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MENDEL AND MEIOSIS Chapter 10 Zebra stripes are like fingerprints. Every zebra’s stripe pattern is unique. Stripes are genetically determined Mendel’s Laws of Heredity Gregor Mendel Austrian monk who carried out studies in heredity Chose pea plants to study Heredity Passing on of characteristics from parents to offspring Traits Inherited characteristics Hair color, eye color Why Pea Plants? Reproduce sexually Usually self pollinate Gametes – male and female sex cells Fertilization – when male gamete units with female gamete Zygote – fertilized cell, develops into a seed Able to control pollination! Pollination – transfer of pollen grains from male to female Mendel used good scientific method Only studied one trait at a time Like plant height, flower color, type of seed coat Used math to calculate data Mendel’s Monohybrid Crosses Mendel crossed a tall plant and a short plant He called the offspring a hybrid Offspring from parents that have different forms of a trait “mono” meaning one trait “hybrid” see above definition Results of monohybrid cross 1st Generation nd 2 Generation Mendel allowed tall plants to self pollinate ¾ were tall ¼ were short Like the short trait had reappeared from nowhere “P” parent “F” filial or son/daughter Terms “P1” – your parents “F1” – you “F2”- any children you have Rule of unit factors Two factors control traits Traits are controlled by genes Genes are locations on chromosomes Different forms of a gene are called “alleles” Like two alleles for height Possibilities Two alleles for tall height Two alleles for short height One allele for tall height and one allele for short height Rule of Dominance Dominant – the trait that you see Recessive – the trait that is not expressed if dominant allele is present Use same letter for allele Capital letter for dominate Lower case letter for recessive Dominant (capital letter) is written first Law of Segregation Every individual has two alleles of each gene and when gametes are produced, each gamete receives one of these alleles Phenotypes and Genotypes Two organisms can look alike (tall pea plants) but have different underlying allele combinations. Phenotype – the way an organism looks and behaves – PHYSICAL Tall plant (phenotype) even though it could be TT or Tt Genotype – the allele combination an organism contains – GENETIC TT or Tt or tt – the actual genetic makeup of organism Homozygous or Heterozygous Homozygous – when alleles for a trait are the same TT or tt – homozygous for tall or short Heterozygous – when alleles for trait are different Tt – one allele for tall height, one allele for short height Practice Give phenotype and genotype of the following (R is dominant for round seed, r is recessive for wrinkled seed) TT Tt Tt RR Rr rr Heterozygous or homozygous? Dihybrid Cross Differing in 2 traits Mendel paired plants that were RRYY (round and yellow) and rryy (wrinkled and green) He found the offspring were in a definite ratio of phenotypes 9 round yellow 3 round green 3 wrinkled yellow 1 wrinkled green Law of Independent Assortment Genes for different traits are inherited independently of each other Having the allele for a green seed color does not effect the type of seed coat or height of plant Punnett Squares A method to predict offspring outcomes based on knowledge of parent genotypes. PROBABILITY Punnett Squares show likelihood, not reality Closer to actual numbers when the number of offspring is large FOIL METHOD FIRST, OUTSIDE, INSIDE, LAST AaBb F – AB O – Ab I – aB L – ab EXAMPLE RRYy X Rryy Practice Complete a Punnett Square for the following crosses List possible genotypes and phenotypes Traits for flower color and plant height P – purple flower p – white flower T – tall t – short PpTt X PPtt MEIOSIS CHAPTER 10 SECTION 2 Genes, Chromosomes and Numbers Pea plants had 14 chromosomes or 7 pairs Most chromosomes appear in pairs One from male parent, one from female parent Diploid Cell that contains a diploid or 2n, number of chromosomes Gametes contain one of each kind of chromosome Haploid Cell that contains one of each kind of chromosome, haploid or n Chromosome Numbers Organism Body cells (2n) Gamete (n) Fruit fly 8 4 Corn 20 10 Human 46 23 Dog 78 39 Homologous Chromosomes Have genes for the same trait But may have different alleles Mendel Pea Plants Get one from each parent Why Meiosis? Difference between Mitosis and Meiosis? End product – Mitosis 2 identical daughter cells, with same genetic makeup as parent and the same number of chromosomes End product – Meiosis Gametes with half the number of chromosomes as parent Why does this work? When gametes unite – you are back to the right number of chromosomes Meiosis Occurs in the specialized body cells of each parent Male gametes – sperm – haploid (n) Female gametes – egg – haploid (n) Joining of sperm and egg – diploid (2n) number of chromosomes Zygote then undergoes mitosis to develop into multicellular organism Sexual Reproduction Recombination of genetic material Phases of Meiosis Meiosis I Interphase Chromosomes replicated, held together by centromere Prophase I DNA thickens Homologous chromosomes line up Four part structure is called a tetrad Crossing Over When chromosomes break and exchange genetic material Approx 2-3 crossovers for each pair of homologous chromosomes, in humans Meiosis I Metaphase I Spindle fiber pulls tetrad to middle of cell Homologous chromosomes are side by side Anaphase I Homologous chromosomes separate and move to opposite ends of the cell Centromeres are still holding sister chromatids together Telophase I Spindles break down Chromosomes uncoil Cytoplasm divides to yield two new cells Cell is still at 2n, because of the replication of DNA Meiosis II We need to get to n, or haploid, to a gamete Prophase II Spindle forms and attach chromotids Metaphase II Chromatids line up randomly at equator Anaphase II Centromere splits, sister chromatids separate Move to opposite poles Telophase II Spindle break down Cytoplasm divides End Products of Meiosis 4 haploid (n) cells are formed These will become gametes Meiosis Provides for Genetic Variation Genetic Variation - reassortment of chromosomes and genetic information they carry by c.o. or independent segregation Crossing Over Provides variation by giving new combinations of alleles Pea Plant Sperm possibilities Seven pairs of chromosomes can each line up 2 different ways, same for eggs 2n = 2 7 = 128 Humans 2n = 223 or 8 million possible egg or sperm Nondisjunction Events usually go smoothy during Meiosis, but, sometimes, homologous chromosomes fail to separate This is called nondisjunction For example, when nondisjunction occurs One gamete has an extra chromosome One gamete is missing a chromosome When the gamete with the extra chromosome is fertilized by a normal gamete Trisomy occurs Now 47 not 46 chromosomes, location chromosome 21, Down Syndrome occurs Turner Syndrome Monosomy – missing a X chromosome, occurs in females Polyploidy More than the usual number of chromosomes Usually results in death for zygote in ANIMALS But, in plants, used to plant breeders advantage Plants are healthier, larger Hexaploid wheat, triploid apples, polyploid chrysanthemums Gene Linkage and Maps If genes are close together, the usually are inherited together They are said to be “linked” All the genes on a chromosome are usually linked Chromosomes themselves follow I.A. Affected by Crossing Over