Bio1151 Chapter 14 Mendel and the Gene Idea
  1. Gregor Mendel studied the              of several characters with peas.

    The garden pea has closed flowers and can self-fertilize, but also allow for manual cross-pollination.

    Mendel studied inheritance of a single character (or trait, such as flower color) by cross-pollinating two true-breeding varieties of the pea.

    This is called a monohybrid cross. When pollen from a white flower fertilizes eggs of a purple flower, the first-generation hybrids (F[1]) all have purple flowers.


      The purple-flowered F[1] hybrids from a monohybrid cross were allowed to self-pollinate.

      Both purple-flowered plants and white-flowered plants appeared in the F[2] generation, in a ratio of about 3 purple : 1 white.

      Similar ratios were observed in other pea characters (traits).

      Mendel called the purple flower trait dominant, and the white flower color trait recessive.

  2. Mendel proposed "heritable          " (now known to be genes) to explain his results based on two principles of inheritance.

    An observable trait, such as flower color, is inherited in units of DNA called genes. Alternative versions of a gene are called alleles. An organism's total collection of genes is its genome.

      For each observable trait (phenotype), an organism inherits 2 alleles, one from each parent. These alleles make up the organism's genotype. Chromosomes that have the same genes (but possibly different alleles) are called homologous chromosomes. If the 2 alleles at a locus (the region on a chromosome where a gene is found) are the same, the organism is homozygous, otherwise it is heterozygous, and the dominant allele determines the organism's phenotype.
    1. Law of              .
      • The           allele determines a heterozygous individual's trait, or            .

        Phenotype and genotype.

        A monohybrid cross yields a 3:1 phenotypic ratio in the F[2], if purple flower color is dominant and white is recessive.

        The genotypic ratio is 1:2:1, since there are 2 types of purple-flowered plants: PP (homozygous) and Pp (heterozygous).

        The true-breeding P generation must have identical alleles for that gene and are homozygous.

        In the heterozygous F[1] and F[2] individuals, the dominant purple allele determines the phenotype.

      • The phenotype ratio of the offspring can be calculated from a              model called the          square.

        Law of segregation.

        Each plant inherits 1 allele for flower color from each parent.

        The 2 alleles segregate (separate) and end up in different gametes during meiosis.

        Random fertilization yields predictable offspring ratios: 3:1 phenotypic ratio, 1:2:1 genotypic ratio.

        These ratios can be modeled by a Punnett square: list all the possible female gametes along one side and all the possible male gametes along the other side, then combine the gametes to produce offspring.

          Random fertilization.

          When a heterozygote (Rr) forms gametes, segregation of alleles is like the toss of a coin.

          We can determine the probability for any genotype among the offspring of 2 heterozygotes by multiplying the individual probabilities of a gamete having a particular allele (R or r).

      • A testcross can be used to determine the           of an organism with the dominant phenotype.
        Monohybrid Cross activity


      An organism exhibiting a dominant trait (purple flowers). can be either homozygous dominant or heterozygous.

      To determine its genotype, the individual with the dominant phenotype is crossed with a recessive phenotype (white flowers), since the latter's genotype has to be homozygous recessive. By observing the phenotypes of the offspring, we can deduce the genotype of the purple-flowered parent.

    2. Law of Independent             .
      • Crossing true-breeding parents differing in two traits produces            in the F1 generation heterozygous for both traits, and       phenotypes in the F2 generation.
        Dihybrid Cross activity

    Law of independent assortment.

    The P plants of this dihybrid cross are true-breeding: one with yellow-round seeds and the other with green-wrinkled seeds.

    The F[1] dihybrids are heterozygous for both characters.

    Self-pollination of the F[1] yields a phenotypic ratio of 9:3:3:1 in the F[2], NOT the 3:1 typical of a monohybrid cross.

    This id due to the independent assortment of the 2 traits during meiosis, assuming they are on different chromosomes.

    One way to ensure you have all combinations of gametes for a dihybrid cross is the FOIL method.

      Using FOIL to multiply two binomial expressions.
    Review: Mendelian genetics activity
  3. Extending Mendelian genetics.
    • In             dominance, the phenotype of                is somewhere between the phenotypes of homozygotes.

      Incomplete dominance.

      When red snapdragons are crossed with white ones, the F[1] hybrids have pink flowers.

      Superscripts indicate alleles for flower color: C^R for red and C^W for white.

      The F[2] generation produces a 1:2:1 ratio for both genotype and phenotype.


    • In              , two dominant alleles affect the phenotype.

      Codominance in ABO blood group.

      The A, B, AB, or O phenotypes are affected by 3 different alleles.

      I^A and I^B alleles produce different antigens on the surface of red blood cells, thus are dominant to the i allele which produces no antigen.

      I^A and I^B are codominant to each other because the RBCs bear both antigens.

    • Some traits exhibit            inheritance which often shows               variation.

      Human skin pigmentation is influenced by multiple genes which produce different melanin pigment molecules and shows quantitative variation.

      This polygenic inheritance also exhibits incomplete dominance.

    • In            , a gene at one locus may alter the phenotypic expression of a gene at a second locus.

    A gene at one locus may affect phenotypic expression of a gene at another locus by epistasis.

    The B/b gene determines the pigment color (B for black and b for brown)

    The epistatic C/c gene controls whether or not any pigment will be deposited in the hair.

    A homozygous recessive cc mouse has no hair pigment and is albino regardless of its B/b genotype.

  4. Inheritance in human families can be studied by analyzing a           showing the inheritance of alleles across generations.

    Pedigree: dominant trait.

    In these family trees, square symbols represent males and circles represent females.

    Shaded symbols represent individuals who exhibit the trait.

    A dominant trait such as widow's peak cannot skip a generation.

    A recessive trait may skip a generation.

      Pedigree: recessive trait.

      A recessive trait such as attached earlobe may skip a generation.

      A dot may be placed in a symbol to represent known heterozygotes (carriers who do not exhibit the recessive phenotype).

      Identify the carriers. ( Hint )