HG - week 3

Overview of Cytogenetics

  • Definition: The branch of genetics studying the structure and function of chromosomes.

  • Techniques: A variety of methods are utilized in this field to analyze chromosomes.

Chromosomes

  • Differences Among Species: Different species have varying numbers of chromosomes.

    • Humans (Homo sapiens): 46 diploid (2n), 23 haploid (n)

    • Chimpanzees (Pan troglodytes): 48 diploid, 24 haploid

    • Gorillas (Gorilla gorilla): 48 diploid, 24 haploid

    • Dogs (Canis familiaris): 78 diploid, 39 haploid

    • Chickens (Gallus domesticus): 78 diploid, 39 haploid

    • Frogs (Rana pipiens): 26 diploid, 13 haploid

    • House Mice (Mus musculus): 40 diploid, 20 haploid

    • Houseflies (Musca domestica): 12 diploid, 6 haploid

    • Fruit Flies (Drosophila melanogaster): 8 diploid, 4 haploid

    • Nematodes (Caenorhabditis elegans): 12 diploid, 6 haploid

    • Onions (Allium cepa): 16 diploid, 8 haploid

    • Corn (Zea mays): 20 diploid, 10 haploid

    • Tobacco (Nicotiana tobacum): 48 diploid, 24 haploid

Chromosome Structure

  • Chromosome Arms: Divided into short (p) and long (q) arms, with centromeres determining the division.

    • Types:

      • Metacentric: Centromere in the middle

      • Submetacentric: Centromere slightly off-center

      • Acrocentric: Centromere near one end

  • Staining Patterns: Distinct patterns help in identifying specific regions and bands on chromosomes.

    • Example Notation: 1q2.4 = Chromosome 1, Arm q, Region 2, Band 4

Karyotype

  • Definition: A display of all chromosomes in a cell, organized into pairs arranged by size.

  • Preparation:

    • Blood collection (4-6 mL)

    • Stimulation of mitosis with phytohemagglutinin

    • Incubation and use of colcemid to stop mitosis at metaphase

    • Preparation of slides to view chromosomes

  • Cells for Analysis:

    • Include lymphocytes, skin cells, tumor cells, amniotic cells, etc.

Chromosomal Aberrations

  • Types of Chromosome Changes:

    • Duplication: Extra genetic material is created.

    • Deletion: Loss of genetic material.

    • Inversion: Portions of chromosomes are flipped.

    • Translocation: Genetic material is swapped between non-homologous chromosomes (reciprocal and Robertsonian translocations).

Non-disjunction Events

  • Definition: Failure of chromosomes to separate properly during cell division, particularly during meiosis.

  • Results: Can lead to aneuploidy (abnormal number of chromosomes).

  • Types of Aneuploidy:

    • Monosomy: Loss of one chromosome (e.g. Turner Syndrome - 45,X).

    • Trisomy: Addition of one chromosome (e.g. Down Syndrome - 47,+21).

Maternal Age and Risk Factors

  • Impact of Maternal Age: Higher age correlates with increased trisomy risks. For example:

    • Mother at age 20: 1 in 2,000 chance of Down Syndrome

    • Mother at age 35: 1 in 111

    • Mother at age 45: 1 in 33

Specific Chromosomal Conditions

  • Turner Syndrome (45,X): Affects females; results in short stature and infertility.

  • Klinefelter Syndrome (47,XXY): Affects males; leads to infertility and developmental issues.

  • XYY Syndrome (47,XYY): Associated with taller stature and potential developmental delays.

  • Trisomy 13 (Patau Syndrome): Severe, often lethal congenital malformations.

  • Trisomy 18 (Edwards Syndrome): Severe and lethal, with various malformations.

  • Trisomy 21 (Down Syndrome): Common developmental disorder with varying severity.

Summary of Chromosome Abnormalities

  • Statistics: Out of 10,000 embryos, many show chromosomal abnormalities.

  • Common Abnormalities: Include extra chromosomes (trisomy) or missing a chromosome (monosomy).

Conclusion

Understanding cytogenetics through the study of chromosomes, their structures, functions, and associated disorders helps identify genetic issues and informs treatment options and genetic counseling.