Mendel's Laws of Inheritance and Genetic Concepts

Mendel's First Law: The Law of Uniformity

Mendel started his experiments by crossing two individuals homozygous for a particular trait. The first law, or the law of uniformity, states that if you cross two pure individuals differing in one character, the offspring is uniform, all showing the dominant character.

Mendel's Second Law: The Law of Segregation

For the F2 generation, Mendel allowed plants with the Aa genotype from the F1 generation to self-fertilize. The second law, the law of segregation, states that the recessive traits that do not occur in the first generation reappear in the second generation in the proportion of three dominant to one recessive.

Mendel's Third Law: The Law of Independent Assortment

The third law, the principle of independent assortment, states that in heterozygotes for two or more characters, each character is transmitted to the next generation independently of any other character.

Key Genetic Concepts

• Genome: The complete set of genes characteristic of a species. It is highly variable.
• Gene: A fragment of DNA that carries information for a particular character.
• Loci: The specific locations of genes on chromosomes. Each determined place is called a locus (plural: loci).
• Alleles: Each character is determined by two alleles. Alleles are located at the same spot on a pair of homologous chromosomes.
• Homozygous: An organism is homozygous for a character if it carries two identical alleles for that character.
• Heterozygous: An organism is heterozygous for a character if it carries two different alleles for that character.
• Genotype: The set of genes an organism has inherited.
• Phenotype: The observable characteristics of an organism, resulting from the interaction of its genotype with the environment.

Early Chromosomal Theory of Inheritance

Every living being has a set of defining characteristics. These characteristics are determined by genes, which can be functional or non-coding DNA, located on chromosomes. A relationship exists between how characteristics are inherited and the processes that form gametes or zygotes.

Environmental Influence on Phenotype

The phenotype is the result of the interaction between genotype and environment. A living being's genotype only indicates its potential phenotype, not necessarily the actual phenotype. The difference between the potential and actual phenotype is due to the influence of the environment on the individual. Phenotype = Genotype + Environment. This relationship can explain the differences between identical twins. The only factor that can affect their physical appearance is the environment, as both have identical genetic information. Examples include food, moisture, light, and temperature.

Codominance and Incomplete Dominance

• Codominance: The process by which an individual expresses two dominant traits simultaneously.
• Incomplete Dominance: The phenotype of the heterozygote shows characteristics midway between those of the two parents.

Sex-Linked Inheritance

Color Blindness: A condition with sex-linked recessive inheritance characterized by the inability to perceive certain colors. Variations of color blindness include:

• Deuteranopia: Green color blindness (malformation in the retina).
• Protanopia: Red color blindness (lack of photopigments in the cones).

The inheritance of characters whose genes are located on the X chromosome has certain features. In men, these characters are controlled by a single allele on the X chromosome. Women carry two alleles for a character, one on each X chromosome. Color blindness and hemophilia are examples of sex-linked inheritance. In both cases, the cause is a recessive allele. X-linked characters are much more common among men, as they only need to receive one X chromosome from their mother with the responsible gene for it to manifest in the phenotype.

Mendel's Experimental Approach

Mendel chose plant varieties whose characteristics remained unchanged over generations. He used plants that self-pollinated, preventing accidental cross-pollination and allowing him to obtain pure breeds. Mendel applied statistical analysis to his experiments on inheritance. He crossed pea plants of two varieties and found a mathematical relationship that allowed him to extract rules explaining the transmission of characters in inheritance.