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Chapter 10 Sexual Reproduction and Genetics Sexual Reproduction and Genetics Section 1: Meiosis Section 2: Section 3: Mendelian Genetics Gene Linkage and Polyploidy Section 10.1 Meiosis 1. Objective: Understand meiosis is an early step in sexual reproduction that produces haploid gametes. 2. Essential Question: At the end of meiosis are there n or 2n number of chromosomes? (13 slides) Karyotype Diagram Human Karyotype- 23 pairs of chromosomes (46 total) What is meiosis? Meiosis = a type of cell division that reduces the number of chromosomes in half. Meiosis produces four haploid daughter cells that are not identical Spermatogenesis = 1 sperm cell 4 sperm cells Females: Oogenesis = 1 egg cell 1 egg cell & 3 polar bodies (total 4) Males: Which cells go . through meiosis? What are gametes? Only the gametes undergo meiosis Gametes are the sex cells, which are the egg (ovum) and the sperm. [pollen for plants] Meiosis produces haploid cells. What does that mean? = (n) a cell that has half the number of chromosomes as a diploid cell. “half” Haploid In humans the haploid # is 23 What is diploid? = (2n) a cell that has 2 copies of each chromosome “double” Diploid In humans the diploid # is 46. What are homologous chromosomes? Homologous chromosomes —one of two paired chromosomes, one from each parent. They have: Same length Same centromere position control the same inherited traits Meiosis separates homologous chromosomes What are the steps of meiosis? There are 2 divisions – Meiosis I and II. Each has the PMAT steps. . What are the steps of Meiosis I? Prophase I - Homologous chromosomes pair. Synapse & crossing over occurs Metaphase I - Homologous chromosomes (pairs) line up at the equator randomly. This increases genetic variation, too. Anaphase I - homologous chromosomes separate and move to opposite poles Telophase I Chromosomes uncoil and form two nuclei Meiosis II – DNA is NOT duplicated again! Prophase II –second phase begins, chromosomes condense again Metaphase II- chromosomes line up individually at the equator. Anaphase II- The sister chromatids separate bringing the cells to a haploid number. Telophase II- four nuclei have formed. Cytokinesis -four haploid cells (gametes –egg or sperm-) have formed. (n number of chromosomes) Why is meiosis important? The gametes need to reduce their chromosome number in half so that when there is fertilization the diploid number (2n) is restored. For humans: 23 + 23 = 46 . How Does Meiosis Provide Genetic Variation? 1. Random assortment when chromosomes line up at the equator 2. Crossing over 3. Random combination of gametes during fertilization. What is synapsis? Synapsis = a process that binds together a pair of homologous chromosomes What is crossing over? Crossing over = homologous chromosomes exchange segments. This increases genetic variation. What about asexual reproduction? The offspring receive ALL the genes from one parent. This is why the offspring look EXACTLY alike. Example: bacteria 10.2 Mendelian Genetics 1. Objective: • 2. Predict combinations of alleles (genotype and phenotype) from the genetic makeup of the parents. Essential Question: • What are the rules for the inheritance of traits with Mendelian genetics? • (10 slides) Who was Gregor Mendel? (1822-1884) The “father” of genetics Austrian Monk Experimented with pea plants Compared 7 contrasting traits. (Ex:height = tall and short) Mendel studied seven different traits Seed or pea color Flower color Seed pod color Seed shape or texture Seed pod shape Stem length Flower position What is heredity? Heredity = passing of traits to the next generation. How do we name generations? The parent generation is also known as the P generation. The offspring of this P cross are called the first filial (F1) generation. The second filial (F2) generation is the offspring from the F1 cross. What did Mendel conclude? Allele = An alternative form of a single gene passed from generation to generation example: alleles for pea color are yellow & green Dominant factor – dominates or masks all other factors Recessive factor – does not show if a dominant factor is present What are Mendel’s Laws? Law of Segregation –a pair of alleles is separated during the formation of gametes (meiosis) Law of independent assortment – alleles are distributed to gametes independently or randomly What are Genotype and Phenotype? We use Mendel’s discoveries to predict genotype and phenotype probabilities. Genotype – the allele pairs (genetic makeup) of an organism Phenotype – the observable physical characteristics (appearance) of an allele pair. Ex: PP or Pp Ex: Purple flowers Phenotype depends on Genotype What are homozygous & heterozygous? Homozygous – both alleles are the same for a trait (AKA purebred) Ex: (homozygous dominant- PP) or (homozygous recessive -pp ) Heterozygous (AKA hybrid)– allele are different for a trait Ex: Pp (heterozygous) Which pair of alleles is heterozygous? A. RR B. Rr C. rr D. yR Describe: Phenotype = Genotype = Purple flower PP or Pp (if purple is dominant) What is the genotype if it is a recessive trait? How do we predict genotype and phenotype? Punnett Squares Monohybrid Cross (1 trait) Be able to identify both the phenotype and genotype ratios Sample Problem A. B. C. D. In rabbits, gray fur (G) is dominant to black fur (g). If a heterozygous male is crossed with a heterozygous female, what is the phenotypic ratio of the possible offspring? 1:1 1:2:1 2:1 3:1 Monohybrid Cross (1 trait) How do we predict genotype & phenotype for two traits? Use FOIL from math to set up gametes! Dihybrid cross (2 traits) Example: YyRr x YyRr produces the phenotypic ratio of 9:3:3:1. 10.3 Gene Linkage and Polyploidy (5 slides) What is Genetic Recombination? Genetic recombination = the new combination of genes produced by crossing over and independent assortment Combinations of genes in a gamete due to independent assortment can be calculated using the formula 2n where n is the number of chromosome pairs. Pea Plants 27 = 128 combinations Examples of Possible Combinations For pea plants there are 7 pairs of chromosomes The possible combinations for each off-spring are 27 (x) 27 = 16,384 possible combinations For humans there are 23 pairs of chromosomes Since any possible male gamete can fertilize any possible female gamete, then the possible combinations are (x) X = more than 70 trillion (without considering the effects of crossing over) What is Gene Linkage? The linkage of genes on a chromosome results in an exception to Mendel’s law of independent assortment because linked genes usually do not segregate independently. What is polyploidy? Polyploidy is the occurrence of one or more extra sets of all chromosomes in an organism Polyploidy ** Occurs in plants—rarely animals** A triploid organism, for instance, would be designated 3n, which means that it has three complete sets of chromosomes.