BackMendelian Genetics: Principles of Heredity and Experimental Foundations
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Basic Principles of Heredity
Subtopic 3.1 – Mendel and the Monohybrid Cross
The foundation of classical genetics was established by Gregor Mendel through his experiments with the garden pea (Pisum sativum). Mendel's work demonstrated how hereditary characteristics are transmitted from one generation to the next, using controlled crosses and careful counting of offspring phenotypes.
Selective Breeding: Mendel used selective breeding to study the inheritance of specific traits in pea plants.
Model Organism: The garden pea was chosen for its easily distinguishable traits and ability to self- or cross-pollinate.
Key Traits Studied: Mendel examined seven characteristics, each with two contrasting forms (e.g., seed shape: round vs. wrinkled; flower color: purple vs. white).
Experimental Approach: Mendel tested hypotheses by counting thousands of plants over ten years, ensuring statistical reliability.
Mendel’s Experimental Design
Mendel’s classic monohybrid cross involved two true-breeding parental strains differing in one trait. The process involved crossing these strains, observing the first filial (F1) generation, and then allowing F1 individuals to self-fertilize to produce the second filial (F2) generation.
P Generation: True-breeding parents (e.g., purple-flowered × white-flowered).
F1 Generation: All offspring display only one parental trait (dominant).
F2 Generation: Offspring show both parental traits in a 3:1 ratio (dominant:recessive).
Key Observations: Traits do not blend; reciprocal crosses yield similar results.
Mendel’s Hypotheses and Genetic Terminology
Mendel proposed that each plant carries two factors (now called alleles) for each trait. One allele may be dominant over the other, and alleles segregate during gamete formation.
Genotype: The genetic constitution (e.g., PP, Pp, pp).
Phenotype: The observable trait (e.g., purple or white flowers).
Dominant Allele: Expressed in the heterozygote (e.g., P for purple).
Recessive Allele: Masked in the heterozygote (e.g., p for white).
Punnett Squares and Predicting Offspring
Punnett squares are used to predict the genotypic and phenotypic ratios of offspring from genetic crosses. In a monohybrid cross, the F2 generation shows a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio.
F1 Genotypes: All heterozygous (Pp).
F2 Genotypes: 1 PP : 2 Pp : 1 pp.
F2 Phenotypes: 3 purple : 1 white.
Testcrosses
A testcross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. The resulting offspring phenotypes reveal the unknown genotype.
Example: Black (dominant, B) vs. chocolate (recessive, b) coat color in dogs. A black dog (B–) crossed with a chocolate dog (bb) can reveal if the black dog is BB or Bb based on offspring ratios.
Subtopic 3.2 – Mendel’s Principle of Segregation
The principle of segregation states that alleles for a trait separate during gamete formation, ensuring that each gamete receives only one allele. This explains the reappearance of recessive traits in the F2 generation.
Mechanism: Separation of homologous chromosomes during meiosis.
Consequence: Heterozygotes produce gametes with either allele, leading to segregation of traits in progeny.
Mendel’s First Law: There is a one-generation lag before the phenotypic consequences of segregation are observed.
Genetic Symbols: Different organisms use different conventions (e.g., uppercase for dominant, lowercase for recessive; wild-type vs. mutant notation in Drosophila).
Subtopic 3.3 – Mendel’s Principle of Independent Assortment
The principle of independent assortment states that alleles of different genes assort independently during gamete formation. This was demonstrated by Mendel’s dihybrid crosses, which produced a 9:3:3:1 phenotypic ratio in the F2 generation.
Dihybrid Cross: Cross between individuals differing in two traits (e.g., seed color and seed shape).
F1 Gametes: Four possible combinations (e.g., YR, Yr, yR, yr).
F2 Phenotypic Ratio: 9:3:3:1 for the four possible phenotypes.
Mechanism: Independent assortment of chromosome pairs during meiosis.
Number of Gamete Types: , where n is the number of homologous chromosome pairs.
Subtopic 3.4 – Chi-square Analysis
Chi-square analysis is used to determine whether observed genetic ratios deviate significantly from expected ratios due to chance. The test compares observed and expected counts and calculates a probability value (P).
Chi-square Formula:
Degrees of Freedom (df): , where n is the number of expected phenotypes.
Interpretation: If , differences are likely due to chance; if , differences are significant.
Example Calculation: For 69 purple and 31 white flowers (expected 75 and 25, respectively):
With 1 degree of freedom, ; thus, the observed ratio does not significantly deviate from the expected 3:1 ratio.
Subtopic 3.5 – Using Pedigrees to Study Human Inheritance
Pedigree analysis is used to study inheritance patterns in humans, where controlled crosses are not possible. Pedigrees use standardized symbols to represent individuals and relationships, allowing geneticists to infer modes of inheritance.
Complications in Humans: No true-breeding lines, long generation times, small family sizes, and ethical constraints.
Pedigree Analysis: Distinguishes between dominant and recessive traits, homozygous and heterozygous individuals, and autosomal vs. sex-linked inheritance.
Autosomal Dominant Traits: Trait appears in every generation; affected individuals have at least one affected parent (e.g., Huntington’s disease).
Autosomal Recessive Traits: Trait may skip generations; affected individuals can have unaffected carrier parents (e.g., cystic fibrosis, albinism).
Trait Type | Pattern in Pedigree | Examples |
|---|---|---|
Autosomal Dominant | Appears in every generation | Huntington’s disease, Achondroplasia |
Autosomal Recessive | May skip generations | Cystic fibrosis, Albinism |
