Chapter 12

Phenotype: The expressed traits of an organism resulting from its genotype and environment.
Dominant allele: An allele that masks the effect of a recessive allele when present in a heterozygous individual.
Genotype: The genetic makeup of an organism, often defined in terms of the alleles present (e.g., TT, Tt, tt).
Recessive allele: An allele that is masked in the presence of a dominant allele and is only expressed in homozygous conditions.
Locus: The specific physical location of a gene or a significant sequence on a chromosome.
Allele: Different forms of a gene that can exist at a specific locus.
Homozygous: Having two identical alleles for a particular gene (e.g., TT or tt).
Heterozygous: Having two different alleles for a particular gene (e.g., Tt).

Segregation: During meiosis, the alleles for each locus segregate (separate) into different gametes.
Independent Assortment: Alleles of different loci assort independently into gametes, leading to the possible recombination of genes and new trait combinations not found in the parents.

P Generation: The parental generation, involving a tall plant (TT) crossed with a short plant (tt).
F₁ Generation: All offspring are tall plants (Tt).
F₂ Generation: From the F₁ interbreeding, the result is a phenotypic ratio of 3 tall plants to 1 short plant.

Monohybrid Cross: A genetic cross that follows the inheritance of a single character, differing at one locus (e.g., TT x tt).
Dihybrid Cross: A genetic cross that follows the inheritance of two independent traits, differing at two loci (e.g., TTSS x ttss).
Test Cross: A cross between an individual of unknown genotype and a homozygous recessive individual to determine genotype of the unknown.

Example of Monohybrid Cross:
- Parental generation: homozygous black (BB) crossed with homozygous brown (bb). The F₁ generation consists entirely of heterozygous offspring (Bb).
- F₂ generation results show the distribution: 1 BB, 2 Bb, and 1 bb (genotypic ratio).

A gamete contains one set of chromosomes (haploid, n) carrying one allele from each homologous chromosome pair.
Upon fusion with another gamete, it produces a zygote (diploid, 2n) possessing two alleles for each gene, with homologous chromosomes present.

Genetic ratios derived from crosses can be expressed as probabilities:
- Product Rule: Used when calculating the combined probability of independent events (e.g., probability of both event A and event B occurring).
- Sum Rule: Used when calculating the combined probability of mutually exclusive events (e.g., probability of event A or event B occurring).

Segregation: Homologous chromosomes separate during meiosis, affecting allele distribution.
Independent Assortment: Orientation of homologous chromosomes during metaphase influences how genes segregate.
Linkage: When genes are located close together on the same chromosome, they do not independently assort, leading to linked inheritance patterns.

Linkage: Genes physically located close to each other on chromosomes tend to be inherited together.
Recombination: Occurs during crossing-over in prophase I of meiosis, enables exchange of genetic material between homologous chromosomes.

Females typically possess two X chromosomes, whereas males have one X and one smaller Y chromosome. The presence of the Y chromosome usually determines male sex in mammals.
Males are hemizygous concerning genes on the X chromosome.

Eggs always contain an X chromosome.
Sperms can carry either an X or a Y chromosome. Purposes:
- X sperm fertilizing an egg results in female offspring (XX).
- Y sperm fertilizing an egg results in male offspring (XY).

Affects the expression of color vision in affected males and females differently:
- Colorblind male genotype: XcY
- Heterozygous female genotype: XXc
- Normal male genotype: XY
- Crosses may result in normal and colorblind offspring depending on parental genotypes.

Incomplete dominance: Heterozygotes exhibit a phenotype that is an intermediate between the two homozygous phenotypes (Example: Four o'clocks plant color).
Codominance: Both alleles are equally expressed in heterozygotes (Example: ABO blood types - both A and B antigens expressed in AB).

Pleiotropy: One allele influences multiple phenotypic traits.
- Example: Sickle-cell allele affects hemoglobin and leads to several health issues (anemia, pain, organ damage).
Gene Interaction: Modifier genes can affect the expression of other genes resulting in different phenotypes (Example: Combs in chickens).

Traits influenced by multiple genes lead to variations in the phenotype.
- Example: Human skin pigmentation is influenced by several genes, portraying a gradient of color.

Norm of reaction: The phenotypic range that a single genotype may produce under different environmental conditions.
- Example: Height in humans can vary with diet, despite genetic predisposition.

Understanding genetic principles such as Mendel's laws, the role of probabilities, and mechanisms such as linkage, pleiotropy, and gene interactions are essential for grasping the complexity of heredity and variation in organisms.