karyotype 2

Human Genetics and Meiosis

Introduction

  • Study of inheritance patterns in humans

  • Focus on specific genetic conditions and karyotyping

Chromosomes

  • Oculocutaneous albinism: Genetic disorder related to pigmentation

Karyotype

  • Definition: A photomicrograph of chromosomes arranged according to a standard classification.

  • Arrangement of chromosomes is digital to match with their homologous or 'partner' chromosome.

    • Homologous chromosomes:

    • Same size

    • Same shape

    • Carry same genes

    • One chromosome inherited from each parent.

  • They are numbered according to size.

Sex Determination with Karyotype

  • A normal human female has 23 exact pairs of chromosomes, with both #23 chromosomes being X.

  • A normal human male contains a Y chromosome in pair #23 along with an X chromosome.

Chromosomal Abnormalities

Trisomy 21

  • Results in Down Syndrome.

  • Characterized by three copies of #21 chromosome.

Correlation Between Mother's Age and Trisomy 21 Incidence

  • The chance of giving birth to a child with Down Syndrome correlates with maternal age:

    • Age 20: 1 in 1925

    • Age 25: 1 in 1205

    • Age 30: 1 in 885

    • Age 35: 1 in 365

    • Age 40: 1 in 110

    • Age 45: 1 in 32

Monosomy X

  • Characterized by having only one copy of the X chromosome, known as Turner’s syndrome.

Obtaining DNA Samples for Karyotypes

  • Methods:

    • Packed red and white blood cells (including lymphocytes) obtained through a centrifuge process.

    • Use of a fixative and staining to prepare white blood cells for analysis.

Methods of Sampling

  1. Amniocentesis: Obtaining amniotic fluid containing fetal cells.

  2. Chorionic Villi Sampling: Removing cells from the chorion that contains fetal tissue.

Meiosis Overview

  • Meiosis is the process of creating sperm or egg cells from a diploid cell.

    • Detection of chromosomal number abnormalities originates during meiosis.

    • Mistakes during chromosome separation can lead to irregularities in gametes, resulting in either having too many or too few chromosomes.

Stages of Meiosis

Meiosis I

  1. Interphase I: Cells undergo DNA replication, forming duplicate chromosomes.

  2. Prophase I: Homologous chromosomes pair to form tetrads.

  3. Metaphase I: Spindle fibers attach to chromosomes.

  4. Anaphase I: Homologous chromosomes are pulled to opposite ends of the cell.

  5. Telophase I and Cytokinesis: Nuclear membranes form and the cell divides into two haploid cells.

Meiosis II

  • Results in four haploid daughter cells, each with half the original chromosome number.

  1. Prophase II: Spindle fibers bind to sister chromatids.

  2. Metaphase II: Chromosomes align similar to mitosis.

  3. Anaphase II: Sister chromatids separate.

  4. Telophase II: Nuclear membranes form around each set of chromatids.

Crossing Over

  • Occurs during Prophase I: where homologous chromosomes randomly exchange genetic material leading to new allele combinations.

Chromosomal Disorders

  • Types of chromosomal disorders:

    • Deletion: Missing parts of chromosome.

    • Duplication: Copied parts of chromosome.

    • Inversion: Parts of chromosomes are flipped.

    • Translocation: Chromosomes swap segments.

Human Genetic Disorders from Deleterious Genes

  • Types of genetic disorders:

    • Sex-linked: Genes found on X or Y chromosome.

    • Recessive: Requires 2 copies to express disorder.

    • Dominant: Requires only 1 copy to express disorder.

Recessive Disorders

  • Example: Gene for sickle-cell anemia is recessive (SS).

    • Probability of inheriting sickle-cell anemia from heterozygous parents (Ss): 25%.

Dominant Disorders

  • Example: Huntington disease is a dominant disorder (Hh).

    • Probability of inheriting from heterozygous parent (Hh): 50%.

Sex-linked Disorders

  • Example: Hemophilia, a recessively inherited disorder on the X chromosome.

    • Inheritance patterns affect male and female progeny differently.

Selected Examples of Human Genetic Disorders

Type

Name of Condition

Effects

X-linked recessive disorders

Hemophilia

Faulty blood clotting

X-linked recessive disorders

Duchenne muscular dystrophy

Wasting of muscles

X-linked recessive disorders

Color Blindness

Inability to distinguish red from green

Autosomal recessive disorders

Albinism

No pigmentation in skin

Autosomal recessive disorders

Sickle-cell anemia

Decreased oxygen to brain and muscles

Autosomal recessive disorders

Cystic fibrosis

Impaired lung function

Autosomal recessive disorders

Phenylketonuria

Mental retardation

Autosomal recessive disorders

Tay-Sachs disease

Nervous system degeneration in infants

Autosomal dominant disorders

Huntington disease

Brain tissue degeneration

Aberrations in chromosome number

Down syndrome

Mental retardation, shortened life span

Pedigrees

  • A pedigree chart displays genetic traits through generations, indicating inheritance patterns.

Symbols in Pedigrees

  • Males are represented by squares and females by circles, with shaded symbols indicating affected individuals.

Conclusion

  • Understanding of these genetic principles is crucial for diagnosing and understanding human genetic disorders and for discussing the impact of genetic variations on health.