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Unit 6 Mendelian Genetics Who is Mendel ? • Gregor Mendel - Mid-nineteenth century Austrian monk who carried out important studies of heredity. • First person to succeed in predicting how traits are transferred from one generation to the next. • “Father of Genetics” What Did He Do? • Experimented with garden peas – reproduce sexually, which means that they produce gametes. What Did He Do? • The male gamete forms in the pollen grain • The female gamete forms in the female reproductive organ. – Pollination – Fertilization – Zygote What Did He Do? • When he wanted to breed, or cross, one plant with another, Mendel opened the petals of a flower and removed the male Remove organs. male parts What Did He Do? • He then dusted the female organ with pollen from the plant he wished to cross it with. Pollen grains Transfer pollen Female Male part parts Cross-pollination What Did He Do? • This process is called cross-pollination. • By using this technique, Mendel could be sure of the parents in his cross. Mendel’s Experiments prefix • Mendel’s first experiments are called monohybrid crosses because mono means “one” and the two parent plants differed from each other by a single trait—height. Mendel’s Experiments • He cross-pollinated this tall pea plant with pollen from a short pea plant. – All of the hybrid offspring grew to be as tall as the taller parent. P1 Short pea plant F1 Tall pea plant Mendel’s Experiments • Mendel allowed the first generation to selfpollinate. – Three-fourths of the plants were as tall as the parent and first generations. P1 Short pea plant F1 Tall pea plant All tall pea plants F2 3 tall: 1 short Mendel’s Experiments • P1 generation = The original parents, the true-breeding plants • F1 generation = The offspring of the parent plants • F2 generation = cross two F1 plants with each other Mendel’s Experiments: The Conclusion • In every case, he found that one trait of a pair seemed to disappear in the F1 generation, only to reappear unchanged in one-fourth of the F2 plants. Mendel’s Experiments: The Conclusion The rule of unit factors • each organism has two factors that control each of its traits. – genes that are located on chromosomes. • Alleles: different forms of a gene Mendel’s Experiments: The Conclusion The rule of unit factors • An organism’s two alleles are located on different copies of a chromosome—one inherited from the female parent and one from the male parent. Mendel’s Experiments: The Conclusion The rule of dominance • Dominant: the observed trait • Recessive: the trait that disappeared Short plant Tall plant • Mendel concluded that the allele for tall plants is dominant to the allele for short plants. t T T F1 t t T All tall plants T t Mendel’s Experiments: The Conclusion The law of segregation • Every individual has two alleles of each gene and when gametes are produced, each gamete receives one of these alleles. • During fertilization, these gametes randomly pair to produce four combinations of alleles. Phenotypes and Genotypes Law of segregation Tt Tt cross • Two organisms can look alike but have different underlying allele combinations. F1 Tall plant Tall plant T T t t F2 Tall T T Tall T t 3 Tall T t Short t 1 t Phenotypes and Genotypes • Phenotype: the way an organism looks and behaves • Genotype: the allele combination an organism contains • An organism’s genotype can’t always be known by its phenotype. Phenotypes and Genotypes • Homozygous: An organism that has two alleles for a trait that are the same. • The true-breeding tall plant that had two alleles for tallness (TT) would be homozygous for the trait of height. Phenotypes and Genotypes • Heterozygous: An organism that has two alleles for a trait that differ from each other. • Therefore, the tall plant that had one allele for tallness and one allele for shortness (Tt) is heterozygous for the trait of height. Determining Genotypes • The genotype of an organism that is homozygous recessive for a trait is obvious to an observer because the recessive trait is expressed. • However, organisms that are either homozygous dominant or heterozygous for a trait controlled by Mendelian inheritance have the same phenotype. Mendel’s Experiments: The Conclusion The law of independent assortment • Mendel’s second law states that genes for different traits—for example, seed shape and seed color—are inherited independently of each other. • This conclusion is known as the law of independent assortment. Can we predict outcomes of offspring?? Yes Punnett Squares • In 1905, Reginald Punnett, an English biologist, devised a shorthand way of finding the expected proportions of possible genotypes in the offspring of a cross. • This method is called a Punnett square. Punnett Squares • If you know the genotypes of the parents, you can use a Punnett square to predict the possible genotypes of their offspring. Monohybrid crosses Heterozygous tall parent T T T t t t T T T t t Heterozygous tall parent t • A Punnett square for this cross is two boxes tall and two boxes wide because each parent can produce two kinds of gametes for this trait. Monohybrid crosses Heterozygous tall parent T T T t t t T T T t t Heterozygous tall parent t • The two kinds of gametes from one parent are listed on top of the square, and the two kinds of gametes from the other parent are listed on the left side. Monohybrid crosses • It doesn’t matter which set of gametes is on top and which is on the side. • Each box is filled in with the gametes above and to the left side of that box. You can see that each box then contains two alleles—one possible genotype. • After the genotypes have been determined, you can determine the phenotypes. Punnett Square of Dihybrid Cross RY RRYY Gametes from RrYy parent Ry rY ry RRYy RrYY RrYy Gametes from RrYy parent RY RRYy RRYy RrYy Rryy RrYY RrYy rrYY rrYy RrYy Rryy rrYy rryy Ry rY ry Dihybrid crosses • A Punnett square for a dihybrid cross will need to be four boxes on each side for a total of 16 boxes. Punnett Square of Dihybrid Cross RY RRYY Gametes from RrYy parent Ry rY ry RRYy RrYY RrYy Dihybrid crosses Gametes from RrYy parent RY RRYy RRYy RrYy Rryy round yellow Ry RrYY RrYy rrYY rrYy rY RrYy ry F1 cross: RrYy ´ RrYy Rryy rrYy rryy round green wrinkled yellow wrinkled green Probability • In reality you don’t get the exact ratio of results shown in the square. • That’s because, in some ways, genetics is like flipping a coin—it follows the rules of chance. • A Punnett square can be used to determine the probability of getting a result Genetics Mendel and Meiosis Meiosis Genes, Chromosomes, and Numbers • Genes do not exist free in the nucleus of a cell; they are lined up on chromosomes. • Typically, a chromosome can contain a thousand or more genes along its length. Diploid and haploid cells • In the body cells of animals and most plants, chromosomes occur in pairs. • Diploid : A cell with two of each kind of chromosome (2n) Diploid and haploid cells • This pairing supports Mendel’s conclusion that organisms have two factors—alleles—for each trait. • Organisms produce gametes that contain one of each kind of chromosome. • Haploid: a cell containing one of each kind of chromosome (n) Homologous chromosomes • Homologous chromosomes: the two chromosomes of each pair in a diploid cell • Each pair of homologous chromosomes has genes for the same traits. Homologous chromosomes • On homologous Homologous Chromosome 4 chromosomes, these genes are arranged in the same a A order, but because there are Terminal Axial different possible alleles for the same gene, the two chromosomes in a Inflated homologous pair are not D d Constricted always identical to each T t other. Tall Short Why meiosis? • When cells divide by mitosis, the new cells have exactly the same number and kind of chromosomes as the original cells. • Each pea plant parent, which has 14 chromosomes, would produce gametes that contained a complete set of 14 chromosomes…. And each mitosis cycle would continue to double the chromosomes……. Why meiosis? • There must be another form of cell division that allows offspring to have the same number of chromosomes as their parents. • Meiosis: a kind of cell division, which produces gametes containing half the number of chromosomes as a parent’s body cell Why meiosis? • Meiosis consists of two separate divisions, known as meiosis I and meiosis II. •The eight phases of meiosis prophase I prophase II metaphase I metaphase II anaphase I anaphase II telophase I telophase II. Why meiosis? • Meiosis I begins with one diploid (2n) cell. • By the end of meiosis II, there are four haploid (n) cells. Sperm: male gametes Eggs: Female gametes ( sperm fertilizes an egg, the resulting zygote is diploid) Why meiosis? Haploid gametes (n=23) • Sexual reproduction: reproduction involving the production and fusion of haploid sex cells Sperm Cell Meiosis Egg Cell Fertilization Diploid zygote (2n=46) Mitosis and Development Multicellular diploid adults (2n=46) The Phases of Meiosis • During meiosis, a spindle forms and the cytoplasm divides in the same ways they do during mitosis. • However, what happens to the chromosomes in meiosis is very different. Prophase I •chromatin coils up into visible chromosomes •The spindles form •Synapsis: the homologous chromosomes line up forming a four part structure called a tetrad •Crossing over may occur – exchange genetic material Metaphase I • Chromosomes become attached to the spindle fibers by their centromeres and tetrads line up on the midline of the cell Anaphase I • Homologous pairs separate, sister chromatids remain attached Telophase I •Chromosomes unwind, spindles break down, cytoplasm divides • Two new diploid cells are formed Prophase II •The spindles form Metaphase II •Chromosomes become attached to the spindle fibers by their centromeres and chromosomes line up on the midline of the cell Anaphase II • The centromere of each chromosome splits and sister chromatids separate and move to opposite poles Telophase II • Chromosomes unwind, spindles break down, cytoplasm divides, nuclei re-form MEIOSIS I MEIOSIS II Possible gametes Possible gametes Chromosome A Chromosome B Chromosome a Chromosome b Meiosis Provides for Genetic Variation • Cells that are formed by mitosis are identical to each other and to the parent cell. • **Crossing over during meiosis, increases the genetic variability due to allele combinations. – Genetic recombination is a major source of variation among organisms Genetic recombination • It is a major source of variation among organisms. MEIOSIS I MEIOSIS II Possible gametes Chromosome A Chromosome B Possible gametes Chromosome a Chromosome b