Unit 6 Mendelian Genetics

Section 10.1 Summary – pages 253-262 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” Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 What Did He Do? Experimented with garden peas reproduce sexually, which means that they produce gametes. Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 What Did He Do? The male gamete forms in the pollen grain The female gamete forms in the female reproductive organ. Pollination Fertilization Zygote Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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 organs. Remove male parts Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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 part Male parts Cross-pollination Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 What Did He Do? This process is called cross-pollination. By using this technique, Mendel could be sure of the parents in his cross. Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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. Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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 Tall pea plant F1 Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 Mendel’s Experiments Mendel allowed the first generation to self-pollinate. Three-fourths of the plants were as tall as the parent and first generations. P1 Short pea plant Tall pea plant F1 All tall pea plants F2 3 tall: 1 short Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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 Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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. Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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 Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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. Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 Mendel’s Experiments: The Conclusion The rule of dominance Dominant: the observed trait Recessive: the trait that disappeared Short plant Tall plant T T t t Mendel concluded that the allele for tall plants is dominant to the allele for short plants. T t F1 All tall plants T t Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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. Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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 Tall Tall Short T T T t T t t t 3 1 Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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. Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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. Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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. Section 10.1 Summary – pages 253-262

Section 13.1 Summary – pages 337 - 340 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. Section 13.1 Summary – pages 337 - 340

Section 10.1 Summary – pages 253-262 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. Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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. Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 Punnett Squares If you know the genotypes of the parents, you can use a Punnett square to predict the possible genotypes of their offspring. Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 Monohybrid crosses 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. Heterozygous tall parent T t T t T t T T t t T t Heterozygous tall parent Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 Monohybrid crosses 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. Heterozygous tall parent T t T t T t T T t t T t Heterozygous tall parent Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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. Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 Punnett Square of Dihybrid Cross Dihybrid crosses Gametes from RrYy parent RY Ry rY ry RRYY RRYy RrYY RrYy A Punnett square for a dihybrid cross will need to be four boxes on each side for a total of 16 boxes. RY RRYy RRYy RrYy Rryy Ry Gametes from RrYy parent RrYY RrYy rrYY rrYy rY RrYy Rryy rrYy rryy ry Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 Punnett Square of Dihybrid Cross Dihybrid crosses Gametes from RrYy parent RY Ry rY ry RRYY RRYy RrYY RrYy RY F1 cross: RrYy ´ RrYy RRYy RRYy RrYy Rryy Ry round yellow Gametes from RrYy parent RrYY RrYy rrYY rrYy round green rY wrinkled yellow RrYy Rryy rrYy rryy ry wrinkled green Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262 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 Section 10.1 Summary – pages 253-262

Genetics Mendel and Meiosis Meiosis Unit Overview – pages 250-251

Section 10.2 Summary – pages 263-273 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. Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 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) Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 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) Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 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. Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 Homologous chromosomes On homologous chromosomes, these genes are arranged in the same order, but because there are different possible alleles for the same gene, the two chromosomes in a homologous pair are not always identical to each other. Homologous Chromosome 4 a A Terminal Axial Inflated D d Constricted T t Short Tall Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 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……. Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 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 Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 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. Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 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) Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 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) Multicellular diploid adults (2n=46) Mitosis and Development Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 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. Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 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 Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 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 Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 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 Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 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 Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 MEIOSIS I MEIOSIS II Possible gametes Possible gametes Chromosome b Chromosome A Chromosome B Chromosome a Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 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 Section 10.2 Summary – pages 263-273

Section 10.2 Summary – pages 263-273 Genetic recombination It is a major source of variation among organisms. MEIOSIS I MEIOSIS II Possible gametes Possible gametes Chromosome A Chromosome B Chromosome a Chromosome b Section 10.2 Summary – pages 263-273