Study Notes on Basic Principles of Heredity

Basic Principles of Heredity

Table of Contents

  • Mendel: Why Pea Plants?

  • Important Genetic Terminology

  • Mendel’s Monohybrid Cross

  • Interpretation of Monohybrid Cross Results

  • Testcross

  • Dihybrid Cross

  • Learning Outcomes

Mendel: Why Pea Plants?

  • Gregor Mendel discovered the basic principles of heredity.

  • Reasons for choosing pea plants:

    • Easy to cultivate: Pea plants are relatively straightforward to grow.

    • Short growing cycle: They can grow quickly, allowing for rapid experimentation.

    • Many offspring produced from matings: This allowed Mendel to gather statistically meaningful data.

    • Many different characteristics can be studied: Pea plants have various characteristics that can easily be observed.

Important Genetic Terminology

  • Gene: Inherited “factor” that determines a characteristic.

  • Locus: Position of a gene on a chromosome.

  • Alleles: Alternate forms of a gene. For example, the gene for seed shape in peas exists as two alleles:

    • Round (R)

    • Wrinkled (r)

  • Alleles are usually represented by italics. In summary:

    • Two alleles exist at a gene locus.

  • Genotype: The combination of alleles possessed by an organism at a gene locus (e.g., genotype for seed shape locus is Rr).

    • Heterozygous: Having two different alleles at a gene locus (e.g., Rr).

    • Homozygous: Having identical alleles at a gene locus (e.g., RR, rr).

  • Characteristic (or character): Any general feature of an organism, which can be physical, physiological, biochemical, or behavioral (e.g., seed shape in peas).

  • Phenotype (or trait): The appearance or manifestation of a characteristic (e.g., round vs. wrinkled seeds).

  • Genetically pure (or pure breeding or true breeding): Homozygous for the trait under study (all alleles are the same).

    • For detailed definitions, refer to Table 3.1.

Mendel’s Monohybrid Cross

  • Monohybrid cross: A cross (mating) between parents that differ in phenotypes for only a single characteristic (e.g., round seeds vs. wrinkled seeds).

  • Parents were genetically pure—Mendel prevented self-fertilization of parental pea plants.

How Did Mendel Prevent Self-Fertilization?

  • Experiment Steps:

    1. Removed the anthers from flowers to prevent self-fertilization.

    2. Dusted the stigma with pollen from a different plant.

    3. The pollen fertilized ova, resulting in the development of seeds.

    4. These seeds grew into plants.

Result of the Cross:

  • P Generation:

    • Homozygous round seeds crossed with homozygous wrinkled seeds.

    • F₁ Generation (First filial generation): All seeds produced were round.

  • Allowed F₁ plants to self-fertilize to produce F₂ generation.

Interpretation of Monohybrid Cross Results

  • All F₁ plants express the phenotype of one parent; however, F₁ must have inherited genetic factors for phenotypes of both parents to express both traits in F₂.

  • Genetic Factors (Alleles):

    • Allele for round seeds: R

    • Allele for wrinkled seeds: r

Gamete Genotype Notation:

  • Gamete genotypes correspond to the phenotypes of the parent from which they derived.

F₂ Generation Results:

  • Phenotype Results:

    • Round: (Phenotypic ratio) 3:1 (¾ round and ¼ wrinkled seeds)

    • Genotype Results: Ratio of genotypes for F₂:

    • RR: 1

    • Rr: 2

    • rr: 1

    • Genotype Ratio: 1:2:1

Principle of Segregation:

  • A diploid organism possesses 2 alleles for any particular characteristic. These alleles segregate during gamete formation:

    • Each allele goes into a separate gamete.

    • The two alleles segregate into gametes in equal proportions.

Testcross

  • Used to determine the genotype of an organism exhibiting a dominant phenotype (e.g. tall plants).

    • Procedure: Cross a plant with a known phenotype (recessive) against a plant exhibiting the dominant phenotype but with an unknown genotype.

Example of Testcross:

  • P Generation: Tall pea plant (unknown genotype) crossed with a short pea plant (tt).

  • By examining offspring's phenotypic ratios, you can deduce the genotype of the tall parent.

Dihybrid Cross

  • A dihybrid cross involves matings between pea plants differing in two characteristics (each encoded by genes on different chromosomes):

    • E.g., Seed Shape: Round (R), Wrinkled (r) and Seed Color: Yellow (Y), Green (y).

Gametes in Dihybrid Cross:

  • F₁ generation: Round yellow seeds, termed “double heterozygote” (RrYy).

  • Independent assortment during Anaphase I of Meiosis allows for various combinations of alleles in gametes.

Results of Dihybrid Cross:

  • Phenotypic Proportions in F₂ Generation:

    • Round Yellow (R-Y-) = 9/16

    • Round Green (R-yy) = 3/16

    • Wrinkled Yellow (rrY-) = 3/16

    • Wrinkled Green (rryy) = 1/16

  • Principle of Independent Assortment states:

    • Alleles at one locus separate independently of other loci during gamete formation.

Learning Outcomes

  • Be able to explain relevant terminology: blending inheritance, gene, allele, genotype, homozygous, heterozygous, characteristic, phenotype, genetically pure, dominant phenotype, recessive phenotype, reciprocal cross, backcross, testcross.

  • Work out simple genetic crosses based on dominance/recessiveness using Punnett squares or otherwise.

  • Understand the purpose of backcross, testcross, and reciprocal cross.

  • Calculate expected ratios and proportions in genetic crosses (both monohybrid and dihybrid).

  • Deduce genotypes of organisms expressing dominant phenotypes using testcrosses.

  • Apply dash notation correctly in genetic crosses.

  • Explain the chromosomal basis of inheritance and the relationship to meiosis.

  • Understand the concept of dominance and the principles of segregation and independent assortment, including molecular basis of allele determination of phenotypes.