Chromosomes and Human Genetics

Pea Plant Genetics

The inheritance of traits in pea plants is a foundational study in genetics, primarily established by Gregor Mendel in the 19th century. His experiments with pea plants led to the formulation of key genetic principles, which include the concepts of dominance, segregation, and independent assortment.

Genotypes:

  • Tall plant with genotype TTGg: Homozygous dominant for height, heterozygous for color.

  • Tall plant with genotype TtGg: Heterozygous for both height and color traits.

Phenotypes:

  • Tall/Green: The phenotype expressed by either TTGG or TTGg or TtGG or TtGg.

  • Tall/White: The phenotype expressed by TTgg or Ttgg.

  • Short/Green: The phenotype expressed by ttGG or ttGg.

  • Short/White: The phenotype expressed by ttgg.

Dominance:

  • Tall (dominant): Represented by genotypes TT or Tt.

  • Short (recessive): Represented by genotype tt.

  • Green (dominant): Represented by genotypes GG or Gg.

  • White (recessive): Represented by genotype gg.

Chromosomes and Genetic Disorders

Definition of Genetic Disorder:

A genetic disorder is a disease caused by defective genes or abnormalities in chromosome number or structure. These disorders may be inherited or arise spontaneously due to mutations.

Gene Therapy:

Gene therapy is a revolutionary technique aimed at correcting abnormal genes responsible for disease development. It entails multiple steps:

  • Removing or replacing mutated DNA: This can potentially halt disease progression.

  • Introducing modified genes into cells, tissues, or organisms: Successful applications have been made for various diseases.

  • First approved for use in 2012: It has shown success in treating over a dozen diseases.

Method of Gene Therapy:

  • Healthy genes cannot simply be inserted directly into cells due to potential complications.

  • Inactivated viruses are frequently utilized as vectors to deliver healthy genes into cells, effectively allowing the correction of genetic defects.

Genetic Mutations and Chromosomal Abnormalities

Genetic Disorders from Mutations:

Genetic disorders arise from changes to DNA sequences either in individual genes or chromosome structures. Each species has a characteristic number of chromosomes; for example, humans possess 23 pairs (46 total).

Types of Chromosomes:

  • Autosomes: Homologous chromosomes that carry the same genes and are equal in length and shape.

  • Sex Chromosomes: Determine biological sex, consisting of X and Y chromosomes. Males have XY while females have XX.

Karyotype:

A karyotype refers to a visual representation of a cell's chromosomes. Chromosomal abnormalities represent deviations in chromosome structure or number, leading to various genetic disorders.

Key Chromosomal Abnormalities

Common Types of Chromosomal Aberrations:

  • Trisomy: Presence of an extra chromosome (e.g., Down syndrome, which is characterized by an extra chromosome 21).

  • Monosomy: Absence of one chromosome from a pair (e.g., Turner syndrome, characterized by one X chromosome).

  • Deletion: Loss of a segment of a chromosome (e.g., Cri du Chat syndrome).

  • Inversion: A segment of a chromosome is reversed end to end.

  • Translocation: A portion of one chromosome is transferred to another chromosome.

  • Duplication: An extra copy of a chromosome segment is present.

Genetic Inheritance Patterns

Mutations can impact either individual genes or entire chromosomes, leading to various inheritance patterns.

Locus vs. Alleles:

  • Locus: The specific location of a gene on a chromosome.

  • Alleles: Variants of a gene at a given locus. Cells may be homozygous (having identical alleles) or heterozygous (having different alleles) at a locus.

Stem Cells and Gene Therapy Process

Process:

  1. Removal of stem cells from the patient: Stem cells are harvested from the patient's own body to reduce risk of rejection.

  2. Mixing with viral vectors containing healthy genes: These vectors will carry the corrected genes to the problematic areas.

  3. Modified cells are tested and injected back into the patient: The success of modified cells is crucial to determine their effectiveness in treating genetic disorders.

Common Genetic Disorders

Examples of Recessive Genetic Disorders:

  • Sickle cell disease: A disorder characterized by the production of abnormal hemoglobin.

  • Tay-Sachs disease: A fatal genetic disorder primarily affecting infants that leads to the destruction of nerve cells in the brain and spinal cord.

  • Cystic fibrosis: A genetic disorder affecting the respiratory and digestive systems.

  • Additional disorders include Gaucher disease, neurofibromatosis type 2, and Huntington disease; symptoms and inheritance patterns vary.

Inheritance of Genetic Disorders

Autosomal Dominant Disorders:

Only one dominant allele is sufficient for the expression of the disorder (AA or Aa). In contrast, homozygous recessive individuals (aa) show no symptoms.

Sex-linked Inheritance:

Genetic diseases are often linked to X or Y chromosomes (e.g., Hemophilia or color blindness). These conditions are more frequently expressed in males due to their possession of only one X chromosome.

Pedigree Analysis

Pedigrees:

Pedigrees are charts that illustrate genetic relationships within families across generations. They are valuable tools for tracking the inheritance patterns of traits or genetic disorders.