History of Medical Genetics & Introduction to the Human Genome

History of Medical Genetics

Key Figures in Medical Genetics

The field of medical genetics has been shaped by several pioneering scientists whose discoveries laid the foundation for our understanding of genetic diseases and inheritance.

• Sir Archibald Garrod (1857–1936):

  • British physician who studied alkaptonuria, a rare inherited disorder.

  • Collected family histories and determined that alkaptonuria was inherited in a recessive manner.

  • Published "The Incidence of Alkaptonuria: a Study in Chemical Individuality," introducing the concept of inborn errors of metabolism—diseases caused by specific inherited enzyme deficiencies.

  • His work established the link between genes and metabolic pathways.

• William Allan (1881–1943):

  • Physician and geneticist at the Bowman Gray School of Medicine.

  • Founded the first Department of Medical Genetics in the United States.

  • The Allan Award (by the American Society of Human Genetics) is named in his honor, recognizing significant contributions to human genetics.

  • Supported eugenics and sterilization as a means to eliminate genetic abnormalities and diseases (a view now considered unethical).

• Victor McKusick (1921–2008):

  • Often called the "Father of Medical Genetics."

  • Professor at Johns Hopkins University.

  • Published the first edition of Mendelian Inheritance in Man, now maintained as the Online Mendelian Inheritance in Man (OMIM) database.

Categories of Genetic Disease

Genetic diseases can be classified into three broad categories based on their underlying causes.

Chromosome Disorders

• Caused by abnormalities in chromosome number or structure (e.g., deletions, insertions, translocations).

• Humans have 46 chromosomes (23 pairs).

• Chromosome disorders are the most common genetic disorders, affecting approximately 7 in 1,000 liveborn infants and accounting for about 50% of all spontaneous abortions in the first trimester.

• Examples include Down syndrome (trisomy 21), Turner syndrome (monosomy X), and Klinefelter syndrome (XXY).

Single Gene Defects

• Result from mutations in a single gene.

• Can be inherited in a recessive (both copies mutated) or dominant (one copy mutated) manner.

• Serious single gene defects occur in about 1 in 300 liveborn infants.

• Prevalence may vary by genetic ancestry.

Condition	Gene (Chromosome Location)	Inheritance Pattern
Congenital Deafness (nonsyndromic)	Connexin26 (13q11)	Recessive
Tay-Sachs Disease	Hexosaminidase A (15q23)	Recessive
Familial Hypercholesterolemia	LDL receptor (19p13)	Dominant
Sickle Cell Anemia	Beta-globin (11p15)	Recessive
Duchenne Muscular Dystrophy	Dystrophin (Xp21)	X-linked Recessive
Cystic Fibrosis	CFTR (7q31)	Recessive
Hemochromatosis	HFE (6p21)	Recessive
Huntington Disease	Huntingtin (4p16)	Dominant

Multifactorial (Complex) Diseases

• Result from the combined effects of multiple genes and environmental factors or behaviors.

• Include most common diseases with a genetic component, such as heart disease, diabetes, and many birth defects (e.g., cleft lip and palate, congenital heart defects).

Introduction to the Human Genome

Genome Organization

• The genome is the complete set of DNA in an organism.

• In eukaryotes, DNA is organized into chromosomes within the nucleus.

• Humans have 46 nuclear chromosomes and mitochondrial DNA (mtDNA).

Nucleotides and DNA Structure

• Nucleotides are the building blocks of nucleic acids (DNA and RNA).

• Each nucleotide consists of:

  • A nitrogenous base (A, T, C, G for DNA; A, U, C, G for RNA)

  • A five-carbon sugar (deoxyribose in DNA, ribose in RNA)

  • A phosphate group

• Nitrogenous bases are classified as:

  • Purines: Adenine (A) and Guanine (G)

  • Pyrimidines: Cytosine (C), Thymine (T, in DNA), and Uracil (U, in RNA)

• DNA contains thymine; RNA contains uracil instead of thymine.

Primary and Secondary Structure of DNA

• The primary structure of DNA is a linear sequence of nucleotides joined by phosphodiester bonds between the 5' phosphate and 3' hydroxyl groups.

• The secondary structure is the famous double helix, stabilized by hydrogen bonds between complementary bases:

  • Adenine pairs with Thymine (A–T) via two hydrogen bonds.

  • Guanine pairs with Cytosine (G–C) via three hydrogen bonds.

• The two DNA strands are antiparallel and complementary.

Chromatin and Chromosome Structure

• DNA is packaged into chromatin, which consists of DNA wrapped around histone proteins to form nucleosomes ("beads on a string").

• Nucleosomes coil to form solenoids, which further fold and attach to scaffolding proteins, ultimately forming chromosomes.

Chromosome Number in Animals

• Different species have characteristic chromosome numbers (e.g., humans: 46, dogs: 78, cats: 38).

Genetics: Chromosomes, Structure, and Inheritance

Karyotyping

• Karyotype: Visualization of chromosomes, used for counting and identifying chromosomal abnormalities.

• Chromosomes can be stained and observed under a light microscope.

• Commonly used to diagnose chromosomal disorders (e.g., trisomies, deletions).

Cell Division: Mitosis and Meiosis

• Genetic material must be accurately replicated and passed to daughter cells.

• Mitosis: Somatic cell division, producing two identical diploid (2n) cells.

• Meiosis: Germline cell division, producing four haploid (n) gametes, each with half the original chromosome number.

• Meiosis introduces genetic diversity through recombination (crossing over) and independent assortment.

Phases of Mitosis

1. Prophase: Chromosomes condense, spindle forms, centrosomes migrate.

2. Metaphase: Nuclear membrane dissolves, chromosomes align at the metaphase plate.

3. Anaphase: Sister chromatids separate and move to opposite poles.

4. Telophase: Chromosomes decondense, nuclear membrane reforms.

5. Cytokinesis: Cytoplasm divides, forming two daughter cells.

Phases of Meiosis

1. Meiosis I: Homologous chromosomes pair, recombine, and separate.

2. Meiosis II: Sister chromatids separate, resulting in four haploid cells.

Each gamete contains a unique combination of genetic material.

The Human Genome Project

Overview

• The Human Genome Project was an international effort to sequence the entire human genome.

• A draft sequence was completed in the early 2000s, providing a reference for human genetic studies.

• Ongoing research continues to identify genetic variability and its significance.

Genome Browsers

• Tools like the UCSC Genome Browser allow researchers to search, visualize, and analyze genomic data.

• These browsers integrate data from multiple sources and support evaluation of genetic and functional significance.

• Data can be downloaded for further analysis.

Additional info: Some details, such as the full list of chromosome numbers in animals and the complete process of chromatin folding, were inferred and expanded for academic completeness.