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I. Pre-Mendelian Views

●Heredity: a) Blending Theory (1800’s): idea that offspring exhibit traits that are a blend of those exhibited by the parents. The

problem with blending theory is that blending would ultimately lead to a single phenotype within a population (natural selection, which relies on variation, would cease to act).

II. Mendelian Revolution ● Gregor Mendel (1858-1866): performed cross-breeding experiments with the garden pea. Mendel selected garden peas for his experiments because: a) Peas are easy to grow & breeding (via self or cross pollination) can be easily controlled. b) Peas produce many offspring (results of the experiments were less likely to be affected by chance). c) Pea plant traits exist in 2 observable, alternate forms (e.g. tall OR short stems, yellow OR green seeds, etc). The

pea plant traits studied by Mendel included: Flower Color: purple or white Flower Position: axial or terminal Seed & Pod Color: green or yellow Seed & Pod Shape: round or wrinkled Stem Length: tall or short

● Prior to starting his hybridization experiments, Mendel took great care in establishing True Breeding lines for each trait he chose to study.

Figure 1: True Breeding Lines (Flower Color)

*Since the plant in the above example is genetically “pure” for the trait of flower color, ALL subsequent generations will exhibit the SAME flower color when allowed to self-fertilize.

●True Breeding: a) F0 /P (Parental Generation): represent the initial true-breeding (genetically pure) individuals that are mated. b) F1 Generation: hybrid offspring of F0 /P generation. c) F2 Generation: offspring of the F1.

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II. Foundations of Heredity Mendel’s Hereditary Factors (Alleles) ● Mendel crossed plants from different true breeding lines (P/F0) with contrasting phenotypes (e.g. true breeding round seeds x true breeding wrinkled). In every case, all members of the F1 Generation exhibited the SAME phenotype & resembled one of the 2 parents (e.g. 100% round seeds).

● The F2 exhibited BOTH parental traits (round & wrinkled seeds) in 3:1 ratio in favor of the dominant phenotype. Figure 3: Early Observations (Interpreting Phenotypic Ratios)

● Mendel hypothesized that, for each trait, there were two alleles controlling it. During reproduction, only ONE of the two alleles from each parent was passed on to offspring. Furthermore, the alleles did not blend, but retained their identity & exhibited Dominance instead. Figure 3.1: Mendel’s Model: Laws of Dominance & Segregation

● As mentioned in fig. 3.1, the term Allele describes alternate forms of a gene found at corresponding points (loci) of homologs. In other words, alleles code for the SAME trait, but DIFFERENT “versions” of the trait. a) The dominant allele is always expressed & is always represented by an uppercase letter. b) The recessive allele is always masked when paired with a dominant allele. Only expressed when paired with another recessive. Always represented by a lowercase letter.

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● As seen in fig. 3.1, the specific combination of alleles for a given trait is known as a Genotype. Specific genotype pairings can be described as: a) Homozygous: genotype in which alleles are identical (e.g. GG –homozygous dominant, gg –homozygous recessive).

b) Heterozygous: alleles in a pair are different. Examples of heterozygous genotypes include:

●The physical expression of a genotype that can be visually observed is known as a Phenotype. Figure 4: Test Crosses

● The genotype of an organism exhibiting the dominant phenotype cannot be determined by direct observation. Thus if a pea plant has a yellow pod phenotype, its genotype may either be GG or Gg.

● In order to determine its genotype, a Test Cross must be performed in which the individual in question is crossed with a known genotype (one exhibiting the recessive phenotype). If ANY of the resulting offspring exhibits the recessive phenotype (green pods), then the individual parent having the dominant phenotype must be heterozygous. III. Genetic Crosses Figure 5: Punnett Square

● Punnett squares are models that are used to determine the probable outcome (genotypes & phenotypes) of a mating.

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Sample Problem 1: Determine the expected phenotypic ratio & genotypic ratio of a cross between a homozygous tall plant & a heterozygous plant.

Phenotypic Ratio: Genotypic Ratio: Sample Problem 2: What percentage of F1 offspring from a cross between a plant having wrinkled seeds & a heterozygote will be expected to show the dominant phenotype?

Expected Outcome: Sample Problem 3: What percentage of F1 offspring from a cross between a plant homozygous for yellow seeds & a plant with green seeds be expected to show the recessive phenotype? What percentage of F2 plants will be expected have green seeds?

F1 Plants: F2 Plants:

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Law of Independent Assortment F0/P: Yellow Pods, Round Seeds (GGRR) x Green Pods, Wrinkled Seeds (ggrr) F1: 100% Yellow Pods, Round Seeds (GgRr) Figure 6: Dependent OR Independent Allele Assortment?

● After conducting the parental cross to obtain the F1 dihybrids, Mendel ask himself the following questions: a) Mendel wondered if the dominant alleles for pod color & seed shape (GR) always packaged together during gamete

production? Likewise, were the recessive alleles for pod color & seed shape (gr) always packaged together during gamete production? Mendel referred to this hypothesis as the Dependent Assortment of alleles.

b) Or, were these alleles packaged randomly during gamete production, a hypothesis he called Independent Assortment of alleles.

● To test these hypotheses, Mendel predicted what he should observe under the circumstances of both dependent & independent assortment: Figure 6.1: Mendel’s Predictions (Gamete Allele Combos)

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F1 Cross: GgRr x GgRr: Expected Phenotype Ratio w/DEPENDENT ASSORTMENT ● Expected Phenotypic Ratio: F1 Cross: GgRr x GgRr: Expected Phenotype Ratio w/INDEPENDENT ASSORTMENT

● Expected Phenotypic Ratio: a) The observed phenotypic ratio obtained by Mendel most closely matched that which was expected if the alleles for

the traits of pod color & seed shape assorted INDEPENDENTLY. IV. Non-Mendelian Inheritance Figure 7: Incomplete Dominance: Japanese Snapdragon Flower Color Alleles

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●Incomplete Dominance:

Figure 7.1: Codominance: Cattle Coat Color

●Codominance:

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Figure 8: Multiple Alleles: Human Blood Type Alleles

Sample Problem 1: A man with blood type AB marries a woman with type O blood. They have two natural children & one adopted child. The children’s blood types are A, B, & O. Which child was adopted?

Sample Problem 2: A couple has 4 children. The 1st has blood type A, the 2nd, blood type O, the 3rd blood type B, & the 4th blood type AB. What are the genotypes & phenotypes of the parents?

Sample Problem 3: A woman with type A blood is claiming that a man with type AB blood is the father of her child, who also has type AB blood. Could this man be the father of this child?

● Multiple Alleles:

Adopted Child:

Parental Genotypes: Parental Phenotypes:

Answer:

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Figure 9: Polygenic Traits: Human Height

Figure 9.1: Polygenic Traits: Human Height Distribution

*Any polygenic trait can be identified if it varies continuously across a population. Thus if plotted as a function of its frequency in the population, the pattern produced will be that of a bell curve.

● Polygenic Inheritance: a) a) Any trait that cannot be classified as “either-or” & varies infinitely throughout a population is most likely

polygenic (height, skin tones, etc).