09 Mendelian Genetics, Meiosis and Evolution


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09 Mendelian Genetics, Meiosis and Evolution

  1. 1. Mendelian Genetics, Meiosis and Evolution • Mendel‟s Laws of Heredity (10.1) • Meiosis (10.2) • The Theory of Evolution (15)
  2. 2. Answer these questions…  What is heredity?   What is genetics?    A seed What is pollination?   Fertilization In a plant, what does a zygote develop into?   Male sex cell + Female sex cell  Fertilized cell Male gamete + Female gameteZygote What is the above process called?   The branch of biology that studies heredity What is sexual reproduction?   The passing of traits from parents to offspring The transfer of pollen grains from a male reproductive organ to a female reproductive organ in a plant What is a hybrid?  The offspring of parents that have different forms of a trait
  3. 3. Gregor Mendel  Old thinking: Blending   Traits from both parents get blended and the offspring is a mixture of both Cross-pollinated pea plants to study how traits are passed from parents to offspring
  4. 4. Mendel’s 3 Conclusions  The rule of unit factors    Each organism has two factors that control each of its traits Factors = genes (located on chromosomes) Genes exist in alternative forms    Alleles Example: A pea plant could have 2 alleles for purple, 2 alleles for white or 1 allele for purple and 1 allele for white An organism‟s two alleles are located on different copies of a chromosome (one from the female parent, one from the male)
  5. 5. Mendel’s 3 Conclusions  The rule of dominance     Purple flower + White flower = Purple flower Only the purple trait is observed, therefore it is dominant The white trait „disappeared‟, therefore it is recessive The law of segregation    Every individual has two alleles of each gene When gametes (sex cells) are produced each gamete receives one of these alleles During fertilization, these gametes randomly pair to produce four combinations of alleles
  6. 6. Answer these questions…  What are the alleles of the purple-flowered plant in the parental generation?   The white-flowered?   2 for purple 2 for white The first gen. offspring?  1 for purple, 1 for white
  7. 7. Phenotypes and Genotypes  The purple parent has 2 purple alleles and the purple offspring has 1 purple and 1 white allele  Therefore, two organisms can look alike, but have different underling allele combinations  The way an organism looks and behaves is called its phenotype  The allele combination an organism contains is called its genotype
  8. 8. Phenotypes and Genotypes  If an organisms alleles are the same for a trait (think purple parent) then the organism is homozygous for that trait  If an organisms alleles are different for a trait (think first gen. purple offspring) then the organism is heterozygous for that trait  Mendel‟s conclusions allow us to…   Predict the probability of the genotype of an offspring when given the parents genotype Determine a phenotype based on genotype
  9. 9. Answer this question…  If two plants are crossed that have two different traits, will the two traits stay together or will they be inherited independently of each other?
  10. 10. The Law of Independent Assortment  Genes for different traits are inherited independently of each other
  11. 11. Answer these questions…  Where are genes located?   How many chromosomes do humans have?   23 from mom, 23 from dad Which cells combine to form a zygote (and then offspring)?   46 (23 pairs) Where do these chromosomes come from?   46 (23 pairs) How many chromosomes in each cell?   Chromosomes Gametes (sex cells) Therefore, how many chromosomes in a gamete?  23
  12. 12. Diploid and Haploid Cells  A cell with pairs of each chromosome is called a diploid cell (2n)  A cell with one of each chromosome is called a haploid cell (n)  The two chromosomes of each pair in a diploid cell are called homologouschromosomes    Not identical Contain information for the same traits Can have different alleles
  13. 13. Typical Animal Life Cycle
  14. 14. Homologous Chromosomes (Homologs)     Chromosomes 1 & 2 are homologous chromosomes Chromosomes 3 & 4 are homologous chromosomes Chromosomes 1 & 3 came from the mother Chromosomes 2 & 4 came from the father
  15. 15. Meiosis – Overview  Meiosis is a special type of cell division that occurs in sexually reproducing organisms  Chromosome number reduced by half, enabling sexual recombination to occur.   Meiosis of diploid cells haploid daughter cells (which may function as gametes) Gametes undergo fertilization, restoring the diploid number of chromosomes in the zygote
  16. 16. Meiosis – Overview  Meiosis and fertilization introduce genetic variation in three ways:   Crossing over between homologous chromosomes at prophase I Independent assortment of homologous pairs at metaphase I   Each homologous pair can orient in either of two ways at the plane of cell division Random chance fertilization between any one female gamete with any other male gamete
  17. 17. Meiosis - Overview  Sexual reproduction in a population should decline in frequency relative to asexual reproduction    Asexual – No males are needed, all individuals can produce offspring Sexual – Only females can produce offspring, therefore fewer are produced Sexual reproduction may exist because it provides genetic variability that reduces susceptibility of a population to pathogen attack  This is the role of sexual reproduction in evolution
  18. 18. Meiosis  2 main stages  Meiosis I   Prophase I, Metaphase I, Anaphase I, Telophase I Meiosis II  Prophase II, Metaphase II, Anaphase II, Telophase II
  19. 19. Meiosis I – Prophase I     The chromosomes condense and become visible The centrioles form and move toward the poles The nuclear membrane begins to dissolve The homologs pair up, forming a tetrad   Each tetrad is comprised of four chromotids - the two homologs, each with their sister chromatid Homologous chromosomes will swap genetic material in a process known as crossing over  Crossing over serves to increase genetic diversity by creating four unique chromatids
  20. 20. Meiosis I – Metaphase I  Microtubules grow from the centrioles and attach to the centromeres  The tetrads line up along the cell equator
  21. 21. Meiosis I – Anaphase I  Homologous chromosomes separate (note that the sister chromatids are still attached)  Cytokinesis begins
  22. 22. Meiosis I – Telophase I  The chromosomes may decondense (depends on species)  Cytokinesis reaches completion, creating two haploid daughter cells
  23. 23. Meiosis II – Prophase II  Centrioles form and move toward the poles  The nuclear membrane dissolves
  24. 24. Meiosis II – Metaphase II  Microtubules grow from the centrioles and attach to the centromeres  The sister chromatids line up along the cell equator
  25. 25. Meiosis II – Anaphase II  The centromeres break and sister chromatids separate  Cytokinesis begins
  26. 26. Meiosis II – Telophase II  The chromosomes may decondense (depends on species)  Cytokinesis reaches completion, creating four haploid daughter cells  https://www.youtube.co m/watch?v=D1_mQS_FZ0
  27. 27. Answer these questions…  How does the number of daughter cells produced from mitosis and meiosis differ?   How does the ploidy of the daughter cells produced from mitosis and meiosis differ?   Mitosis produces diploid (2n) cells. Meiosis produces haploid (n) cells. Do the daughter cells produced from mitosis contain identical genetic complements?   When mitosis is complete, there are two daughter cells. When meiosis is complete, there are four. Yes, the purpose of mitosis is to produce two identical cells Do any of the daughter cells produced from meiosis contain identical genetic complements?  No, the genetic information swapped between homologous chromosomes during crossing over insures that each daughter cell produced during meiosis will be unique
  28. 28. Answer these questions…  When do the homologous chromosomes separate during mitosis?   When do the homologous chromosomes separate during meiosis?   Homologs separate during Anaphase I, when the tetrads break When do sister chromatids separate during mitosis?   Never, they are never joined during mitosis (no tetrads are formed) Sister chromatids separate during Anaphase. When do sister chromatids separate during meiosis?  Sister chromatids separate during Anaphase II.
  29. 29. The Consequences of Meiotic Mistakes  Nondisjunctions occur when homologous chromosomes fail to separate at meiosis I or when chromatids fail to separate at meiosis II.
  30. 30. The Consequences of Meiotic Mistakes  Nondisjunctions occur when homologous chromosomes fail to separate at meiosis I or when chromatids fail to separate at meiosis II    Fertilization can result in embryos that are 2n + 1 (a "trisomy") Abnormal copy numbers of one or more chromosomes is usually, but not always, fatal (Example: Down syndrome) Polyploidy can occur when whole sets of chromosomes fail to separate at meiosis I or II   The resulting 2n gametes, if fertilized by normal sperm, create 3n zygotes (triploid) Organisms with an odd number of chromosome sets cannot produce viable gametes (Example: seedless fruits)
  31. 31. Answer these questions…  How do mutations drive evolution?   Mutations  change traits of and organism  if change helps the organism survive the greater the chance of that organism living long enough to reproduce  trait gets passed down through generations How does sexual reproduction drive evolution?  Sexual reproduction may exist because it provides genetic variability that reduces susceptibility of a population to pathogen attack
  32. 32. Evolution  All of the similarities and dissimilarities among groups of organisms that are the result of the branching process creating the great tree of life, were viewed by early 19th century philosophers and scientists as a consequence of omnipotent design.
  33. 33. Evolution  In 1859, Charles Darwin published his famous On the Origin of Species  Patterns in the distribution and similarity of organisms had an important influence of Darwin's thinking
  34. 34. Darwin’s Theory of Evolution  Species (populations of interbreeding organisms) change over time and space. The representatives of species living today differ from those that lived in the recent past, and populations in different geographic regions today differ slightly in form or behavior. These differences extend into the fossil record, which provides ample support for this claim.
  35. 35. Darwin’s Theory of Evolution  All organisms share common ancestors with other organisms. Over time, populations may divide into different species, which share a common ancestral population. Far enough back in time, any pair of organisms shares a common ancestor. For example, humans shared a common ancestor with chimpanzees about eight million years ago, with whales about 60 million years ago, and with kangaroos over 100 million years ago. Shared ancestry explains the similarities of organisms that are classified together: their similarities reflect the inheritance of traits from a common ancestor.
  36. 36. Darwin’s Theory of Evolution  Evolutionary change is gradual and slow in Darwin‟s view. This claim was supported by the long episodes of gradual change in organisms in the fossil record and the fact that no naturalist had observed the sudden appearance of a new species in Darwin‟s time.  Since then, biologists and paleontologists have documented a broad spectrum of slow to rapid rates of evolutionary change within lineages.  The primary mechanism of change over time is natural selection
  37. 37. The Process of Natural Selection  Variation  Organisms (within populations) exhibit individual variation in appearance and behavior   Some traits show little to no variation among individuals   Body size, hair color, facial markings, voice properties etc. Number of eyes in vertebrates Inheritance   Some traits are consistently passed on from parent to offspring Other traits are strongly influenced by environmental conditions and show weak heritability
  38. 38. The Process of Natural Selection  High rate of population growth    Most populations have more offspring each year than local resources can support leading to a struggle for resources. Each generation experiences substantial mortality. Differential survival and reproduction  Individuals possessing traits well suited for the struggle for local resources will contribute more offspring to the next generation.
  39. 39. Final Thoughts on Evolution  In order for natural selection to operate on a trait, the trait must possess heritable variation and must confer an advantage in the competition for resources. If one of these requirements does not occur, then the trait does not experience natural selection  “…as natural selection acts by competition for resources, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates” (Charles Darwin, On the Origin of Species, 1859).
  40. 40. Final Thoughts on Evolution  Variations arise by mutation  Mutations arise by chance and without foresight for the potential advantage or disadvantage of the mutation.  In other words, variations do not arise because they are needed.