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Chapter 10: Genetics Meiosis (Reduction Division) MEIOSIS is the way many organisms produce gametes through a type of cell division where the chromosome number is halved (HAPLOID) Only occurs in eukaryotic cells in phases similar to the phases of mitosis. I. Chromosome Number A. In most organisms, gamete (sex cells) can either be EGG OR SPERM B. Humans have 46 chromosomes or 23 PAIRS of chromosomes. 1. Egg cell would carry 23 chromosomes 2. Sperm cell would carry 23 chromosomes C. Genes are located on chromosomes in the cell nucleus. D. Mendel's principles of genetics require at least two things: 1. Each organism must inherit a single copy of every gene from both its “parents.” Because each organism has two “parents,” each organism must carry two complete sets of genes. 2. When an organism produces its own gametes, those two sets of genes must be separated from each other so that each gamete contains just one set of genes. D. Diploid cells (Body cells) 1. Homologous: chromosomes that came from the male parent has a corresponding chromosome from the female parent. 2. Diploid: A cell that contains both sets of homologous chromosomes. The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N. Diploid cells contain two complete sets of chromosomes and two complete sets of genes. E. Haploid cells 1. The gametes of sexually reproducing organisms, including fruit flies and peas, contain only a single set of chromosomes, and therefore only a single set of genes. These are haploid (one set) and represented by N. F. How are haploid (N) gamete cells produced from diploid (2N) cells? Meiosis must occur II. Phases of meiosis Meiosis: Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Meiosis usually involves two distinct divisions called meiosis I and meiosis II. A. Meiosis I: Homologous chromosomes are separated and crossing over occurs B. Meiosis II: 1. Sister Chromatids of each chromosomes are separated. 2. By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells. Interphase I Meiosis I (Prophase I Metaphase I Anaphase I Telophase I) Meiosis II (Prophase II Metaphase II Anaphase II Telophase II) A. Meiosis I: In Meiosis I, the division of the cell is very similar to mitosis. The end result is the splitting of the chromosomes from their homologous pair. 1. Prophase 1:Each chromosome pairs with its corresponding homologous chromosome. This creates a TETRAD. a. There are 4 chromatids in a tetrad. b. Crossing-Over: During meiosis I, while paired up with their sister chromatids, alleles can exchange material between homologous chromosomes to form new combinations of alleles. 1. Creates Genetic Variation!! Synapse Tetrad MEIOSIS I Interphase Prophase I Cell doubles in size, DNA and organelles replicate (G1, S, G2) Chromosomes condense and spindle forms. Chromosomes come together =SYNAPSE Chromosomes line up in tetrads Crossing over occurs *GENETIC VARIATION Metaphase I Anaphase I Telophase I/ Cytokinesis Homologous pairs (tetrads) line up in the middle of the cell Homologous pairs of chromosomes pulled toward opposite sides of the cell Law of segregation Chromosomes uncoil, cells separate cells divide. Each new cell is haploid B. Meiosis II: 1. At this point, each of the cells has a haploid number of chromosomes and each chromosome has 2 chromatids (a copy of the original). 2. At the end of Meiosis II, you get four haploid (N) daughter cells. Meiosis II Prophase II NO REPLICATION NO CROSSING OVER Metaphase II Condensing again Chromosomes line up just as the Anaphase II Telophase II/Cytokinesis line up in mitosis – in the middle The sister chromatid are pulled to opposite ends (poles) The end product are four haploid cells that may develop into gametes C. Meiosis vs. Mitosis: Mitosis results in two genetically identical diploid cells. Meiosis produces four genetically different haploid cells. MITOSIS DAUGHTER CELLS WITH FULL SET DIPLOID MEIOSIS DAUGHTER CELLS WITH HAPLOID N 2N 2 CELLS 1 CELL DIVISION GENETICALLY IDENTICAL 4 CELLS 2 CELLS DIVISION GENETICALLY DIFFERENT III. FORMATION OF GAMETES A. Meiosis occurs within the Reproductive Organs, 1. Reproductive organs a. TESTES (Boys) b. OVARIES. (Girls) B. Development of Sperm Cells 1. Each cell produces FOUR Haploid Cells called SPERMATIDS 2. Spermatids develop into sperm in a process of SPERMATOGENESIS C. Development of Egg Cells 1. Each cell produces ONE Haploid egg and three non-functional polar bodies because during each division – most of the cytoplasm and nutrients goes to one cell (egg) 2. Mature egg cells or ova develop in a process called OOGENESIS The work of Gregor Mendel I. The experiments of Gregor Mendel A. Genetics: the study of heredity B. Gregor Mendel: 1. An Austrian Monk who was born in 1822. 2. He was educated in Math and Science. 3. worked as a high school teacher. 4. Worked in the monastery garden and took a particular interest in the pea plants. ` a. Pea plants reproduce quickly b. Small c. Self pollinating C. Fertilization: Process in sexual reproduction in which male and female reproductive cells (gametes) join to form a new cell D. Self-pollination: The sperm cells in pollen fertilize the egg cells in the same flower. The seeds that are produced by self-pollination inherit all of their characteristics from the single plant. E. True Breeding: that if they were allowed to self-pollinate, they would produce offspring identical to themselves F. Cross-pollination: Taking the sperm from one plant to fertilize the egg of another plant. This produces seeds that have two different plants as parents. G. Trait: a specific characteristic, such as seed color or plant height, that varies from one individual to another. F. Hybrid: The offspring of crosses between parents with different traits. II. Genes and Alleles When conducting genetic crosses the following terms are important. A. P generation: (Parental Generation) the original pair of crossed plants. B. F1 generation: (First Filial Generation) Filius and filia are the Latin words for “son” and “daughter.” C. Hybrid: The offspring of crosses between parents with different traits. D. Genes: Factors that are passed down from parent to offspring E. Alleles: Different form of a gene. 1. Seed color: yellow, green. Each color is an allele, a different form of the gene. F. Mendel’s Conclusions: 1. Biological inheritance is determined by factors that are passed from one generation to the next. 2. We now know the factors Mendel was referring to are genes. Each gene is made up of two alleles (one of a number of different forms of a gene). 3. Some alleles are dominant and some are recessive. a. Dominant: An organism with a dominant allele for a particular form of a trait will always exhibit that form of the trait. 1. (Denoted with a CAPITAL LETTER) b. Recessive: An organism with a recessive allele for a particular form of a trait will exhibit that form only when the dominant allele for the trait is not present. 1. (Denoted with a lower case letter) G. Segregation: The separation of alleles during gamete formation. 1. Gamete: Specialized cells involved in sexual reproduction a. egg or sperm cells. b.The alleles for the gene that controls plant height segregate from each other so that each gamete carries only a single allele of each gene. c. Each F1 plant produces two types of gametes—those with the allele for tallness and those with the allele for shortness. Applying Mendel’s Principles The principles of probability can be used to predict the outcomes of genetic crosses I. Probability and Punnett Squares A. Probability: the mathematic likelihood an event will occur 1. (flipping a coin : 50% heads, 50% tails) B. Punnett squares: can be used to predict and compare the genetic variations that will result from a cross. C. Homozygous: Organisms that have two identical alleles for a particular trait 1. TT or tt D. Heterozygous: Organisms that have two different alleles for the same trait 1. Tt E. Phenotype: The physical appearance of a trait (Tall or short). **not always visible (can’t see colorblindness) F. Genotype: The genetic make up (alleles) of an organism (Tt, TT or tt). I. Independent Assortment A. After Mendel showed that alleles segregate during gamete formation he wondered if the segregation of one pair of alleles (such as plant height) affected the segregation of another set of alleles (such as seed color) B. Two-Factor Cross F1 1. Mendel crossed true breeding round and yellow seeded plants (RRYY) with true breeding wrinkled and green seeded plants (rryy) RRYY x rryy RY RY RY RY ry RrYy RrYy RrYy RrYy ry RrYy RrYy RrYy RrYy ry RrYy RrYy RrYy RrYy ry RrYy RrYy RrYy RrYy 2. Results a. All plants were heterozygous RrYy and all are round and yellow b. Provided hybrid plants to breed for F2 generation C. Two-Factor Cross: F2 1. Mendel crossed the F1 plants to produce F2 offspring RrYy x RrYy RY Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy 2. Results a. A 9:3:3:1 ratio was found b. Law of independent assortment was discovered. 1. Different traits can segregate independently, one trait does not influence each other. III. Summary A. Inheritance of biological factors is determined by genes that are passed down from parent to offspring B. Two or more forms of an allele can exist and can be dominant or recessive. C. In most sexually reproducing organisms, each adult has two copies of a gene. One from each parent D. Alleles for different genes sort independently Other Patterns of Inheritance There are some exceptions to Mendel’s principles. Genetics is complicated. A majority of genes have more than one allele Many traits are controlled by more than one gene. (skin color, height, eye color, fingerprint patterns) Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes. I. Incomplete dominance. (Think—Intermediate) A. One allele is not completely dominant over another allele B. The heterozygous phenotype lies somewhere between the two homozygous phenotypes. C. Example in flowers: Cross between two four o’clock plants show one of these complications. 1. When you cross the red and white plant the offspring is pink in the F1 generation a. It “seems” that it is simply a blending of the dominant (red) and recessive (white), 2. BUT when you get the F2 generation, red, white and pink show up. The hybrid is a phenotypic mix between dominant and recessive. D. Example in humans: hypercholesterolemia (high blood cholesterol levels) 1. Causes: a. Total cholesterol - all the cholesterols combined b. High density lipoprotein (HDL) cholesterol 1. often called "good" cholesterol c. Low density lipoprotein (LDL) cholesterol 1. often called "bad" cholesterol 1. The LDL cholesterol needs to be broken down, the only way this can be done is with the help of LDL receptors to help mediate the endocytosis of the “bad cholesterol” d. Homozygous dominant (HH) person does not have hypercholesterolemia, e. Heterozygous Hh have a mild case of this disease, f. Homozygous recessive person (hh) has a severe cholesterol problem. II. Codominance—(Think Cooperate A. Both alleles (dominant and recessive) contribute to the phenotype of the organism. B. Example: Feather color certain varieties of chicken a. The allele for black feathers is codominant with the allele for white feathers 1.Heterozygous offspring are described as erminette or “speckled. 2. Cross a black feathered chicken with a white feathered chicken a. FBFB = Black feathers b. FwFw = White feathers c. FBFw =Speckled feathers FB FB Fw FB Fw FB Fw Fw FB Fw FB Fwolyg III.Multiple Alleles A. Many genes have more than two alleles. B. Blood type is an example of this. 1. The alleles that make up all blood types are A, B or i 2. Type A and B are codominant to each other, both are dominant to type O Polygenic Traits – poly = many genic = genes. Some traits are controlled by the interaction of two or more genes. Different combinations of these alleles produce very different traits. Eye color is an example, as is height. o Height AABBCC = 6 ft tall o aabbcc = 5 ft tall o AaBbCc = 5 ft 6 inches AS DOES AABbcc IV Genes and the Environment Characteristics of an organism are not solely determined by genes, environment plays a role. A. Western White Butterfly (Ponita occidentalis) 1. Butterflies that hatched in summer had a different color pattern than those that hatched in spring. a. Those that hatched in spring had greater levels of pigment in their wings. b. ENVIRONMENT in which butterflies developed influenced expression of genes. 1. The change in pigment influenced the temperature of the butterfly. The darker pigment helped increase the temp, which is important for flight effectiveness. Meiosis MEIOSIS is the way many organisms produce gametes through a type of cell division where the chromosome number is halved (HAPLOID) Only occurs in eukaryotic cells in phases similar to the phases of mitosis. I. Chromosome Number A. In most organisms, gamete (sex cells) can either be EGG OR SPERM B. Humans have 46 chromosomes or 23 PAIRS of chromosomes. 1. Egg cell would carry 23 chromosomes 2. Sperm cell would carry 23 chromosomes C. Genes are located on chromosomes in the cell nucleus. D. Mendel's principles of genetics require at least two things: 1. Each organism must inherit a single copy of every gene from both its “parents.” Because each organism has two “parents,” each organism must carry two complete sets of genes. 2. When an organism produces its own gametes, those two sets of genes must be separated from each other so that each gamete contains just one set of genes. D. Diploid cells 1. Homologous: chromosomes that came from the male parent has a corresponding chromosome from the female parent. 2. Diploid: A cell that contains both sets of homologous chromosomes. The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N. Diploid cells contain two complete sets of chromosomes and two complete sets of genes. E. Haploid cells 1. The gametes of sexually reproducing organisms, including fruit flies and peas, contain only a single set of chromosomes, and therefore only a single set of genes. These are haploid (one set) and represented by N. F. How are haploid (N) gamete cells produced from diploid (2N) cells? II. Phases of meiosis Meiosis: Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Meiosis usually involves two distinct divisions called meiosis I and meiosis II. A. Meiosis I: Homologous chromosomes are separated and crossing over occurs B. Meiosis II: 1. Sister Chromatids of each chromosomes are separated. 2. By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells. Interphase I Meiosis I (Prophase I Metaphase I Anaphase I Telophase I) Meiosis II (Prophase II Metaphase II Anaphase II Telophase II) A. Meiosis I: In Meiosis I, the division of the cell is very similar to mitosis. The end result is the splitting of the chromosomes from their homologous pair. 1. Prophase 1:Each chromosome pairs with its corresponding homologous chromosome. This creates a TETRAD. a. There are 4 chromatids in a tetrad. b. Crossing-Over: During meiosis I, while paired up with their sister chromatids, alleles can exchange material between homologous chromosomes to form new combinations of alleles. 1. Creates Genetic Variation!! MEIOSIS I Interphase Prophase I Cell doubles in size, DNA and organelles (G1, S, G2) Chromosomes condense and spindle forms. Chromosomes come together =SYNAPSE Chromosomes line up in tetrads Crossing over occurs *GENETIC VARIATION Metaphase I Anaphase I Telophase I/ Cytokinesis Homologous pairs (tetrads) line up in the middle of the cell Homologous pairs of chromosomes pulled toward opposite sides of the cell Independent Assortment Chromosomes uncoil, cells separate cells divide. Each new cell is haploid B. Meiosis II: 1. At this point, each of the cells has a haploid number of chromosomes and each chromosome has 2 chromatids (a copy of the original). 2. At the end of Meiosis II, you get four haploid (N) daughter cells. Meiosis II Prophase II Metaphase II Anaphase II Telophase II/Cytokinesis NO REPLICATION Condensing again Chromosomes line up just as the line up in mitosis – in the middle The sister chromatid are pulled to opposite ends (poles) The end product are four haploid cells that may develop into gametes C. Meiosis vs. Mitosis: Mitosis results in two genetically identical diploid cells. Meiosis produces four genetically different haploid cells. MITOSIS DAUGHTER CELLS WITH FULL SET MEIOSIS DAUGHTER CELLS WITH HAPLOID 2N N 2 CELLS 1 CELL DIVISION GENETICALLY IDENTICAL 4 CELLS 2 CELLS DIVISION GENETICALLY DIFFERENT III. FORMATION OF GAMETES A. Meiosis occurs within the Reproductive Organs, 1. Reproductive organs a. TESTES b. OVARIES. B. Development of Sperm Cells 1. Each cell produces FOUR Haploid Cells called SPERMATIDS 2. Spermatids develop into sperm in a process of SPERMATOGENESIS C. Development of Egg Cells 1. Each cell produces ONE Haploid egg and three non-functional polar bodies because during each division – most of the cytoplasm and nutrients goes to one cell (egg) 2. Mature egg cells or ova develop in a process called OOGENESIS FORMATION OF GAMETES Meiosis occurs within the Reproductive Organs, in the TESTES or OVARIES. In the development of Sperm Cells, each cell produces FOUR Haploid Cells called SPERMATIDS which develop into sperm in a process of SPERMATOGENESIS OOGENESIS is the production of Mature Egg Cells or OVA. In the development of eggs, each cell produces ONE Haploid egg and three non-functional polar bodies because during each division – most of the cytoplasm and nutrients goes to one cell (egg) 11-5 Linkage and Gene Maps The Law of Independent Assortment states that chromosomes assort independently during gamete formation (not individual genes) HOWEVER – if genes are on the same chromosome they are inherited together. – ie. All of the chromosomes on chromosome 1 are inherited together because they are all on the same chromosome. Gene Maps If genes are found on the same chromosome – they can be mapped out. This shows the relative positions of the genes on the chromosomes. Linkage and Gene Maps: Thomos Hunt Morgan realized, in working with fruit flies, that it is the chromosomes that assort independently of one another rather than the genes (as Mendel thought) Gene Linkage: genes for different traits are located on the same chromosome Gene Map: Shows the relative locations of each known gene on one chromosome. These maps can be constructed by measuring the frequencies of crossing-over between genes.