The Principles of Heredity
Gregor Mendel, through his experiments with pea plants, laid the foundation for modern genetics. His work established that traits are passed from parents to offspring through discrete units we now call genes.
Key Terminology
Allele: A variant form of a gene (e.g., the allele for purple flowers vs. the allele for white flowers).
Dominant Allele: An allele that expresses its trait even when only one copy is present. Represented by a capital letter (e.g., P for purple).
Recessive Allele: An allele that only expresses its trait when two copies are present. Represented by a lowercase letter (e.g., p for white).
Genotype: The genetic makeup of an organism, referring to the combination of alleles it has (e.g., PP, Pp, or pp).
Phenotype: The observable physical or biochemical characteristics of an organism (e.g., purple flowers or white flowers).
Homozygous: Having two identical alleles for a particular gene (e.g., PP or pp).
Heterozygous: Having two different alleles for a particular gene (e.g., Pp).
Mendel's Laws
1.Law of Segregation: During gamete formation (meiosis), the two alleles for a heritable character separate (segregate) from each other, so that each gamete ends up with only one allele. This explains how a Pp parent can produce both P and p gametes.
2.Law of Independent Assortment: Genes for different traits assort independently of one another during gamete formation. This applies to genes located on different chromosomes or far apart on the same chromosome. For example, the inheritance of seed color does not affect the inheritance of seed shape.
Punnett Squares: Predicting Outcomes
A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross.
Monohybrid Cross (e.g., Pp x Pp):
Possible offspring genotypes: 1 PP : 2 Pp : 1 pp (ratio 1:2:1)
Possible offspring phenotypes: 3 Purple : 1 White (ratio 3:1)
Dihybrid Cross (e.g., YyRr x YyRr):
A cross involving two traits demonstrates independent assortment, famously resulting in a 9:3:3:1 phenotypic ratio (9 dominant/dominant, 3 dominant/recessive, 3 recessive/dominant, 1 recessive/recessive).
Beyond Simple Dominance
Incomplete Dominance: The heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. (e.g., Red flower (RR) x White flower (rr) → Pink flower (Rr)).
Codominance: Both alleles are fully and separately expressed in the heterozygous phenotype. (e.g., Human ABO blood types, where the AB blood type expresses both A and B antigens).