Genetics Fundamentals: Mendelian Laws & Human Conditions

Basic Concepts of Genetics

Genetics studies the transmission of hereditary traits.

• Gene: A fragment of DNA containing the information needed to make proteins or to determine a specific characteristic.
• Genome: The complete set of genes from an organism.
• Genotype: The genetic makeup of an organism (e.g., represented by letters like AA, Aa, aa).
• Phenotype: The observable traits or characteristics resulting from the genotype and its interaction with the environment.

Mendelian Genetics Principles

Mendelian genetics applies the methodology practiced by Gregor Mendel, which involves making crosses between individuals with known phenotypes to infer their genotypes.

Mendel's Laws of Inheritance

Mendel formulated three fundamental laws governing the transmission of characteristics to offspring. His studies focused on seven distinct characters in pea plants:

• Shape and color of the seeds
• Color and position of flowers
• Color and shape of the pods
• Stem length

Mendel meticulously studied pure breeds, ensuring genetic consistency through self-pollination (autofecundation).

Mendel's First Law: Law of Uniformity

This law states that when two homozygous individuals are crossed for a single character, all the offspring of the first filial generation (F₁) will be uniform and heterozygous hybrids.

Mendel's Second Law: Law of Segregation

When individuals from the F₁ generation are crossed, a separation of antagonistic alleles occurs in the F₂ generation, leading to predictable phenotypic and genotypic ratios.

Mendel's Third Law: Law of Independent Assortment

Each hereditary character is transmitted to offspring completely independent of others, provided the genes are located on different chromosomes or are far apart on the same chromosome.

Human Genetics and Chromosomes

Karyotype: The complete set of human chromosomes within a cell.

Karyogram: A graphic representation of human chromosomes, grouped in homologous pairs and sorted in descending order of size.

Common Genetic Disorders

Dominant Genetic Disorders (Autosomal Dominant)

These disorders manifest when only one copy of the altered gene (A) is present. Examples include:

• Polydactyly: Having more than the normal number of fingers or toes (e.g., 6 fingers).
• Neurofibromatosis: Characterized by benign tumors that grow on nerves throughout the body.
• Achondroplasia: A common form of dwarfism.
• Double Tongue: A rare condition where the tongue appears to be split or duplicated.

Recessive Genetic Disorders (Autosomal Recessive)

These disorders only manifest when an individual inherits two copies of the altered gene (aa), one from each parent. Examples include:

• Albinism: A lack of melanin in the skin, hair, and eyes.
• Congenital Deafness: Deafness present from birth.
• Phenylketonuria (PKU): The body cannot metabolize the amino acid phenylalanine. If untreated, it interferes with normal brain development, causing severe mental retardation. Management involves a strict diet avoiding phenylalanine.
• Cystic Fibrosis: Characterized by high secretion of thick mucus in various cavities, facilitating frequent lung infections and other organ damage.