1. MEIOSIS & MENDELIAN GENETICS– CHAPTER Freshman Biology; Semester Two

2. Chromosomes  Def – the genetic information passed down from parent to offspring  Each/every human body cell has 46 chromosomes  44 = non-sex chromosomes (22 pairs)  2 = sex chromosomes  X or Y (1 pair)  All body cells (except sex cells) go through mitosis  Mitosis produces cells that are:  Clones/genetically identical to parent

5. BEFORE chromosome replication AFTER chromosome replication

6. Cell Cycle Review  Asexual reproduction that occurs in body (somatic) cells; not sex cells  Interphase  G1 SS  G2  Mitosis  Prophase  Metaphase  Anaphase  Telophase  Cytokinesis  End product is 2 cells that are genetically identical to the parent (original) cell

7. Characteristics of Meiosis  Meiosis occurs in gametes (sex cells) ONLY  TWO divisions with 4 phases each (8 phases total) creating 4 unique cells  Cells start out diploid and end haploid

8. Initial Comparison MitosisMeiosis # of cells produced 24 Daughter cells vs. parent cells IdenticalNot identical (Why? crossing over) # of chromosomes Same (46  46 in humans) Cut in ½ (46  23 in humans) PurposeTo produce new cells (growth, repair old/damaged cells) To produce gametes -egg and sperm (for sexually reproducing organisms)

9. Diploid vs. Haploid  Diploid (2n) cells have two sets of chromosomes  One inherited from mom; one from dad  All somatic (body) cells are diploid (all cells except sex cells)  Humans’ diploid number is 46, but other species have other numbers.  The chromosomes that are alike from each set are called homologous chromosomes.  Haploid (1n) cells have one set of chromosomes  Gametes (sex cells) are haploid  Humans’ haploid number is 23, but other species have different numbers.  When fertilization occurs, the organism will again be diploid.  23 chromosomes from male parent + 23 chromosomes from female parent = 46 total (diploid)

10. Meiosis I: Prepping for Meiosis  Interphase I  Cells replicate DNA ONCE, forming duplicate chromosomes  There will only be ONE interphase during the whole process of meiosis.  Meiosis I has four stages:  Prophase I  Metaphase I  Anaphase I  Telophase I  Cytokinesis  Page 273, Figure 5

11. Meiosis I: Stage One  Prophase I  Each chromosome (2 sister chromatids) pairs with its corresponding homologous chromosome to form a tetrad  Crossing-over occurs Result: the exchange of alleles between homologous chromosomes and produces new combos of alleles

13. Meiosis I: Stage Two  Metaphase I  Spindle fibers attach to the chromosomes  Still attached at the centromere  Forms tetrad (2 homologous c’somes lined up at equator)

14. Meiosis I: Stage Three  Anaphase I  Spindle fibers pull apart homologous chromosomes toward opposite ends of the cell  Sister Chromatids are still connected at the centromere!

15. Meiosis I: Stage Four & Cytokinesis  Telophase I  Nuclear membranes form  Sister Chromatids may not be identical due to crossing over  Cytokinesis  The cytoplasm separates (just like in mitosis)  Cell splits into two haploid (n) cells

16. Meiosis II  Meiosis II is very similar to mitosis; however there is NO chromosome replication that takes place before it begins (no interphase II)  Both haploid (n) cells created in meiosis I divide  Ends with four new haploid (n) cells  Sperm or egg cells  Four stages:  Prophase II  Metaphase II  Anaphase II  Telophase II  Cytokinesis MEIOSIS II

17. Meiosis II: Stage One  Prophase II  The two haploid (n) daughter cells that were produced at the end of meiosis I have half the number of chromosomes as the original cell  NO REPLICATION OF CHROMOSOMES happens during meiosis II

18. Meiosis II: Stage Two  Metaphase II  Chromosomes line up in the center of each cell  Spindle fibers are attached at centromeres of sister chromatids  Like metaphase in mitosis. Why?

19. Meiosis II: Stage Three  Anaphase II  Spindle fibers shorten  Sister chromatids separate and move apart toward opposite ends of each cell

20. Meiosis II: Stage Four & Cytokinesis  Telophase II  Nuclear envelopes reform in both cells  Cytokinesis  The cytoplasm in both cells splits to form 4 haploid (n) daughter cells with HALF the number of chromosomes as the original cell  So if parent cell has 46 chromosomes, each cell at the end of meiosis II would have 23 chromosomes.  Result: Sexual Reproduction allows for genetic variation  Crossing over makes 4 possibilites!

21. Spermatogenesis & Oogenesis  Spermatogenesis  Formation of sperm  Starts at puberty  Forms 4 sperm during each meiosis  Men will make 5 to 200 million sperm per day!!  Oogenesis  Formation of the egg  Meiosis starts inside the womb and continues in some during every cycle after puberty  1 egg and 3 polar bodies are created after every meiosis  The egg must contain a lot of cytoplasm to support the developing embryo after fertilization  Mitosis/Meiosis Video Mitosis/Meiosis Video

22. Genetics  Genetics is the study of traits and how they are passed from one generation to the next.  BrainPop BrainPop  Greatest Discoveries Greatest Discoveries

23. Gregor Mendel  Austrian monk  Performed genetic experiments in the 1850’s and 1860’s  Considered the “Father of Genetics”  His work was performed with no knowledge of DNA, cells, or meiosis!

24. Mendel’s Experiments  Worked with pea plants in the monastery gardens  Followed the inheritance patterns of seven different traits (Ex.: Green seed Vs. Yellow seed) in the plants

27. Creating the F 1 Generation  For each trait:  Mendel used a true-breeding plant for each form of the trait for the parent (P) generation Ex- True-breeding purple flower x true-breeding white flower  Cross-pollinated the plants to produce offspring  Created F 1 generation which only displayed one form of the trait (hybrids or heterozygous) Ex- all F 1 plants were purple flowered

29. Conclusions  Pea plants were passing a chemical message from one generation to the next that was controlling the trait (Ex- flower color)  This is a gene (Ex- gene for flower color) Genes are sections of DNA on chromosomes that code for a trait  Different forms of a trait are called alleles There is a purple and a white allele for flower color

30. More Conclusions  Principle of Dominance  One allele is dominant over the other  Dominant will always be displayed when present  Recessive is only seen when it is the only allele present

31. Creating the F 2 Generation  For each trait  Mendel self-pollinated plants from the F 1 generation Ex- F 1 purple flower is crossed with itself  Created the F 2 generation which displayed both traits in a 3:1 ratio For every 4 flowers, 3 were purple flowered and one was white flowered

33. Conclusions  Each pea plant has two copies of every gene  Each copy is found on one of the homologous chromosomes  Each individual has three possible types of combinations Two dominant alleles- homozygous dominant Two recessive alleles- homozygous recessive One of each- heterozygous

34. More Conclusions  Principle of Segregation  The two copies of a gene that an individual has separate (segregate) from each other during gamete formation (Meiosis)  The copy to be put in the gamete is chosen at random  This happens during Anaphase I when the tetrads separate

35. Tetrad Separation (Segregation)

36. Predicting Inheritance Outcomes  Probability- rules that predict the likelihood of an event occurring  Punnett squares- tool used in genetics to figure out the probability of a genetic cross  Monohybrid cross- Punnett square showing the outcome of the inheritance of one trait  Dihybrid cross- Punnett square showing the outcome of the inheritance of two traits

37. Information About Traits  Physical form of the trait seen is the phenotype (show either dominant or recessive)  Genotype is the alleles that an individual has for a trait (2 alleles/trait)  Represented by letters (capital for dominant, lower-case for recessive)  Letter is chosen based on dominant allele  Possibilities (using flower color as example) Homozygous dominant PP Heterozygous Pp Homozygous recessive pp  Heredity Heredity

38. Setting Up a Punnett Square  One parent’s possible gametes go on the top  Other parent’s possible gametes go on the side  Squares are filled in with the column and row header  Dominant letter is written first

39. Mendel’s Dihybrid Experiment  Mendel crossed two plants that were true-breeding for two traits  Ex- True-breeding round and yellow peas (RRYY) x True- breeding wrinkled and green peas (rryy)  F 1 generation phenotype: all round and yellow  F 1 generation was self-pollinated to create F 2  F 2 generation showed all 4 possible phenotype combinations in a 9:3:3:1 ratio

41. Conclusions  Law of Independent Assortment  Each gene segregates on its own  The inheritance of one trait does not influence the inheritance of another; each trait is chosen randomly and independent from each other For example, a pea plant that inherited the dominant yellow pea color did not automatically inherit the round (dominant) pea shape.

43. Setting Up a Dihybrid Punnett Square  All possible allele combinations from one parent are placed along the top (4 total)  For example- an F 1 round and yellow pea plant (RrYy) could produce RY, Ry, rY, and ry gametes  All possible allele combinations from the other parent are placed along the side (4 total)  Square are filled with the column and row headers (16 squares)  Letters from one trait go first, then the other  Capital letter for that trait are put in front

44. Dihybrid Punnett Square

45. Uses for Punnett Squares  Give all possible outcomes for a cross between two different parents  Predicts expected (not actual) ratios among the offspring