Chapter 12 pt. 2

Key Concepts in Mendelian Genetics

Dominant Trait: A trait that appears in the phenotype of the organism regardless of its genotype. In Mendel's experiments, purple flower color was the dominant trait.
Recessive Trait: A trait that is masked by the presence of a dominant trait and only appears when the organism is homozygous for that trait; thus, white flower color was recessive.

Mendel started with true breeding plants with distinct traits (e.g., flower color, seed shape).
F1 Generation: The first generation of offspring. All plants exhibited the dominant trait.
F2 Generation: The second generation, resulting from the self-fertilization of F1 hybrids, showed a reappearance of the recessive trait at approximately a 3:1 ratio (dominant:recessive).

Phenotypic Ratio in F2: 3 dominant : 1 recessive (e.g., purple : white).
Genotypic Ratio in F2: 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive (1:2:1).
It is crucial to differentiate between genotype (the genetic composition) and phenotype (the physical expression of traits).

Homozygous: Organisms that have two identical alleles for a trait (e.g., BB or bb).
Heterozygous: Organisms that have two different alleles for a trait (e.g., Bb).
Complete Dominance: When the dominant allele completely masks the effect of the recessive allele in heterozygous individuals.

Each individual has two alleles for each trait, and these alleles segregate during gamete formation so that each gamete carries only one allele for each trait.

A tool used to predict the probability of offspring's genotypes and phenotypes based on the parental alleles.
Example: A 2x2 Punnett square to show F1 cross results in 1 homozygous dominant, 2 heterozygous, 1 homozygous recessive.

Incomplete Dominance: The phenotype of heterozygotes is a blend of the two parental traits (e.g., red and white flowers producing pink flowers).
Codominance: Both alleles in a heterozygous individual contribute equally to the phenotype (e.g., AB blood type).
Multiple Alleles: More than two alleles exist for a gene (e.g., ABO blood group system with A, B, and O alleles).
Pleiotropy: A single gene can influence multiple phenotypic traits (e.g., sickle cell anemia affecting various body functions).
Polygenic Inheritance: Many genes contribute to a single trait, leading to a continuous range of phenotypes (e.g., skin color).

The genetics of many traits can be more complex than Mendel's original experiments suggested, involving multiple genes, environmental influences, and interactions between alleles.
Techniques like genome sequencing have provided further insights into the nature of alleles and their effect on traits.

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