UNIT 3 GENETICS Chapter 6 Meiosis and Mendel 6.1 - Chromosomes and Meiosis 6.1 - Chromosomes and Meiosis A. You have body cells and gametes. 1. Cells can be divided into two major groups: a. Somatic Cells - body cells. 1) Make up most of your body tissues and organs. 2) DNA is not inherited. b. Gametes - sex cells 1) Derived from germ cells in reproductive organs, i.e., ovaries or testes 2) Form egg and sperm cells 3) DNA can be inherited 2. Each species has a characteristic number of chromosomes per cell. a. Number not linked to complexity of the organism. b. Cells differ due to way genes are expressed, not because they contain different genes. B. Your cells contain autosomes and sex chromosomes. 1. Your body cells have 23 pairs of chromosomes. a. Each pair referred to as a homologous pair. b. Homologous chromosomes are two chromosomes one from mother and one from father. c. Each pair contains copies of same genes, but the copies may differ. 2. Autosomes - chromosome pairs numbered 1-22. (they are homologous). 3. Sex chromosomes - a pair of chromosomes that directly control development of sexual characteristics. a. Very different in humans b. XX chromosomes female c. XY chromosomes male d. Considered 23rd pair, but X and Y not homologous Sex chromosomes C. Body cells are diploid (2n); gametes are haploid (n). 1. Sexual reproduction involves fusion of two gametes. a. Results in genetic mixture of both parents. b. Fertilization - Fusion of egg and sperm. c. Each egg and sperm cell has half the usual number of chromosomes. 2. Diploid and Haploid cells a. Body cells are diploid (2n) - two copies of each chromosome. b. Gametes are haploid (n) - have one copy of each chromosome. 3. Maintaining the correct number of chromosomes is important to the survival of all organisms. 4. Germ cells (sex cells) undergo the process of meiosis to form gametes (egg / sperm). a. Diploid nucleus divides into haploid nucleus. b. Sometimes called reduction division. Haploid cells Chapter 6 Meiosis and Mendel 6.2 - Process of Meiosis 6.2 - Process of Meiosis A. Meiosis begins with a diploid germ cell containing chromosomes replicated during S-phase of Interphase. B. Cells go through two rounds of division in meiosis. 1. Nuclear division produces four haploid cells from one diploid cell. 2. Two rounds of cell division Meiosis I and Meiosis II, each with four phases. C. Homologous Chromosomes and Sister Chromatids 1. Need to distinguish between the two to understand meiosis. 2. Homologous chromosomes - two separate chromosomes: one from mother, one from father. a. Very similar to each other, e.g., same length and carry same genes (but are not copies of each other). b. Each half of duplicated chromosome is called a chromatid. (together called sister chromatids). 1). Homologous chromosomes divided in meiosis I. 2). Sister chromatids not divided until meiosis II. Sister chromatids 3. Results in two haploid (n) cells each containing replicated chromosomes, i.e., sister chromatids still held together at centromere. D. Meiosis I (first of two rounds of cell division): 1. Occurs after DNA was duplicated during interphase (during Synthesis phase). 2. Separates homologous pairs of chromosomes in four phases. (PMAT) E. Meiosis II (second of two rounds of cell division): 1. Sister chromatids divide in four phases. (PMAT) 2. DNA is not replicated between meiosis I and meiosis II. F. Meiosis differs from mitosis in significant ways. 1. Meiosis has two cell divisions while mitosis has one. 2. In mitosis, homologous chromosomes never pair up. 3. Meiosis results in haploid cells; mitosis results in diploid cells. G. Haploid cells develop into mature gametes. 1. Gametogenesis - production of mature gametes. 2. Differs between the sexes a. Sperm - male gamete, smaller than female egg. 1) Develops flagellum 2) Contains no organelles b. Egg - female gamete 1) Uneven cell divisions result in one large egg cell and three polar bodies. 2) Egg provides organelles to embryo. Chapter 6 Meiosis and Mendel 6.3 - Mendel & Heredity 6.3 - Mendel and Heredity A. Mendel laid the groundwork for genetics. 1. Traits - distinguishing characteristics that are inherited, e.g., eye color. 2. Genetics - study of biological inheritance patterns and variation. 3. In 1800s scientists thought traits were blended in offspring. 4. Gregor Mendel showed traits are inherited as discrete units from parental generation. B. Mendel’s data revealed patterns of inheritance. 1. Mendel studied plant variation in a monastery garden. 2. Mendel made three key decisions in his experiments: a. Control over breeding b. Use of purebred plants c. Observation of “eitheror” traits (only appear in two alternate forms). 3. Experimental design a. Mendel chose pea plants - they reproduce quickly; he could control how they mate. b. Pea plant flowers contain both male and female reproductive organs (called a complete flower). c. Used cross pollination to cross different pea plants. d. Studied seven traits, each with either-or characteristics. 4. Results: a. Crossed (mated) two purebred (homozygous) pea plants, e.g., crossed white-flowered with purebred purple-flowered plants . 1). Called parental, or P generation. 2). Resulting plants (first filial or F1 generation) all had purple flowers (heterozygous). b. Allowed F1 generation (e.g., all purple) to selfpollinate/fertilize. 1). Produced F2 generation - some had all purple flowers, others had all white flowers. 2). Trait for white had been “masked” in the F1 generation, it did not disappear. c. Began to observe patterns - i.e., each cross yielded similar ratios of traits in F2 generation (e.g., he counted three plants with purple flowers for every one with white flowers (3:1)) 4. Mendel made three important conclusions a. Traits are inherited as discrete units. (explained why individual traits persisted without being blended or diluted over successive generations) b. Two other key conclusions are collectively called the law of segregation. 1). Organisms inherit two copies of each gene, one from each parent. 2). Organisms donate only one copy of each gene in their gametes. (two copies of each gene segregate, or separate, during gamete formation) Chapter 6 Meiosis and Mendel 6.4 – Traits, Genes & Alleles 6.4 - Traits, Genes, and Alleles A. The same gene can have many versions. 1. Gene - a “piece” of DNA that provides instructions to a cell to make a specific protein. . a. Locus - location on chromosome where a gene resides. b. Most genes exist in many forms. c. Allele - alternative form of a gene. d. You have two alleles for each gene, one on each homologous chromosome. (e.) Each parent donates one allele for every gene. 2. Homozygous - means two of the same allele at a locus. 3. Heterozygous - two different alleles at a locus. B. Genes influence the development of traits 1. Genome - all the organism’s genetic material. 2. Genotype - genetic makeup of a specific set of genes. 3. Phenotype - physical characteristics of an organism (e.g., white or purple flower, tall or short plant, round or wrinkled seed). C. Dominant and Recessive Alleles 1. Dominant alleles - allele that is expressed when two different alleles or two dominant alleles are present (represented by Capital letters). 2. Recessive alleles - expressed when two copies of recessive allele present (represented by lower-case letters). 3. Dominant does not mean stronger; it is expressed (makes protein) when it’s without recessive allele for same trait. 2. Alleles represented by letters, one letter per allele; uppercase = dominant, lowercase = recessive. a. Homozygous dominant = e.g., TT b. Heterozygous = e.g., Tt c. Homozygous recessive = e.g., tt D. Alleles and Phenotypes 1. Both homozygous dominant and heterozygous genotypes yield a dominant phenotype. 2. Most traits occur in a range and do not follow simple dominantrecessive patterns. . Chapter 6 Meiosis and Mendel 6.5 – Traits & Probability 6.5 - Traits and Probability A. Punnett squares illustrate genetic crosses. 1. Used to predict possible genotypes resulting from a cross. a. Axes of grid represent possible gamete genotypes of each parent. b. Boxes show all possible genotypes of offspring from those parents. c. Can determine ratio of genotypes in that generation. (the ratio has to add up to the total # of boxes) d. Can also be used to determine ratio of phenotypes. Punnett Square parents gametes Dominant Allele Possible offspring Recessive allele homozygous heterozygous B. Monohybrid cross - involves one trait 1. Homozygous dominant X Homozygous recessive Genotypic ratio = 4 : 0 (100% Ff) Phenotypic ratio = 4 purple : 0 white (100% purple) 2. Heterozygous X Heterozygous Genotypic ratio = 1FF : 2Ff : 1ff (25% : 50% : 25%) Phenotypic ratio = 3 purple : 1 white 3. Heterozygous X Homozygous recessive Genotypic ratio = 2 Ff : 2 ff (can reduce to 1Ff : 1ff) (50% : 50%) Phenotypic ratio = 2 purple : 2 white (1 purple : 1 white) C. Test Cross - cross between organism with an unknown genotype and an organism with a homozygous recessive phenotype. (?? x rr) D. Dihybrid cross involves two traits 1. Dihybrid cross - predicts inheritance of two different traits, e.g., seed color and shape. 2. Mendel discovered phenotypic ratio in F2 generation was always 9:3:3:1, regardless of combination of traits used. (hetero x hetero) A dihybrid cross involves two traits. 3. Presence of one trait does not affect inheritance of another trait. 4. Led to Mendel’s second law of genetics, the Law of Independent Assortment. 5. Law states that allele pairs separate independently of each other during meiosis. E. Heredity patterns can be calculated using probability. 1. Probability - likelihood that a particular event will happen. Probability = number of ways a specific event can occur number of total possible outcomes a. Predicts average number of occurrences. b. Can calculate probability of two independent events happening at same time by multiplying probability of each individual event. 2. Probability also applies to meiosis and fertilization events. What do you think the probability would be? Chapter 6 Meiosis and Mendel 6.6 – Meiosis & Genetic Variation 6.6 - Meiosis and Genetic Variation A. Sexual reproduction creates unique combination of genes. 1. Sexual reproduction provides much genetic variation within species. 2. Variation results from: a. Independent assortment of chromosomes in meiosis. b. Random fertilization of gametes. 3. Meiosis can produce human gametes (egg or sperm) containing 223 (about 8 million) different combinations of chromosomes. 4. Due to random fertilization, a human zygote can contain 223 X 223 (more than 64 trillion) different combinations of chromosomes. B. Crossing over during meiosis increases genetic diversity. 1. Crossing over - exchange of chromosome segments between homologous chromosomes during Prophase I of Meiosis I. 2. Results in new combination of genes. C. Linked genes - genes located on the same chromosome that are inherited together. 1. Some genes on same chromosome are close together while others are far apart. 2. The farther apart two genes are the more likely they will be separated during crossing over. 3. The closer together two genes are on the same chromosome, the more likely they will be inherited together. a. Genes inherited together are called linked genes. b. Gene linkage used to calculate distance between genes on a chromosome.