OCT 29

Homozygous: An individual with two identical alleles for a given trait.
- Example: Homozygous dominant (AA) or homozygous recessive (aa).
Heterozygous: An individual with two different alleles for a trait (Aa).
Hemizygous: A term for genes that are present as a single copy in diploid organisms, such as those located on the X chromosome in males.
Genotype: The genetic makeup of an individual, represented by the alleles they carry (e.g., AA, Aa, aa).
Phenotype: The observable characteristics or traits of an individual, which are expressed as a result of the genotype.

Monohybrid Cross: A genetic cross that considers only one trait. In this case, Mendel studied flower color in pea plants, crossing purple and white flowers.
- Parental Generation (P): The original pair of organisms being crossed (e.g., purple and white flowers).
- First Filial Generation (F1): The first generation of offspring resulting from the P generation.
- All F1 individuals resulting from a purple and white cross are hybrids and display the dominant phenotype (purple).
- Second Filial Generation (F2): The second generation of offspring, produced by crossing F1 individuals. Results in a phenotypic ratio of approximately 3:1 (purple:white).
Law of Segregation: Mendel's first law stating that during the formation of gametes, the two alleles responsible for a trait separate from each other. As a result, every gamete carries only one allele for each trait.

In a monohybrid cross between a homozygous dominant (AA) and a heterozygous (Aa), the possible offspring ratios reflect Mendel's Law of Segregation.
- Offspring phenotypic ratio: 3:1 - where 3 are dominant (purple) and 1 is recessive (white).
- Offspring genotypic ratio: 1:2:1 - where there is 1 homozygous dominant (AA), 2 heterozygous (Aa), and 1 homozygous recessive (aa).
Mendel noted that his experiments showed consistency with a 3:1 ratio concerning traits like flower color (purple to white) and seed shape (round to wrinkled).

Dihybrid Cross: A genetic cross that considers two traits simultaneously. Mendel tested daylily seed color (yellow or green) and seed shape (round or wrinkled).
- Phenotypic Ratio from Dihybrid Cross: 9:3:3:1.
- Results show that the traits assort independently of one another during gamete formation.

A test cross is used to determine an individual's genotype by crossing it with a homozygous recessive individual.
- If all offspring display the dominant phenotype, the test subject is homozygous dominant.
- If offspring show a 1:1 ratio of dominants to recessives, the test subject is heterozygous.

Dominant Trait: A trait that is expressed when at least one dominant allele is present (e.g., purple flowers in pea plants).
Recessive Trait: A trait that is only expressed when both alleles are recessive (e.g., white flowers require two recessive alleles).

Alleles: Different versions of a gene that can produce various traits. Each plant inherits two alleles for a trait, one from each parent.
- Example: for flower color, alleles can be purple (dominant) or white (recessive).
Enzymatic Activity Impact on Phenotype: The presence of an enzyme that synthesizes purple pigment leads to the flower's purple phenotype, while a non-functioning allele results in no color expressed (white).

Mendel's experiments with over 5,000 seeds established reliable ratios for dominance and inheritance patterns.
Observations of ratios in his studies across multiple traits reinforced the idea of independent assortment, leading him to formulate his second law of inheritance.

The consistent ratios observed from Mendel's experiments were crucial in deducing basic principles of genetic inheritance, including segregation and independent assortment. His analytical approach laid the groundwork for modern genetics and our understanding of inheritance patterns across various species.