PUNNET SQUARE

Until approximately 1900, many scientists adhered to the blending theory of inheritance.
In the 1860s, Gregor Mendel conducted systematic studies on the patterns of inheritance using garden peas, known scientifically as Pisum sativum.
Focus of the study involved specific heritable features, referred to as characters, which had alternative forms or traits (character differences).
Establishment of discrete hereditary factors (termed genes) that are passed from parents to offspring.
Mendel noted that certain parental traits remained unchanged in offspring, while others would appear and then seemingly disappear in subsequent generations.
The inheritance patterns observed are attributed to the segregation of chromosomes during gamete formation in meiosis.

The selection of the garden pea for Mendel's experiments was directly related to its ability to self-fertilize, or self-pollinate.
To facilitate cross-fertilization (cross-pollination), Mendel cut off the anthers of the plants to prevent self-fertilization and used pollen from different plants.
True-breeding plants (pure-breeding) were initially chosen for the experiments.

Crossing Technique:
- Remove the anthers from one parent (the flower with white petals) to prevent self-fertilization.
- Transfer pollen from the male parent plant (purple-flowered) to the stigma of the female plant (white-flowered).
- Result: Cross-fertilized plants produce seeds that can then be assessed for seed traits such as shape (e.g., round vs. wrinkled) and color (e.g., purple vs. white flower).
Seeds were grown into adult plants to evaluate adult characteristics.

Parental generation (P generation): The true-breeding plants used in the initial cross.
First filial generation (F1 generation): Offspring produced from the parental cross.
Second filial generation (F2 generation): The result of self-pollination from the F1 generation.

From the initial crosses involving purple and white flowers:
- All F1 generation plants exhibited purple flowers, indicating that the white flower trait was not expressed in this generation.
- In the F2 generation, both flower traits appeared: approximately ¾ of the plants had purple flowers and ¼ had white flowers, producing a ratio of approximately 3:1.
- The consistent 3:1 ratio was observed for each character tested by Mendel.

Adult plants have a pair of alleles (factors) for each trait governing inheritance.
In cases where alleles are different, one will be dominant while the other remains recessive.
During gamete formation, alleles segregate so that half of the gametes carry one allele and the other half carries the other (Mendel’s Principle of Segregation).

Homozygote:
- A true-breeding individual with the same alleles for a gene (e.g., AA or aa).
- Produces only one type of gamete.
Heterozygote:
- An individual with two different alleles of a gene (e.g., Aa).
- Produces two types of gametes: half contain one allele, half contain the other.
Monohybrid:
- The F1 progeny resulting from a single-character cross.
Monohybrid cross:
- A genetic cross between two heterozygous individuals for the same pair of alleles.

Genotype:
- The genetic constitution of an organism concerning its genes and alleles.
Phenotype:
- The observable appearance or expression of a trait in an organism.
Example of flower color alleles:
- Dominant allele (P) = Purple flower color.
- Recessive allele (p) = White flower color.
Genotype PP (homozygous dominant) and Pp (heterozygous) lead to purple phenotype; genotype pp (homozygous recessive) results in white phenotype.

The principle explains how allele pairs separate during gamete formation, leading to offspring inheriting different combinations of alleles.

A tool used for predicting the ratios of offspring and their phenotypes in genetic crosses.

Dihybrid:
- The F1 hybrid produced from a cross that involves two characters, displaying heterozygosity for both traits (e.g., AaBb).
Dihybrid cross:
- A genetic cross wherein both parents are heterozygous for the alleles of two traits.

An F2 generation produced from a cross between true-breeding plants for round yellow seeds (RRYY) and wrinkled green seeds (rryy) yielded a phenotypic ratio of 9:3:3:1.
Key principle: The inheritance of one trait (e.g., seed shape) is independent of another trait (e.g., seed color), demonstrating Mendel’s Law of Independent Assortment.

The principle asserts that alleles of different genes assort independently from one another during gamete formation.

Promoted by Sutton, this theory posits that genes and alleles reside on chromosomes.
The locus of a gene: The specific location of a gene on a chromosome, typically correlating with a particular DNA sequence that influences the phenotype.

Different alleles of a gene vary due to genetic sequence differences, causing potential functional dissimilarities in the resulting protein.