Mendel and the Gene Idea

Chapter 14: Mendel and the Gene Idea

Introduction to Mendel's Principles of Heredity

  • Key Figure: Gregor Mendel, known as the father of genetics.

  • Methodology: Utilized garden peas in controlled breeding experiments to discover principles of heredity.

  • Aim: Understanding how traits are transmitted from parents to offspring.

Basic Concepts and Definitions

  • Character (Gene): A heritable feature that varies among individuals (e.g. flower color).

  • Trait (Allele): Each variant for a character (e.g. purple or white flowers).

  • Alleles: Variants of a gene that occupy a specific locus on chromosomes.

Mendel's Experimental Approach

  • True Breeding Varieties: Started with true-breeding plants which produce offspring of the same variety when self-pollinated.

  • Peas Advantages:

    • Short generation time.

    • Large offspring numbers.

    • Controlled mating: self-pollination or cross-pollination possible.

Mendel's Hybridization Process

  1. Parental Generation (P): Two contrasting true-breeding varieties are mated.

  2. First Filial Generation (F1): Hybrid offspring produced from the P generation.

  3. Second Filial Generation (F2): Resulted by self-pollination or cross-pollination of F1 individuals.

  4. Hybridization Process:

    • Example: Mating purple-flowered (PP) and white-flowered (pp) plants.

    • Resulting F1 generation all had purple flowers.

    • F2 generation exhibited a ratio of 705 purple-flowered to 224 white-flowered plants.

Mendel's Observations on Dominance

  • Dominant Trait: The purple flower color in F1 hybrids, which masks the presence of white.

  • Recessive Trait: The white flower color, which reappears in the F2 generation, indicating it was not destroyed.

  • Conclusion: Dominant traits mask the expression of recessive traits when both are present.

Mendel's Models of Inheritance

The Law of Segregation

  1. Alleles Separate: During gamete formation (meiosis), alleles segregate, resulting in each gamete carrying only one allele for each character.

  2. OMeiosic Distribution: This phenomenon corresponds to the distribution of homologous chromosomes.

  3. Homozygote vs. Heterozygote:

    • Homozygote: Organism with two identical alleles (e.g. PP or pp).

    • Heterozygote: Organism with two different alleles (e.g. Pp).

    • Distinction between phenotype (physical appearance) and genotype (genetic makeup).

Phenotypic Ratios

  • F2 Generation Ratios: Observed ratio of 3:1 for phenotypes (3 purple: 1 white).

  • Genotypic Ratios: 1 PP (homozygous): 2 Pp (heterozygous): 1 pp (homozygous recessive).

The Testcross

  • To determine unknown genotype, a testcross involves crossing with a homozygous recessive individual.

The Law of Independent Assortment

  • Definition: States that each pair of alleles segregates independently during gamete formation.

  • Applicability: This law applies only to genes located on different, nonhomologous chromosomes or those far apart on the same chromosome.

Methods of Probability in Genetics

The Multiplication and Addition Rules

  • Multiplication Rule: Probability of independent events occurring together is the product of their individual probabilities.

  • Addition Rule: Probability of any one of two or more mutually exclusive events is calculated by adding their individual probabilities.

Complex Genetics Problems Using Probability

  • Multicharacter Crosses: Equivalent to performing several independent monohybrid crosses simultaneously.

  • Example calculations demonstrated to understand genotype proportions in offspring.

Extensions of Mendelian Genetics

Incomplete Dominance and Codominance

  • Complete Dominance: Heterozygote phenotype matches the dominant homozygote.

  • Incomplete Dominance: F1 hybrids exhibit a phenotype that is intermediate between the parental varieties.

  • Codominance: Both alleles express distinct phenotypic effects simultaneously.

Multiple Alleles and Pleiotropy

  • Multiple Alleles: Most genes have more than two allelic forms (e.g. ABO blood types determined by IA, IB, i).

  • Pleiotropy: One gene affects multiple phenotypic traits (e.g. cystic fibrosis).

Polygenic Inheritance

  • Definition: Quantitative characters that vary along a continuum due to the additive effects of multiple genes (e.g. height, skin pigmentation).

Effects of Environmental Influence

  • Phenotypic Range: Broadest for polygenic traits; traits are influenced both by genotype and environmental factors.

Genetic Disorders and Inheritance Patterns

Recessively Inherited Disorders

  • Condition Examples: Cystic fibrosis, albinism, sickle-cell disease.

  • Carriers: Heterozygotes are phenotypically normal but carry a recessive disorder.

Dominantly Inherited Disorders

  • Examples: Achondroplasia, Huntington’s disease.

  • Implications: Dominant disorders typically arise from mutations.

Genetic Counseling and Testing

  • Genetics Counseling: Helps assess risks of having children with heritable disorders.

  • Genetic Tests: Amniocentesis, chorionic villus sampling (CVS), and newborn screenings to detect genetic disorders.

Conclusion

  • Mendel's principles lay the foundation for understanding genetic inheritance, emphasizing the laws of segregation and independent assortment, and paving the way for modern genetics understanding through extensions, complexities, and applications in counseling and testing.