. Basic Concepts of Genetics

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1 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: Weekly Online Quizzes Marks Oct. 14 Example Quiz 2** for logging in Oct. 21 Quiz 1 2 Oct. 28 Quiz 2 2 Nov. 4 Quiz 3 2 Nov. 10 Quiz 4 2 WebCT Quizzes Log in: Topics: Mendelian Genetics - 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 1

2 Mitosis Meiosis 2n 2n 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 2n n Meiosis (overview) diploid (2N) > haploid (N) gametes Mitosis Chromosome replication once > 2 nuclear divisions (meiosis I, II) one nucleus > 4 nuclei Meiosis I (reductional division) Meiosis II (equational division) Pairing of homologous chromosomes Continued MEIOSIS ANIMATION (Textbook website) 4 products 2

3 Mitosis Mitosis somatic cells Comparison (Fig. 4-24) Meiosis cells of sexual cycle one doubling one doubling Meiosis I (reductional division) Meiosis II (equationaldivision) 1 division 2 cells 2 divisions 4 cells same amount of DNA ½ amount of DNA Pairing of homologous chromosomes 4 products Mitosis Meiosis # chrs doesn t change #chrs. halved No pairing of chr. Synapsis of homologs Centromeres divide at Not at anaphase I, anaphase but at anaphase II Conservative Comparison (continued) Variation Genetic Terminology Genes: hereditary elements Alleles: forms of a gene: A, a b, b + Genotypes Gene pairs Heterozygote: Aa bb + Homozygotes: AA aa bb b + b + Genes on Chromosomes Genes Meiosis I Expect behaviour of genes to correlate with the behaviour of chromosomes: genes Diploid (2n) AA, Aa, aa chromosomes pairs Haploid (n) A one set 1/2 1/2 A A a a A a A A a a B b Correlation of genes and Chromosomes during meiosis OR Meiosis II A a b B 3

4 Mendelian Genetics Genes - cannot be observed directly Phenotypes - observed directly ** inheritance of phenotypes used to infer the inheritance of genes Requirements: Mendelian Genetics 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 4

5 Genetical Analysis (pea shape) Mendel s Experimental Approach suitable experimental organism examine few traits in each experiment accurate quantitative records analyzed data------> formulated hypotheses Parental round X wrinkled cross F 1 (filial) round self F 2 3/4 round 1/4 wrinkled (3:1) Hypothesis to explain results 1. Hereditary determinants (genes) 2. Each adult plant has a gene pair (alleles) F 1 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 aa A a gametes F 1 Aa Self Aa X Aa Self F 1 Aa X Aa F 2 1/2 A 1/2 a 1/2 A 1/4 AA 1/4 Aa 1/2 a 1/4 Aa 1/4 aa equal segregation F 2 F 2 Genotypes 1/4 AA 2/4Aa 1/4 aa (1:2:1) F 2 Phenotypes 3/4 A- 1/4 aa (3:1) 5

6 Test of Equal Segregation Hypothesis (Test Cross) round wrinkled R r X r r r 1/2 R 1/2 R r round 1/2 r 1/2 r r wrinkled Mendel s First Law Equal segregation of two alleles of a gene pair during gamete formation 1:1 round:wrinkled 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 Experimental: Summary 1. Two pure breeding lines 2. Cross > F 1 hybrid 3. Self F > F 2 Summary Results: 1. F 1 one phenotype 2. F 2 3:1 ratio of 2 phenotypes Inference: Summary 1. Single major gene 2. dominant phenotype 3. equal segregation 4. existence of genes inferred 6

7 P F 1 F 2 AA x aa Aa ---- ¾ AA, Aa ¼ aa observed Fig. 5-2 Mendelian genetics applies to all organisms Correlation of genes and chromosomes Bb Bb Dominant Phenotype Expected 3 : 1 Virtual Fly Parents F 1 X F 2 Parental AA x aa 3 wildtype.. 1 vestigial F1 Hybrid Aa Aa x Aa F2 AA Aa aa F2 ratio 1 : 2 : 1 (¼ ½ ¼ ) 3 : 1 A a A AA Aa a Aa aa 7

8 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 dark Offspring orange dark 11 0 orange Examples Examples: 2. Paternity 3. Detecting Hybrids: Families mother offspring father? AA all AB BB AA 26AA, 24 AB AB AA 48 AA, 2 AB AA + BB? Species 1 Species 2 AA x BB Hybrids AB Mytilus Hybrid tross. edulis AB AA BB BB Sex-linked Inheritance Sex Linkage reciprocal crosses Correlation between inheritance of genes and sex Round wrinkled X X wrinkled Round 8

9 Drosophila melanogaster (T. H. Morgan) Red Eye Drosophila White eye Red eye (wild) X White eye (mutant) Cross A red female X white male Cross B white female X red male F 1 all red F 1 females males F 2 red : white 3 : 1 white all male red 2 : 1 female : male F 2 females males 1 : 1 : 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 Males: 1 pair of sex chromosomes (XX) 3 pairs of autosomes 1 heteromorphic pair (XY) 9

10 Chromosomes Explanation Male Eye colour gene on X chromosome Wildtype (red) dominant to white autosomes sex chromosomes eye-colour genotype Female Females X X WW Ww ww Males X Y W w Red white Cross A Cross B Alternative Notation FEMALE MALE eye-colour genotype Females w + w + w + w ww Males w + Y wy Eye colour Fig. 5-8 F 1 Cross A X Cross B X Cross B ww X w + Y F 1 ww + wy F 2 F 2 w Y w ww wy w + ww + w + Y 1 : 1 : 1 : 1 10

11 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 Heterogametic klinefelter turner * Sterile Sex Determination Humans: Y chr > maleness Drosophila: sex -----> X/A ratio Normal mitosis X w,m X +,+ Gynandromorphs Nondisjunction during Early development (mitotic cell division) X w,m X +,+ 2X/2A = > X w,m X +,+ X w,m X +,+ X w,m X w,m X +,+ X/2A = > Wild female X w,m Xw,m X +,+ Gynandromorphs X w,m X w,m X +,+ Nondisjunction during Early development Other sex determining systems Birds & Moths homogametic ZZ heterogametic ZW 11

12 Sex Linkage (X Y) - Y chromosome lacks homology with X Summary Traits controlled by a single gene: - precise Mendelian ratios: 3:1, 1:1, 1:2:1 - alleles on X expressed in males - hemizygous: a gene present in one copy X w Y - ratios due to chromosome segregation during meiosis - sex linked inheritance sex chromosomes 12