L4 Basic Genetics

🧠 Concepts Introduced

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Learning Outcomes:

• Define chromosomes and describe their structure.
• Explain the types of chromosomal variations and their consequences.
• Discuss mechanisms of sex determination in humans and other species.
• Relate chromosomal abnormalities to genetic disorders

Chromosome Introduction

Definition: Thread-like structures made of DNA and proteins (histones).

Location: Found in the nucleus of eukaryotic cells.

Function: Carry genetic information in the form of genes.

Human cell: 46 chromosomes (23 pairs).

Chromosomes act as vehicles of inheritance. Each contains hundreds to thousands of genes. The number and shape of chromosomes are species-specific.

Chromosome structure

Components: DNA, histone proteins, non- histone proteins. +. Eight histone proteins form an octamer core to create a nucleosome—the basic structural unit of chromatin ("beads on a string").

Key regions:

1. Centromere
2. Telomeres : Specialized repetitive DNA sequences (e.g., TTAGGG in humans) located at the ends of linear chromosomes. They protect the end of the chromosome from degradation, prevent fusion with neighbor chromosomes, and preserve genomic integrity across successive cellular replications.
3. p and q arms = p arm (short arm of chromosome), q arm (long arm of chromosome)

Chromatin types: Euchromatin (active) and heterochromatin (inactive)

Explain chromatin condensation and role of centromeres in segregation during mitosis/meiosis. ​Chromatin Condensation: During interphase, chromatin is relaxed to permit gene expression and replication. As the cell enters prophase of mitosis/meiosis, chromatin undergoes dramatic condensation mediated by protein complexes like condensin and post-translational histone modifications. This compacts long DNA molecules into dense, distinct chromosomes to prevent tangling or shearing during cell division.

​Role of Centromeres in Segregation: During metaphase, chromosomes align along the metaphase plate. Kinetochore proteins attach to spindle microtubules at the centromere region. In anaphase, sister chromatids (in mitosis/meiosis II) or homologous chromosomes (in meiosis I) are pulled toward opposite cell poles as microtubules shorten. The centromere ensures equal and accurate distribution of genetic material to each daughter cell.

Types of Chromosome based on centromere position

Metacentric - centromere in the middle.

Submetacentric - slightly off-center.

Acrocentric - near one end.

Telocentric - at the end.

Show diagram comparing chromosome shapes. Explain relevance in karyotyping. ​Classification: Categorizing chromosomes into metacentric, submetacentric, acrocentric, and telocentric types is fundamental for karyotyping.

​Analysis: Understanding these distinct centromere positions and shapes allows for proper structural evaluation and organization during karyotyping procedures.

​Classification: Categorizing chromosomes into metacentric, submetacentric, acrocentric, and telocentric types is fundamental for karyotyping.
​Analysis: Understanding these distinct centromere positions and shapes allows for proper structural evaluation and organization during karyotyping procedures.

Chromosome number & karyotype

Human karyotype: 22 pairs autosomes + 1 pair sex chromosomes. Karyotyping technique overview. Importance in identifying abnormalities.

Describe how karyotyping is used in prenatal diagnosis (e.g., amniocentesis) Amniocentesis: A procedure in which amniotic fluid is removed from the uterus Usually done between week 15 and 20 of pregnancy Chromosomal abnormalities can be diagnosed before birth by analyzing cells in the amniotic fluid

Chromosomal variation overview

Definition: Numerical or structural changes in chromosomes.

Two main types: Numerical variations (aneuploidy, polyploidy). Structural variations (deletion, duplication, inversion, translocation).

1. Numerical Variations - Aneuploidy

Gain or loss of individual chromosomes.

Examples: Trisomy 21 (Down syndrome) extra chromosome 21 Monosomy X (Turner syndrome) (XO)missing one X chromosome, affects females only XXY (Klinefelter syndrome) extra X chromosome in males, results in tall stature and reduced fertility

Explain nondisjunction during meiosis as a major cause non-disjunction - occurs during meiosis where there is incomplete separation of chromosomes which leads to one daughter cell having one extra chromosome and on daughter cell having one less chromosome which is also known as aneuploidy

2. Numerical Variations - Polyploidy

Triploidy (3n), tetraploidy (4n), etc. Common in plants, rare in humans. Causes developmental arrest in mammals.

Discuss role of polyploidy in evolution and plant breeding.

• Rapid Speciation: Polyploidy can create immediate reproductive isolation from diploid ancestors, giving rise to a new species in a single generation. Roughly 30% to 70% of flowering plants (angiosperms) have a polyploid history.

• Mutation Buffering: Extra copies of genes act as a genetic safety net. If one gene copy mutates or fails, functional copies compensate, allowing the plant to tolerate genetic changes.

• Environmental Adaptation & Novel Traits: Whole-genome duplication alters cell size, stomatal density, and physiology. This helps polyploids colonize new or extreme ecological niches (such as colder, drier, or disturbed habitats) better than their diploid relatives.

• Increased Heterozygosity: Allopolyploids combine distinct parental genomes, capturing high levels of genetic diversity that fuel evolutionary flexibility.

3. Structural Variations - Deletions and Duplications

Deletion: Loss of chromosome segment (e.g., Cri-du-chat). Duplication: Repetition of chromosome segment.

Use diagrams to show how errors during crossing-over lead to duplication/deletion

4. Structural Variations - Inversions and Translocations

Inversion: Reversal of a segment (paracentric - includes centromere or pericentric - excludes centromere). Translocation: Exchange between non-homologous chromosomes (reciprocal or robertsonian)

Emphasize role in genetic diseases and cancers (e.g., Philadelphia chromosome in leukemia

• Philadelphia Chromosome (9 & 22): A well-known reciprocal translocation between chromosome 9 and chromosome 22.
• This swap fuses the (BCR) gene from chromosome 22 with the (ABL1) gene from chromosome 9.
• The resulting (BCR)(ABL1) fusion gene encodes a constitutively active tyrosine kinase, which drives the uncontrolled proliferation of white blood cells.
• It is present in over 90% of Chronic Myeloid Leukemia (CML) cases and a subset of Acute Lymphoblastic Leukemia (ALL) cases.
• More broadly, oncogenic translocations can place proto-oncogenes under the control of active immunoglobulin or T-cell receptor promoters (e.g., MYC in Burkitt lymphoma), triggering malignancy.

5. Chromosomal Aberrations - Clinical Examples

Down syndrome (Trisomy 21). Turner syndrome (45,X). Klinefelter syndrome (47,XXY). Cri-du-chat syndrome (5p deletion)

Highlight physical and developmental characteristics.

Introduction to Sex Determination

Definition: Process by which an organism develops as male or female.

Importance: Ensures balanced sex ratio and reproductive diversity.

Two types: Genetic and environmental

Chromosomal Basis of Sex Determination

XX-XY system (humans, mammals). ZZ-ZW system (birds, reptiles) XO system (grasshoppers).

Compare systems using diagram; emphasize universal principle: specific genes drive gonadal differentiation

Human Sex Chromosomes

X chromosome: Large, carries ~800 genes. Y chromosome: Small, carries ~70 genes (SRY gene critical). X-linked inheritance examples: Hemophilia, color blindness.

Role of SRY Gene

SRY = Sex-determining Region on Y chromosome. Function: Triggers testis development. Mutation or deletion can lead to sex reversal (XY female).

Explain SRY pathway → testis → testosterone male phenotype.

Mechanisms of Dosage Compensation

Females have two X chromosomes; males have one. Solution: X-inactivation (Barr body formation).

X-inactivation permanently silences one X chromosome into a Barr body in each female mammalian cell during early development to equalize gene dosage with males. This inactivation tag is erased in eggs, meaning children always inherit a fully active maternal X chromosome. Because the silencing process is completely random, females are genetic mosaics. If a female carries both a healthy and an unhealthy X chromosome, she usually remains an unaffected "carrier" because roughly 50% of her cells utilize the healthy copy. However, it is entirely possible for the healthy X to be inactivated instead; if this happens disproportionately by chance or survival (known as skewed X-inactivation), the female can express the genetic condition similarly to a male. If both inherited X chromosomes are completely healthy, the random inactivation choice has no impact on health.

Example: Calico cat coat color mosaicism

Abnormalities in Sex Chromosomes

Turner syndrome (X0). Klinefelter syndrome (XXY). Triple X females (XXX). XYY males.

Discuss how abnormal sex chromosome numbers affect fertility and phenotype.

Mosaicism and Chimerism

Mosaicism: Presence of two or more genetically different cell lines from one zygote. Chimerism: Derived from fusion of two different zygotes.

Clinical relevance: Affects diagnosis of chromosomal disorders.

Mosaicism and chimerism both describe individuals who possess two or more genetically distinct cell lines, but they differ fundamentally in their biological origins and impact on clinical diagnostics. Mosaicism occurs when a single fertilized egg (zygote) undergoes a post-zygotic mutation or cell division error during development, branching into a healthy cell line alongside a mutated or chromosomally abnormal line. Conversely, chimerism occurs when two or more separate zygotes fuse together—or exchange blood cells in utero—resulting in an individual composed of entirely distinct genetic blueprints. In clinical practice, both phenomena heavily complicate the diagnosis of chromosomal disorders, as standard genetic testing can easily miss abnormal cell lines depending on which tissue is sampled, requiring highly sensitive methods like SNP arrays or next-generation sequencing for accurate detection.

Environmental Sex Determination

Definition: Sex determined by environmental conditions, not chromosomes.

Examples: Temperature-dependent sex in reptiles (turtles, crocodiles). Hormonal influence in some fish species.

Evolution of Sex Chromosomes

Concept: Sex chromosomes evolved from autosomes through differentiation.

Suppression of recombination between proto-X and proto-Y led to degeneration of Y chromosome

Techniques in Chromosomal Study

Karyotyping: Chromosome visualization. FISH: Detects specific DNA sequences. CGH & NGS: Identify microdeletions, duplications.

Application: Used in prenatal and cancer diagnostics

• Karyotyping:

  • Function: Involves isolating, staining (such as G-banding), and visually examining metaphase chromosomes under a microscope.
  • Resolution: Best for large-scale changes greater than 5–10 megabases (Mb) (e.g., full extra or missing chromosomes, large structural rearrangements, and balanced translocations).
  • Limitation: Requires 4–7 days of cell culture and has a lower resolution limit.

• FISH (Fluorescence In Situ Hybridization):

  • Function: Uses fluorescently tagged single-stranded DNA probes to bind complementary sequences on specific regions of chromosomes.
  • Resolution: Detects microdeletions, small translocations, and gene duplications down to 200 kilobases (kb) to 5 Mb.
  • Advantage: Does not require dividing/metaphase cells and can be performed quickly on interphase cells.

• CGH (Comparative Genomic Hybridization / aCGH) & NGS (Next-Generation Sequencing):

  • Function: Molecular and genomic methods that compare test DNA against reference DNA or sequence base-by-base.
  • Resolution: Identifies sub-microscopic copy number variations (CNVs), microdeletions, microduplications, and single-gene or lower-level structural variations across the entire genome.
  • Advantage: High-throughput and high-resolution detection of subtle genomic imbalances that traditional microscopy misses.

Clinical Applications

• Prenatal Diagnostics: Used on samples like amniotic fluid or chorionic villi to rapidly screen for or confirm common aneuploidies (e.g., Trisomy 21, 13, 18), sex chromosome abnormalities, and sub-microscopic microdeletions. You can read more about combined applications in prenatal settings via
• Cancer Diagnostics (Oncology): Used for tumor subtyping, identifying gene fusions (such as BCR-ABL in chronic myeloid leukemia), monitoring treatment response, and detecting recurrent genomic gains or losses. Learn more about general testing strategies through

Summary

Chromosomes carry hereditary material essential for inheritance. Variations (numerical/structural) lead to diversity or disease.

Sex determination involves genetic and environmental factors. SRY gene, X-inactivation, and chromosome anomalies explain sex-linked traits