chapter 14

Bio1151 Chapter 14 9

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.
  continue
- 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.
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
  - Testcross.
    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
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.
    Exercise:
- 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.
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 )