Darwin s dilemma 8/14/2014. Meiosis & Inheritance Lecture 18 Summer Mitosis & Meiosis. The Modern Synthesis

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1 Darwin s dilemma 1 Meiosis & Inheritance Lecture 18 Summer 2014 How do organisms pass heritable traits to their offspring? The Modern Synthesis 2 Mitosis & Meiosis Darwin essay on Natural Selection 1859 On the Origin of Species Mendel published Mitosis worked out 1890s - Meiosis worked out Early 1900 s - Chromosomal Theory of Inheritance 1953 Structure of DNA discovered Homologous Chromosomes & Sister Chromatids 4 Meiosis The process of cell division that produces genetically unique haploid gametes from diploid cells. 5 Start with one cell with Homologous pair of chromosomes (2 sets of chromosomes) End with 4 cells with 1 set of chromosomes each How does this happen? 1

2 Meiosis is the process where the two sets of homologous chromosomes are separated into different cells - egg or sperm. 6 A Comparison of Mitosis & Meiosis -movie 7 Fig Mitosis vs. Meiosis 8 A Comparison of Mitosis & Meiosis 9 Metaphase of Mitosis Metaphase of Meiosis 1 Fig A Comparison of Mitosis & Meiosis 10 Sexual Reproduction & Genetic Diversity Three ways: Crossing over/genetic recombination Independent assortment of chromosomes Random Fertilization 11 Fig

3 Crossing over / Genetic Recombination Homologous chromosomes pair up and exchange segments Occurs between nonsister chromatids 12 Crossing over / Genetic Recombination See Fig Genetic Variation Independent assortment of chromosomes 2 n possibilities 2 23 = ~ 8 million possibilities 14 Independent Assortment 15 Fig Random fertilization 16 Mendel s Work 17 Who was Mendel and what did he discover? Sperm cell ~ 8 million combinations Egg cell ~ 8 million combinations 64 trillion possibilities!!! 3

4 Alleles and Chromosomes Does dominant mean a phenotype is more common? Wild type Does dominant mean a phenotype is normal? 18 Mendel s Work What happens when true breeding lines are crossed? Monohybrid cross: cross between parent plants that differ in only 1 characteristic 19 Fig. 9.5 Punnett Square The Chromosomal Basis of Inheritance 20 worksheet Fig Dominant & Recessive Disorders If disease is caused by a single gene locus with a dominant/recessive pattern, the probability of an offspring having that disease can be determined by a Punnett square Carriers Deafness caused by recessive allele 21 Science (2010), v.328:

5 Sickle-cell disease kills ~100,000 people worldwide annually No cure, only treatment 23 What can explain the pattern of overlap between the presence of malaria and the presence of sickle cell in certain human populations? 1 in 500 African- American children have sickle-cell disease 1 in 10 are heterozygous Sickle-cell Allele 26 Biology/ecology of organisms involved Variations on Mendel s Laws codominance and pleiotrophy Mutation Natural Selection Point mutation single base substitution Sickle-cell Disease Pleiotrophy The control of more than one phenotype by a single gene Result of sickle-cell allele Misformed hemoglobin, sickled cells Result of sickle-cell disease Misformed hemoglobin, sickled cells, clumped cells, kidney failure, anemia, etc 27 Sickle-cell Disease Sickle cell alleles are Codominant Codominance Both alleles contribute equally to phenotype E.g., blood type See Fig

6 Malaria Lifecycle of protist that causes malaria 29 Malaria, Sickle-Cell disease & Natural Selection Malaria is large threat to human health in equatorial regions of the Earth Some human populations show resistance to the disease 30 Malaria (Plasmodium falciparum) Malaria, Sickle-Cell disease & Natural Selection 31 Individuals who are heterozygous for sickle-cell anemia are more resistant to malaria Punnett squares Selective advantage to be heterozygous for sickle-cell anemia 6