Mendelian Genetics & Concepts of Dominance

Introduction to Heritability

Genetics is the study of heredity, the process by which traits are passed from parents to offspring. The concept of heritability refers to the proportion of observed variation in a particular trait that can be attributed to inherited genetic factors in contrast to environmental ones.

• Heritable Traits: Characteristics that can be transmitted from one generation to the next through genes.

• Evidence for Heritability: Observations such as "like begets like" (offspring resemble their parents) and controlled breeding experiments.

• Example: Physical traits such as eye color, hair color, and certain genetic diseases are heritable.

Mendel’s Laws of Inheritance

Gregor Mendel and His Experiments

Gregor Mendel, through his work with pea plants, established the foundational principles of inheritance. His experiments demonstrated predictable patterns in the transmission of traits.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete carries only one allele for each gene.

• Law of Independent Assortment: Genes for different traits assort independently of one another in gamete formation.

• Example: Mendel observed that crossing purple-flowered and white-flowered pea plants resulted in a 3:1 ratio of purple to white flowers in the F2 generation.

Mendelian Ratios

Mendel consistently observed a 3:1 ratio of dominant to recessive traits in the F2 generation of his pea plant crosses.

• Dominant Trait: The trait that appears in the F1 generation (e.g., purple flowers).

• Recessive Trait: The trait that is masked in the F1 generation but reappears in the F2 generation (e.g., white flowers).

• Genotypic Ratio: 1:2:1 (homozygous dominant : heterozygous : homozygous recessive)

• Phenotypic Ratio: 3:1 (dominant : recessive)

Example Punnett Square:

• Cross: Aa x Aa

• Genotypes: AA, Aa, aa

• Phenotypes: Dominant (AA, Aa), Recessive (aa)

Concepts of Dominance

Dominant and Recessive Alleles

Dominance describes the relationship between alleles, where one allele masks the expression of another in the heterozygote.

• Complete Dominance: The heterozygote (Aa) has the same phenotype as the dominant homozygote (AA).

• Recessive Allele: Only expressed when two copies are present (aa).

Variations on Dominance

Not all allelic relationships are strictly dominant or recessive. Some genes exhibit codominance or incomplete dominance.

• Codominance: Both alleles in a heterozygote are fully expressed, resulting in a phenotype that shows both traits.

• Example: ABO blood group system. Individuals with genotype IAIB express both A and B antigens.

• Incomplete Dominance: The heterozygote displays a phenotype intermediate between the two homozygotes.

• Example: In snapdragons, crossing red (RR) and white (rr) flowers produces pink (Rr) flowers in the F1 generation.

Variable Expressivity and Incomplete Penetrance

Definitions and Examples

Some genetic traits do not show uniform expression among individuals with the same genotype.

• Variable Expressivity: Individuals with the same genotype display a range of phenotypes.

• Incomplete Penetrance: Some individuals with the mutant genotype fail to display the mutant phenotype.

• Example: Polydactyly (extra fingers or toes) can show variable expressivity and incomplete penetrance; not all individuals with the gene show the trait, and those who do may show it to different extents.

Summary Table: Types of Dominance

Key Equations

• Genotypic Ratio (Monohybrid Cross):

• Phenotypic Ratio (Monohybrid Cross):

Additional info: The study notes expand on the brief points in the original materials, providing definitions, examples, and tables for clarity and completeness.