Mendel's Laws of Inheritance

Study Notes

Female plants were pollinated using pollen from male plants with contrasting traits, then bagged to prevent further pollination.
Collected seeds were sown at the appropriate time.
Focus on one inherited character at a time for analysis.
Statistical analysis was employed to interpret results.
Reciprocal and test crosses were conducted to confirm findings.

Mendel proposed three key laws of heredity known as Mendel's Laws:
- Law of Dominance
- Law of Segregation
- Law of Independent Assortment

Dominant traits appear in the F1 generation from a cross between two homozygous individuals with contrasting traits.
Recessive traits are masked in the presence of dominant traits.
Dominant genes produce functional enzymes, while recessive genes may lead to non-functional polypeptides.
Heterozygous and homozygous individuals display similar phenotypes due to dominance.
Cross-breeding experiments validated the law across various species including peas and maize.

Dominant traits can mask harmful recessive traits, allowing them to persist in a population without expression.
Examples include recessive traits like idiocy, diabetes, and hemophilia in humans.

Each allele pair remains distinct in a heterozygote, separating during gamete formation.
Example: Pure tall plants (TT) and pure dwarf plants (tt) produce hybrids (Tt) that are tall but carry a recessive trait.

Applications in breeding improve livestock, agriculture, and new breeds of animals through crossbreeding.
Eugenics is a scientific field emerging from Mendelian principles aimed at enhancing human populations.

Lethal Allele Combinations: Homozygous recessive lethal alleles can eliminate certain progeny classes.
Multiple Alleles: Genes can exist in more than two forms leading to varied phenotypes (e.g., ABO blood groups).
Pleiotropy: A single gene can affect multiple traits.
Phenocopies: Environmental influences can create traits resembling genetic conditions.
Maternal Inheritance: Traits passed from females to offspring irrespective of male contribution (e.g., mitochondrial genes).
Linkage: Deviations from expected Mendelian ratios in dihybrid crosses indicate linked genes.

Incomplete dominance occurs when neither allele is fully dominant, leading to a blended phenotype.
Codominance occurs when both alleles contribute distinctly to the phenotype.

One gene can mask the expression of another, with no new phenotypes produced.
Recessive Epistasis Example: In Labrador Retrievers, fur color depends on two genes (B for color and E for expression). The presence of "ee" results in golden retrievers irrespective of the B gene.
Dominant Epistasis Example: Squash fruit color shows that the W allele dominates over G, leading to white fruit regardless of the G allele.

Discontinuous variation represents distinct categories with limited environmental influence.
Continuous variation suggests multiple genes are involved, often coupled with environmental effects.
Discontinuous traits, like blood types, have fewer categories and are mainly genetic.