1. II. Meiosis and the Chromosomal Theory A.Overview B.Costs and Benefits of Asexual and Sexual Reproduction C.Mixing Genomes D.Meiosis 1. Overview 2n 1n REDUCTIONDIVISION

2. II. Meiosis and the Chromosomal Theory A.Overview B.Costs and Benefits of Asexual and Sexual Reproduction C.Mixing Genomes D.Meiosis 1. Overview 2. Meiosis I (Reduction) There are four replicated chromosomes in the initial cell. Each chromosomes pairs with its homolog (that influences the same suite of traits), and pairs align on the metaphase plate. Pairs are separated in Anaphase I, and two cells, each with only two chromosomes, are produced. REDUCTION

3. II. Meiosis and the Chromosomal Theory A.Overview B.Costs and Benefits of Asexual and Sexual Reproduction C.Mixing Genomes D.Meiosis 1. Overview 2. Meiosis I (Reduction) 3. Transition 4. Meiosis II (Division) Each cell with two chromosomes divides; sister chromatids are separated. There is no change in ploidy in this cycle; haploid cells divide to produce haploid cells. DIVISION

4. 5. Modifications in anisogamous and oogamous species

5. II. Meiosis and the Chromosomal Theory A.Overview B.Costs and Benefits of Asexual and Sexual Reproduction C.Mixing Genomes D.Meiosis E.Sexual Reproduction and Variation 1. Meiosis and Mendelian Heredity: The chromosomal theory of inheritance

6. D.Meiosis E.Sexual Reproduction and Variation 1. Meiosis and Mendelian Heredity: The chromosomal theory Saw homologous chromosomes separating (segregating). If they carried genes, this would explain Mendel’s first law. Theodor Boveri Walter Sutton Aa

7. D.Meiosis E.Sexual Reproduction and Variation 1. Meiosis and Mendelian Heredity: The chromosomal theory And if the way one pair of homologs separated had no effect on how others separated, then the movement of chromosomes would explain Mendel’s second law, also! They proposed that chromosomes carry the heredity information. Theodor Boveri Walter Sutton Aa Aa bBBb ABabAbaB OR

8. D.Meiosis E.Sexual Reproduction and Variation 1. Meiosis and Mendelian Heredity: The chromosomal theory 2. Solving Darwin’s Dilemma Independent Assortment produces an amazing amount of genetic variation. Consider an organism, 2n = 4, with two pairs of homologs. They can make 4 different gametes (long Blue, Short Red) (Long Blue, Short Blue), (Long Red, Short Red), (Long Red, Short blue). Gametes carry thousands of genes, so homologous chromosomes will not be identical over their entire length, even though they may be homozygous at particular loci. Well, the number of gametes can be calculated as 2 n or

9. D.Meiosis E.Sexual Reproduction and Variation 1. Meiosis and Mendelian Heredity: The chromosomal theory 2. Solving Darwin’s Dilemma Independent Assortment produces an amazing amount of genetic variation. Consider an organism with 2n = 6 (AaBbCc) …. There are 2 n = 8 different gamete types. ABCabc AbcabC aBCAbc AbCaBc

10. D.Meiosis E.Sexual Reproduction and Variation 1. Meiosis and Mendelian Heredity: The chromosomal theory 2. Solving Darwin’s Dilemma Independent Assortment produces an amazing amount of genetic variation. Consider an organism with 2n = 6 (AaBbCc) …. There are 2 n = 8 different gamete types. And humans, with 2n = 46?

11. D.Meiosis E.Sexual Reproduction and Variation 1. Meiosis and Mendelian Heredity: The chromosomal theory 2. Solving Darwin’s Dilemma Independent Assortment produces an amazing amount of genetic variation. Consider an organism with 2n = 6 (AaBbCc) …. There are 2 n = 8 different gamete types. And humans, with 2n = 46? 2 23 = ~ 8 million different types of gametes. And each can fertilize ONE of the ~ 8 million types of gametes of the mate… for a total 2 46 = ~70 trillion different chromosomal combinations possible in the offspring of a single pair of mating humans.

12. D.III. Meiosis E.Sexual Reproduction and Variation 1. Meiosis and Mendelian Heredity: The chromosomal theory 2. Solving Darwin’s Dilemma 3. Model of Evolution – circa 1905 Sources of VariationCauses of Change Independent Assortment  VARIATION  NATURAL SELECTION

13. Heredity, Gene Regulation, and Development I. Mendel's Contributions II. Meiosis and the Chromosomal Theory III. Allelic, Genic, and Environmental Interactions

14. Heredity, Gene Regulation, and Development I. Mendel's Contributions II. Meiosis and the Chromosomal Theory III. Allelic, Genic, and Environmental Interactions A. Overview: The effect of a gene is influenced at three levels: - Intralocular (effects of other alleles at this locus) - Interlocular (effects of other genes at other loci) - Environmental (the effect of the environment on determining the effect of a gene on the phenotype) A a GENOME Environment PHENOTYPE

15. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions A a

16. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions 1. Complete Dominance: - The presence of one allele is enough to cause the complete expression of a given phenotype.

17. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions 1. Complete Dominance: 2. Incomplete Dominance: - The heterozygote expresses a phenotype between or intermediate to the phenotypes of the homozygotes.

18. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions 1. Complete Dominance: 2. Incomplete Dominance: 3. Codominance: - Both alleles are expressed completely; the heterozygote does not have an intermediate phenotype, it has BOTH phenotypes. ABO Blood Type: A = ‘A’ surface antigens B = ‘B’ surface antigens O = no surface antigen from this locus PhenotypeGenotypes AAA, AO BBB, BO OOOAB codominance AB Phenotype

19. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions 1. Complete Dominance: 2. Incomplete Dominance: 3. Codominance: 4. Overdominance : – the heterozygote expresses a phenotype MORE EXTREME than either homozygote TEMPEnzyme Activity “T” TEMPEnzyme Activity “t” TT = tall (grows best in warm conditions) tt = short (grows best in cool conditions) Tt = Very Tall (has both alleles and so grows optimally in cool and warm conditions)

20. IV. Modifications to Mendelian Patterns A. Intralocular Interactions - Summary and Implications: populations can harbor extraordinary genetic variation at each locus, and these alleles can interact in myriad ways to produce complex and variable phenotypes. -Consider this cross: AaBbCcDd x AABbCcDD Assume: The genes assort independently A and a are codominant B is incompletely dominant to b C is incompletely dominant to c D is completely dominant to d How many phenotypes are possible in the offspring?

21. IV. Modifications to Mendelian Patterns A. Intralocular Interactions - Summary and Implications: populations can harbor extraordinary genetic variation at each locus, and these alleles can interact in myriad ways to produce complex and variable phenotypes. -Consider this cross: AaBbCcDd x AABbCcDD Assume: The genes assort independently A and a are codominant B is incompletely dominant to b C is incompletely dominant to c D is completely dominant to d How many phenotypes are possible in the offspring? ABCD 2x3 x3 x1 = 18 If they had all exhibited complete dominance, there would have been only: 1x2 x2 x1 = 4 So the variety of allelic interactions that are possible increases phenotypic variation multiplicatively. In a population with many alleles at each locus, there is an nearly limitless amount of phenotypic variability.

22. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions C. Interlocular Interactions The genotype at one locus can influence how the genes at other loci are expressed.

23. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions C. Interlocular Interactions: 1. Quantitative (Polygenic) Traits: There may be several genes that produce the same protein product; and the phenotype is the ADDITIVE sum of these multiple genes. Creates continuously variable traits. So here, both genes A and B produce the same pigment. The double homozygote AABB produces 4 ‘doses’ of pigment and is very dark. It also means that there are more ‘intermediate gradations’ that are possible.

24. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions C. Interlocular Interactions: 1. Quantitative (Polygenic) Traits: 2. Epistasis: This is where one gene affects how another gene is expressed; the genes don’t just add together to affect the phenotype, they interact. -example: in a enzymatic process, all enzymes may be needed to produce a given phenotype. Absence of either may produce the same alternative ‘null’. Process: enzyme 1 enzyme 2 Precursor1 precursor2 product (pigment)

25. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions C. Interlocular Interactions: 1. Quantitative (Polygenic) Traits: 2. Epistasis: -example: in a enzymatic process, all enzymes may be needed to produce a given phenotype. Absence of either may produce the same alternative ‘null’. For example, two strains of white flowers may be white for different reasons; each lacking a different necessary enzyme to make color. Process: enzyme 1 enzyme 2 Precursor1 precursor2 product (pigment) Strain 1: enzyme 1 enzyme 2 Precursor1 precursor2 no product (white) Strain 2: enzyme 1 enzyme 2 Precursor1 precursor2 no product (white)

26. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions C. Interlocular Interactions: 1. Quantitative (Polygenic) Traits: 2. Epistasis: -example: in a enzymatic process, all enzymes may be needed to produce a given phenotype. Absence of either may produce the same alternative ‘null’. For example, two strains of white flowers may be white for different reasons; each lacking a different necessary enzyme to make color. So there must be a dominant gene at both loci to produce color. GenotypePhenotype aaB-white aabbwhite A-bbwhite A-B-pigment So, what’s the phenotypic ratio from a cross: AaBb x AaBb ?

27. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions C. Interlocular Interactions: 1. Quantitative (Polygenic) Traits: 2. Epistasis: -example: in a enzymatic process, all enzymes may be needed to produce a given phenotype. Absence of either may produce the same alternative ‘null’. For example, two strains of white flowers may be white for different reasons; each lacking a different necessary enzyme to make color. So there must be a dominant gene at both loci to produce color. GenotypePhenotype aaB-white aabbwhite A-bbwhite A-B-pigment So, what’s the phenotypic ratio from a cross: AaBb x AaBb ? 9/16 pigment (A-B-), 7/16 white

28. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions C. Interlocular Interactions D. Environmental Effects: The environment can influence whether and how an allele is expressed,and the effect it has.

29. D. Environmental Effects: 1. TEMPERATURE - Siamese cats and Himalayan rabbits – dark feet and ears, where temps are slightly cooler. Their pigment enzymes function at cool temps. - Arctic fox, hares – their pigment genes function at high temps and are responsible for a change in coat color in spring and fall, and a change back to white in fall and winter.

30. D. Environmental Effects: 1. TEMPERATURE 2. TOXINS - people have genetically different sensitivities to different toxins. Certain genes are associated with higher rates of certain types of cancer, for example. However, they are not ‘deterministic’… their effects must be activated by some environmental variable. PKU = phenylketonuria… genetic inability to convert phenylalanine to tyrosine. Phenylalanine can build up and is toxic to nerve cells. Single gene recessive disorder. But if a homozygote recessive person eats a diet low in phenylalanine, no negative consequences develop. So, the genetic predisposition to express the disorder is influenced by the environment. Nutri-sweet contains phenylalanine, which is dangerous for PKU homozygotes to consume. So, the Food and Drug Administration requires that products containing nutri-sweet be labeled as such.

31. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions C. Interlocular Interactions D. Environmental Interactions E. The “Value” of an Allele 1. There are obvious cases where genes are bad – lethal alleles 2. But there are also ‘conditional lethals’ that are only lethal under certain conditions – like temperature-sensitive lethals. 3. And for most genes, the relative value of one allele over another is determined by the relative effects of those genes in a particular environment. And these relative effects may be different in different environments.

32. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions C. Interlocular Interactions D. Environmental Interactions E. The “Value” of an Allele Survivorship in U.S., sickle-cell anemia (incomplete dominance, one gene ‘bad’, two ‘worse’) SSSsss

33. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions C. Interlocular Interactions D. Environmental Interactions E. The “Value” of an Allele Survivorship in U.S., sickle-cell anemiaSurvivorship in tropical Africa (incomplete dominance, one gene ‘bad’, (one gene ‘good’, two ‘bad’) two ‘worse’) SSSsss

34. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions C. Interlocular Interactions D. Environmental Interactions E. The “Value” of an Allele Survivorship in U.S., sickle-cell anemiaSurvivorship in tropical Africa (incomplete dominance, one gene ‘bad’, (one gene ‘good’, two ‘bad’) two ‘worse’) Malaria is still a primary cause of death in tropical Africa. The malarial parasite can’t complete development in red blood cells with sickle cell hemoglobin… so one SC gene confers a resistance to malaria without the totally debilitating effects of sickle cell. SSSsss Survival in U. S.Survival in Tropics

35. III. Allelic, Genic, and Environmental Interactions A. Overview: B. Intralocular Interactions C. Interlocular Interactions D. Environmental Interactions E. The “Value” of an Allele Survivorship in U.S., sickle-cell anemiaSurvivorship in tropical Africa (incomplete dominance, one gene ‘bad’, (one gene ‘good’, two ‘bad’) two ‘worse’) SSSsss As Darwin realized, selection will favor different organisms in different environments, causing populations to become genetically different over time.