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CHAPTER 10.1 MEIOSIS Chromosomes & Chromosome Number Meiosis The Human body has 46 chromosomes The reproductive cells (Egg and Sperm) have 23 chromosomes So each parent contributes 23 chromosomes to = the 46 chromosomes we need The process of reducing the number of chromosomes from 46 to 23 is called Meiosis Homologous chromosomes: are a pair of chromosomes. The 46 chromosomes create 23 pairs of homologous chromosomes: 1. same length 2. same centromere position 3. Carry genes that control Inherited traits Homologous chromosomes: are a pair of chromosomes. The 46 chromosomes create 23 pairs of homologous chromosomes: 1. same length 2. same centromere position 3. Carry genes that control Inherited traits An organism produces gametes to maintain the same number of chromosomes from generation to generation Human gametes contain 23 chromosomes. Gametes are sperm or egg. A cell that contains 2n (46) chromosomes is called a diploid cell 23 Chromosomes -haploid (n) 46 Chromosomes -diploid (2n) Meiosis Reduces the chromosome number by half through the separation of homologous chromosomes Involves 2 consecutive cell divisions: 1. Meiosis I & 2. Meiosis II Meiosis 1 INTERPHASE Chromosomes replicate. PROPHASE 1 Homologous chromosomes pair up Each chromosome consists of two chromatids The nuclear envelope breaks down Spindle forms Metaphase 1 Chromosome centromeres attach to spindle fibers. Homologous chromosomes line up at the equator. Anaphase 1 Homologous chromosomes separate and move to opposite poles of the cell Telophase 1 The spindle breaks down The cell divides Meiosis II Prophase II A second set of phases begin as the spindle apparatus forms and the chromosomes condense. Metaphase II A haploid number of chromosomes line up at the equator Anaphase II The sister chromatids are pulled apart at the centromere by spindle fibers and move toward the opposite poles of the cell Telophase II The chromosomes reach the poles, and the nuclear membane and nuclei reform Cytokinesis: results in four haploid cells, each with n number of chromosomes. Meiosis Results in: Produces 4 haploid daughter cells that are NOT identical Skim Read: pg. 277-282 Homologous chromosomes: are a pair of chromosomes. The 46 chromosomes create 23 pairs of homologous chromosomes: 1. same length 2. same centromere position 3. Carry genes that control Inherited traits Meiosis Provides Variation Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result. Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine. In Prophase I: Crossing over produces exchange of genetic information. Crossing over—chromosomal segments are exchanged between a pair of homologous chromosomes. Genetic Variation increases a species ability to survive and adapt Meiosis = 4 Different Gametes Crossing over increases the Genetic Variation of the gametes Sexual Reproduction: Two different sets of genes are creating a new individual. One from MOM and the other from DAD Unlike asexual reproduction, where the organism inherits all of its chromosomes from a single parent (genetically identical to parent.) How Genetics Began: Gregor Mendel, and Austrian monk and plant breeder He studied how traits are passed on in pea plants Flowering plants are often able to self- fertilize and cross-pollination. Cross- pollination is reproduction between 2 plants. For pea plants Mendel had to do this by hand. The passing of traits to the next generation is called inheritance, or heredity. Mendel started the branch of science we call Geneticsthe science of heredity In Mendel’s Experiments: He studied seven different traits. Pea plants often form offspring with a specific trait such as yellow seeds. He cross-pollinated pea plants with different traits to see what the outcome would be……. The seven traits he looked at are Seed or pea color Flower color Seed pod color Seed shape or texture Seed pod shape Stem length Flower position Mendel’s Experiments with True – breeding pea plants The parent generation is known as the P generation. The offspring of this P cross are called the F1 generation. The offspring of the F1 generation are called the F2 generation. True- breeding pea plants that are cross- pollinated Mendel concluded from his experiments: That there must be two forms of a each trait in the pea plants. Such as yellow seed color and green seed color. Those two forms are controlled by alleles. Alleles are an alternative form of a single gene passed from generation to generation Alleles are found on chromosomes • Alleles One of the alleles is dominate over the other. Dominate traits mask recessive traits What is the Dominate trait in this cross? The dominate trait is yellow seed color. In the F1 generation the only seed color you see is yellow. Showing you that it is dominate over the green seed color. Dominate traits are written using a Capitol Letter Recessive traits are written using a lower case letter The dominate yellow seed would be represented by Y The recessive green seed would be represented by a y Alleles on chromosomes can have: . the same alleles for a particular trait is called homozygous. • OR 2. two different alleles for a particular trait is called heterozygous. 1 The Alleles on the chromosome determine the: Genotype: what an organisms allele types are (the genes) ex.YY ,Yy ,yy Phenotype: what an organism looks like Mendel’s Law of Segregation Two alleles for each trait separate during meiosis. During fertilization, two alleles for that trait unite. • Heterozygous organisms are called hybrids.Yy Monohybrid Cross A cross that involves hybrids for a single trait is called a monohybrid cross. In this example the Single trait is seed color. DIHYBRID CROSS Predicting the inheritance of two or more traits in the same cross. EXAMPLE: Looking at seed color and Seed Color- Yellow Seed type Y Round R YyRr (genotype) Yellow seed & round (phenotype) seed type. green y wrinkled r X yyRR (genotype) Green seed & round (phenotype) Law of Independent Assortment Random distribution of alleles occurs during gamete formation Genes on separate chromosomes sort independently during meiosis Each allele combination is equally likely to occur. Punnett Squares – Monohybrid Are used to predict the possible offspring of a cross between two known genotypes Punnett Square—Dihybrid Cross Four types of alleles from the male gametes and four types of alleles from the female gametes can be produced. The resulting phenotypic ratio is 9:3:3:1. Probability Each time you perform a cross you have the same probability of each outcome occurring. Like flipping a coin- You have a 50% chance of getting heads or tails each time you flip a coin. What you flipped before also does not matter. CHAPTER 10.3 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. Polyploidy is the occurrence of one or more extra sets of all chromosomes in an organism A triploid organism, For instance, would be designated 3n, which means that it has three complete sets of chromosomes