Heredity

  1. Mendel’s Insight Into Patterns of Inheritance
    1. Inheritance has always been intriguing to humans.
      1. "Blood" theory… Tribal belief or heredity.
      2. "Pangenes" and other theories attempted to examine the mystery of heredity scientifically.
      3. By the late nineteenth century, natural selection suggested that a population could evolve if members showed variation in heritable traits. Variations that improved survival chances would be more common in each generation–in time, the population would change or evolve.
      4. The theory of natural selection did not fit with the prevailing view of inheritance–blending.
        1. Blending would produce uniform populations–such populations could not evolve.
        2. Many observations did not fit blending–for example, a white horse and a black horse did not produce only gray offspring.
    2. Mendel’s Experimental Approach
      1. Gregor Mendel used experiments in plant breeding and knowledge of mathematics to form his hypotheses.
      2. Mendel used the garden pea in his experiments.
        1. This plant can fertilize itself; true-breeding varieties were available to Mendel.
        2. Peas can also be cross-fertilized by human manipulation of the pollen.
      3. Mendel cross-fertilized true-breeding garden pea plants having clearly contrasting traits (example: white vs. purple flowers).

    3. Some Terms Used in Genetics
      1. Genes are units of information about specific traits.
      2. Each gene has a locus on a chromosome.
      3. Alleles are various molecular forms of a gene for the same trait.
      4. True-breeding lineage occurs when offspring inherit identical alleles, generation after generation; non-identical alleles produce hybrid offspring.
      5. When both alleles are the same, the condition is called the homozygous condition; if the alleles differ, then it is the heterozygous condition.
      6. When heterozygous, one allele is dominant (A), the other is recessive (a).
      7. Homozygous dominant = AA, homozygous recessive = aa, and heterozygous = Aa.
      8. Genotype is the molecular source of the traits or the "genes", and phenotype is the physical appearance caused by the genes.
      9. P = parental generation; F1 = first-generation offspring; F2 = second-generation offspring.
  2. Monohybrid Crosses

   TT x tt  (Homozygous Tall x Homozygous short)

Punnett Square T T
 

t

 

Tt Tt
 

t

 

Tt Tt

 

  1.   Dominance Relations
    1. Codominance

  2.   Multiple Effects of Single Genes
    1. Sometimes the expression of alleles at one location can have effects on two or more traits; this is termed pleiotropy (sickle-cell anemia).

  3.    Interactions Between Gene Pairs
    1. One gene pair can influence other gene pairs, with their combined activities producing some effect on phenotype; this called epistasis.

  4. .   Examples of Environmental Effects on Phenotype
    1. Fur on the extremities of certain animals will be darker because the enzyme for melanin production will operate at cooler temperatures but is sensitive to heat on the rest of the body.