Chapter 9

Principles of Genetics: Genetics is the study of inheritance and genes that determine traits.
Key Concepts:
- Genes determine traits.
- Diploid cells contain two copies of each gene.
- Genotype influences phenotype.
- Dominant alleles can control phenotypes
- Gene mutations lead to new alleles.
- Controlled crosses elucidate inheritance patterns.

Mendel's Experiments: Gregor Mendel's work with pea plants in the 1860s laid the foundation for modern genetics, although it was largely neglected for 30 years.

Genotype vs. Phenotype:
- Genotype: genetic makeup (e.g., RR, Rr, rr).
- Phenotype: observable characteristics (e.g., tall or short plants).
Homozygous vs. Heterozygous:
- Homozygous: two identical alleles (RR/rr).
- Heterozygous: two different alleles (Rr).

True-Breeding Plants: "Parent" plants that produce offspring with the same traits.
Single-Trait Crosses: Mendel tracked one trait using monohybrid crosses resulting in a 3:1 ratio of dominant to recessive phenotypes in offspring.
Law of Segregation: Each gene segregates into different gametes during meiosis, with each offspring receiving one allele from each parent.

Law of Independent Assortment: Genes for different traits can segregate independently during gamete formation (applies when genes are on different chromosomes).
Two-Trait Crosses: Mendel's dihybrid crosses led to a 9:3:3:1 ratio in the F2 generation, showing combinations of traits.
- Recombinant Phenotypes were observed in dihybrids.

Incomplete Dominance: Both alleles blend to create an intermediate phenotype (e.g., red and white flowers producing pink offspring).
Codominance: Both alleles are equally expressed in the phenotype (e.g., ABO blood types).
Pleiotropy: A single gene affecting multiple traits (e.g., albinism affecting skin color and vision).
Epistasis: Phenotypic effects of one gene depend on the presence of alleles from another gene.
Polygenic Traits: Traits that are influenced by multiple genes, leading to continuous variation (e.g., skin color, height).

Environmental Factors: External conditions can modify the expression of genes (e.g., temperature affecting fur color in Siamese cats).

Investigation into the Romanov family using genetic principles revealed claims of lost princess Anastasia to be unfounded.

Allele: Different forms of a gene.
Genotype: The genetic makeup of an organism.
Phenotype: The expressed traits of an organism.
Homozygous: Having identical alleles for a gene.
Heterozygous: Having different alleles for a gene.
Monohybrid/Dihybrid Cross: Breeding experiments tracking one or two traits, respectively.
Punnett Square: A diagram used to predict offspring genetics.

Questions focused on understanding the principles of inheritance, applying Mendel’s laws, recognizing patterns of dominance, and understanding phenotypic expression.