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Ch. 10 – Sexual Reproduction and Genetics Meiosis provides variation! Meiosis – type of cell division that produces haploid cells (gametes). Meiosis produces cells that reduces the number of chromosomes by half. Meiosis occurs in the reproductive structures of organisms that reproduce sexually. haploid cells (n) – have half the number of chromosomes diploid cells (2n) – formed after fertilization. Meiosis produces 4 daughter cells that are not identical (How does this compare with mitosis?) Meiosis involves two consecutive cell divisions Genetic Recombination – 2 ways meiosis gives us variability in gametes: 1. Independent assortment – genes for traits may come from either parent due to random assortment during meiosis. 2. Crossing over- homologous chromosomes and their sister chromatids are so closely bound together that an exchange of genetic material takes place. See diagram pg. 272. This results in new combinations of alleles on chromosomes. Humans have 46 chromosomes (or 23 pairs) numbered 1 – 23. Autosomes – chromosomes #1 – 22 Sex Chromosomes – 23rd pair of chromosomes in humans determine the individuals sex. If 23rd pair of chromosomes is: XX = female (Females can only produce gametes with X chromosomes) XY = male (Males can produce gametes with either X or Y chromosomes) Homologous chromosomes – contain DNA that codes for the same genes. The organism gets one chromosome from each parent. Homologous chromosomes are not identical!! Problems that can occur during meiosis: polyploidy – more than the usual number of chromosome sets. Rare in animals because it almost always causes death. Frequent in plants causing larger and healthier flowers and fruits. Wheat (6n), sugar (8n) Triploid (3n) Asexual reproduction – the offspring inherits all of its chromosomes from a single parent. Genetically identical to parent (i.e. bacteria, plants can reproduce asexually and sexually) Heredity – the passing of characteristics from parents to offspring Traits – characteristics that are inherited Genes – segments on DNA that control production of proteins which determine characteristics of an individual. Genetics – the study of heredity Gametes – sex cells (sperm and egg). These cells are haploid (n) produced by meiosis. Zygote – fertilized cell Fertilization – process of combining n cells from female parent with n cells from male parent to produce 2n offspring. What about HUMANS? Genetic variation in humans: One human (with 23 chromosomes from each parent) can produce 8 million different gametes, or 70 trillion different zygotes! Gene linkage – genes that are located close to each other on a chromosome are said to be linked and usually travel together during gamete formation. Mendelian Genetics Gregor Mendel – Austrian monk who cross-pollinated pea plants to study genetics. He crossed tall pea plants with short pea plants. The result of these crosses is called a hybrid. Mendel concluded that each organism has two factors that control each of its traits. We now know these are called genes and are located on chromosomes. These different forms for the same gene are called alleles. Allele – The alternative form of a single gene that can be passed down. Most genes have 2 alleles – a dominant and a recessive. For example a red flowered plant (red is dominant) may have a red allele and a white allele. The Rule of Dominance In the F1 generation only one trait was observed – this is the dominant trait. The trait that was not observed in the F1 generation is the recessive trait. The dominant trait is shown with a capital letter (T for tall). The recessive trait is shown with a lower case letter (t for short). The same letter is used for the same trait. P1 – “parent” generation F1 “filial” generation results from crossing plants in P1 generation. F2 – second filial generation – results from crossing plants in F1 generation. The Law of Segregation The two alleles for each trait separate during meiosis During fertilization the two alleles for that trait unite Phenotype – the way an organism looks and behaves (tall). Genotype – the allele combination that an organism contains (TT or Tt) Homozygous – when an organism’s alleles are the same (TT or tt). Short plants are always homozygous. Heterozygous – when an organism has two different alleles (Tt). Also known as a hybrid. 1905 – Reginald Punnett (English biologist) came up with a shorthand way to represent the expected proportions of possible genotypes in offspring – the Punnett Square. At first, Mendel performed many different crosses for several different traits but he only tested one trait at a time: Dihybrid cross - crosses of plants with two different traits. Mendel concluded that genes for different traits are inherited independent of each other, this is known as the Law of Independent Assortment: Random distribution of alleles during gamete formation Punnett Square for a dihybrid cross: Do guinea pig worksheet example together. Long fur is dominant o LL or Ll = long hair (dominant) o ll = short hair (recessive) Black fur is dominant o BB or Bb = black (dominant) o bb = white (recessive) Third Problem: BbLl X bbLl 1st: Make 4 possible gametes for each parent: BbLl: bbLl: 2nd: Draw a table with 16 squares 3rd: Put gametes along the top and side of square 4th: Fill in squares