Foundations of Genetics: Mendelian Inheritance and the Modern Evolutionary Synthesis

Theories of Inheritance

Blending vs. Particulate Inheritance

Early theories of inheritance attempted to explain how traits are passed from parents to offspring. Two major models were proposed: blending inheritance and particulate inheritance.

• Blending Inheritance: Traits in offspring are an average (blend) of parental traits. This model suggested hereditary information from all parts of the body ("gemmules") accumulated in reproductive organs and was transferred to offspring.

• Problems with Blending: Blending inheritance could not explain the persistence of variation or the reappearance of traits after skipping generations. It also implied that natural selection would be ineffective, as advantageous traits would be diluted over time.

• Particulate Inheritance: Traits are inherited as discrete units (now known as genes), which retain their identity across generations.

• Historical Experiments: Francis Galton's experiments on rabbits contradicted Darwin's blending model, leading to its decline.

Weismann's Germ Plasm Theory

August Weismann (1834–1914) proposed a new model for inheritance, emphasizing the distinction between germ cells and somatic cells.

• Germ Plasm Theory: Hereditary information is transmitted only by germ cells (sperm and eggs) in the gonads. Somatic cells (body cells) do not transmit genetic information to offspring.

• Key Points:

  • Germ cells produce somatic cells anew each generation.

  • Germ cells segregate from somatic cells early in development; information flows from germ to soma, not the reverse.

  • Somatic cells are disposable; germ plasm is potentially immortal.

• Exceptions: In some organisms (e.g., plants, corals, sponges), germ and somatic cells can both arise from stem cells, allowing mutations in somatic cells to be inherited if they occur in cells that later become germ cells.

Mendelian Inheritance of Simple Traits

Gregor Mendel and the Foundations of Genetics

Gregor Mendel (1822–1884) is considered the founder of modern genetics. His experiments with pea plants established the basic principles of heredity.

• Experimental Design: Mendel used true-breeding pea plants with discrete traits (e.g., flower color, seed shape) and performed controlled crosses.

• Key Observations: Traits did not blend but appeared in predictable ratios in offspring. For example, crossing purple and white flowered plants yielded all purple F1 offspring, but white reappeared in the F2 generation in a 3:1 ratio.

• Particulate Model: Mendel proposed that "factors" (now called genes) are inherited as discrete units, with each individual carrying two factors for each trait, one from each parent.

Modern Genetic Terminology

• Gene: A specific sequence of DNA that codes for a gene product (RNA or protein).

• Locus: The specific location of a gene on a chromosome.

• Allele: Different versions of a gene at a given locus.

• Homozygote: An individual with two identical alleles at a locus.

• Heterozygote: An individual with two different alleles at a locus.

• Genotype: The genetic makeup of an individual.

• Phenotype: The observable traits of an individual.

Mendel's Laws

• Law of Dominance: In heterozygotes, one allele (dominant) masks the effect of the other (recessive).

• Law of Segregation: During gamete formation, the two alleles for a gene separate so that each gamete receives only one allele.

• Law of Independent Assortment: Alleles of different genes assort independently during gamete formation, provided the genes are on different chromosomes.

Punnett Squares and Probability

Punnett squares are used to predict the genotypic and phenotypic ratios of offspring from genetic crosses.

• Product Rule: The probability of two independent events both occurring is the product of their individual probabilities: $Pr(A \text{ and } B) = Pr(A) \times Pr(B)$

• Sum Rule: The probability of either of two mutually exclusive events occurring is the sum of their probabilities: $Pr(A \text{ or } B) = Pr(A) + Pr(B)$

Complex Patterns of Inheritance

Beyond Simple Dominance

• Incomplete Dominance: The heterozygote phenotype is intermediate between the two homozygotes (e.g., red and white flowers producing pink offspring).

• Codominance: Both alleles are fully expressed in the heterozygote (e.g., AB blood type in humans).

• Pleiotropy: A single gene affects multiple phenotypic traits.

• Multiple Alleles: More than two alleles exist for a gene in a population (e.g., ABO blood group).

• Epistasis: One gene affects the expression of another gene.

• Linkage: Genes located close together on the same chromosome tend to be inherited together, violating independent assortment.

• Recombination: Crossing over during meiosis can separate linked genes, producing new allele combinations.

Polygenic Inheritance and Quantitative Traits

Many traits (e.g., height, skin color) are controlled by multiple genes (polygenic inheritance) and show continuous variation.

• Polygenic Traits: Traits influenced by two or more genes, often resulting in a bell-shaped distribution of phenotypes.

• Environmental Influence: The environment can further modify the expression of polygenic traits, producing a range of phenotypes.

Population Genetics and the Hardy-Weinberg Principle

Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle provides a mathematical model to study genetic variation in populations under certain assumptions.

• Equation: For a gene with two alleles (A and a) with frequencies p and q, the genotype frequencies in the next generation are given by: $p^2 + 2pq + q^2 = 1$

• Assumptions:

  • Random mating

  • No mutation

  • No migration

  • Infinitely large population (no genetic drift)

  • No selection

• Interpretation: If observed genotype frequencies deviate from Hardy-Weinberg expectations, it suggests that one or more assumptions are not met, indicating evolutionary processes at work.

Calculating Allele and Genotype Frequencies

• Allele Frequency: The proportion of a specific allele among all alleles at a locus in a population.

• Genotype Frequency: The proportion of a specific genotype among all individuals in a population.

• Example Calculation: If 59% of a bear population is AA, 22% is AW, and 19% is WW, the allele frequencies can be calculated as: Frequency of A = $(2 \times 0.59 + 0.22) / 2 = 0.70$ Frequency of W = $(2 \times 0.19 + 0.22) / 2 = 0.30$ Additional info: These calculations assume diploid organisms and two alleles at the locus.

Applications of Hardy-Weinberg

• Predicting Carrier Frequencies: For recessive diseases, the frequency of carriers (heterozygotes) can be estimated using $2pq$.

• Estimating Allele Frequencies from Phenotype Data: If a recessive phenotype occurs at frequency $q^2$, then $q = \sqrt{q^2}$, and $p = 1 - q$.

• Example: If a recessive disease occurs in 1 out of 12,000 individuals, $q = \sqrt{1/12,000} \approx 0.0091$.

Summary Table: Key Genetic Terms

Term	Definition	Example
Gene	Unit of heredity; sequence of DNA coding for a product	Gene for flower color
Allele	Variant form of a gene	Purple vs. white flower allele
Locus	Location of a gene on a chromosome	Position of flower color gene
Genotype	Genetic makeup at a locus	PP, Pp, or pp
Phenotype	Observable trait	Purple or white flowers
Homozygote	Two identical alleles at a locus	PP or pp
Heterozygote	Two different alleles at a locus	Pp

Conclusion

The study of inheritance has evolved from early blending theories to the modern understanding of genes, alleles, and population genetics. Mendel's principles laid the foundation for genetics, while the Hardy-Weinberg principle provides a framework for studying genetic variation in populations. Complex inheritance patterns, including polygenic traits and gene interactions, explain the diversity observed in nature.