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Meiosis and Mendel CHAPTER 6 PAGE 156 A. Chromosomes and Meiosis 1. Body cells and gametes a. Many types of specialized cells b. Somatic cells (body cells) 1) Make up tissues and organs 2) Spleen, kidney, eyeballs c. Germ cells 1) In reproductive organs (ovaries and testes) 2) Develop into gametes a) Females – eggs (ova) b) Males – sperm (spermatozoa) 3) Have DNA that can be passed to offspring 2. Autosomes and sex chromosomes a. Humans have 23 pairs of chromosomes 1) Homologous pair – same structure 2) One from each pair comes from each parent 3) Have same genes but possibly different traits b. More chromosomes does NOT mean more complex 1) Fruit flies – 8 chromosomes 2) Fern – 1200+ chromosomes c. Autosomes – pairs 1 – 22 d. Sex chromosomes 1) X and Y a) Not homologous b) X – larger and more genes 2) Pair 23 3) Determines gender 4) XX = female 5) XY = Male 3. Body cells = diploid; gametes = haploid a. Sexual reproduction – fusion of 2 gametes b. Fertilization – actual fusion of egg and sperm c. Diploid (2n) 1) 2 copies of each chromosome 2) Half from each parent d. Haploid (n) 1) 1 copy of each chromosome 2) Have 22 autosomes and 1 sex chromosomes e. Chromosome number is maintained by mitosis and meiosis 1) Mitosis – body cell division 2) Meiosis a) Divides diploid cells into haploid cells b) Germ cells c) Produce gametes d) Reduces chromosome number and increases diversity B. Process of Meiosis 1. Cells divide twice a. Homologous chromosomes 1) 2 separate chromosomes 2) Same length and carry same genes 3) Separated in meiosis I b. Sister chromatids 1) Duplicated chromosomes attached at centromere 2) Divided during meiosis II 2. Meiosis I (DNA already duplicated) a. Prophase I 1) Nuclear membrane breaks down 2) Centrioles move to opposite sides 3) Spindle fibers form 4) Homologous chromosomes pair up 5) Crossing over a) Parts of chromatids break off and exchange parts w/adjacent chromatids b) Gives lots of genetic variation b. Metaphase I 1) Chromosomes meet in the middle 2) Line up in PAIRS (½ from mom and ½ from dad) c. Anaphase I 1) Homologous pairs separate and move to poles 2) Sister chromatids remain together d. Telophase I 1) Chromosomes reach poles 2) Nucleus reforms 3) Cytokinesis takes place 3. Meiosis II a. Prophase II – same as prophase I but NO crossing over b. Metaphase II – chromosomes meet in the middle (NO pairs) c. Anaphase II 1) Chromatids move toward poles 2) Chromatids now called chromosomes d. Telophase II 1) Nucleus reforms 2) Spindle fibers disappear 3) Cytokinesis form 4 cells 4. Mitosis vs. Meiosis a. Meiosis = 2 cell divisions; Mitosis = 1 b. Mitosis homologous chromosomes never pair up c. Meiosis = haploid cells; Mitosis = diploid cells 5. Haploid cells developing into mature gametes a. Gametogenesis – production of gametes b. Males 1) Sperm become motile by flagella 2) Primarily contribute DNA to embryo 3) 4 functional cells c. Females 1) Eggs contribute DNA, cytoplasm, and organelles for embryo 2) Meiosis I forms 1 cell and 1 polar body 3) Meiosis II forms 1 egg and 2 polar bodies 4) 1 functional egg 3 nonfunctional polar bodies 5) Polar body a) Cell with little more than DNA b) Eventually is broken down C. Mendel and Heredity Mendel laid groundwork for genetics a. Austrian monk b. Studied pea plants for 9 years c. AKA – Father of Genetics 2. Traits – distinguishing characteristics that are inherited (eye color, tail length, leaf shape) 3. Genetics – study of biological inheritance patterns and variation 1. 4. Mendel revealed patterns of inheritance a. Key decision of Mendel 1) Used purebred plants – only produce 1 trait 2) Control breeding 3) 7 “either-or” traits b. P – parental generation c. F1 – offspring of parents d. Mendel’s traits Seed Shape Round Wrinkled Round Seed Color Yellow Green Yellow Seed Coat Color Gray Pod Shape Smooth White Constricted Gray Inflated Pod Color Green Yellow Green Flower Position Axial Terminal Axial Plant Height Tall Dwarf Tall e. Mendel’s Experiment 1) Parental generation crossed to produce offspring 2) Prevented the self-pollination process by removing male flower parts 3) Mendel allowed the resulting plants to selfpollinate a) F1 generation i. All plants had purple flowers ii. Heterozygous – both traits b) F2 – Some purple, some white Mendel controlled the fertilization of his pea plants by removing the male parts, or stamens. He then fertilized the female part, or pistil, with pollen from a different pea plant. 4) Mendel noticed patterns (3:1) f. Mendel’s Conclusions 1) Traits are passed down as genes 2) Organisms inherit 2 copies of each gene – one from each parent 3) 2 copies segregate during gamete formation D. Traits, Genes, and Alleles 1. Same gene can have many versions a. Gene – piece of DNA that directs a cell to make a certain protein b. Locus – specific position on a pair of homologous chromosomes c. Allele – alternative forms of a gene that may occur at a specific locus (yellow or green) d. Homozygous 1) 2 of the same alleles 2) TT or tt e. Heterozygous 1) 2 different alleles 2) Tt 2. Genes influence the development of traits a. Genome – all of an organism’s genetic material b. Genotype – genetic makeup of specific genes c. Phenotype – physical appearance d. Dominant 1) Allele that is expressed 2) Not always most common e. Recessive – allele which is masked unless 2 are present (t) Polydactyly Having too many fingers or toes Dominant trait Assigning genotypes - Dominant trait gets capital letter - Recessive trait get lowercase of the same letter - RR Genotype Phenotype RR Round seed Yy Yellow pod gg Green seed E. Traits and Probability 1. Punnett Squares a. Axes represent parent genotypes b. Boxes represent offspring genotypes c. Show ratio of possible genotypes 2. Monohybrid cross a. Only one trait is looked at b. Cross – mating of two organisms c. Homozygous/homozygous d. Heterozygous/heterozygous e. Heterozygous/homozygous recessive 3. Testcross – cross b/w organism w/unknown genotype and an organism with the recessive phenotype 4. Dihybrid cross a. Involves 2 traits b. Heterozygous plants yield 9:3:3:1 phenotype ratio c. Law of independent assortment – allele pairs separate independently of each other during meiosis 5. Patterns can be calculated a. Probability – likelihood of something happening 1) Predicts an ave. number of occurrences, not an exact number 2) Applies to random events like meiosis and fertilization b. Equation Probability = number of ways a specific event can occur number of total possible outcomes F. Meiosis and Genetic Variation 1. Sexual reproduction creates unique combinations of genes a. Independent assortment b. Random fertilization of gametes c. 1 couple can produce 70 trillion different combinations of chromosomes d. Unique phenotypes give a reproductive advantage to some organisms 2. Crossing over increases diversity a. Exchange of chromosome segments b/w homologous chromosomes b. Results in new combinations of genes c. Happens in prophase I d. Farther apart genes = more likely to be separated e. Genetic linkage 1) Genes being inherited together b/c they are located close together on a chromosome 2) Allows calculation of distance b/w 2 genes