USMLE Guide: Chromosomes and Genes

Introduction

This guide aims to provide a comprehensive understanding of chromosomes and genes, which are fundamental concepts in genetics. It covers their structure, function, inheritance patterns, and associated disorders. It will help you prepare for the United States Medical Licensing Examination (USMLE) and enhance your knowledge in genetics.

Table of Contents

1. Chromosomes
   • Definition and Structure
   • Types of Chromosomes
   • Karyotyping
2. Genes
   • Definition and Structure
   • Gene Expression
   • Inheritance Patterns
3. Chromosomal Abnormalities
   • Numerical Abnormalities
   • Structural Abnormalities
4. Genetic Disorders
   • Autosomal Dominant Disorders
   • Autosomal Recessive Disorders
   • X-Linked Disorders

1. Chromosomes

Definition and Structure

• Chromosomes are thread-like structures made up of DNA and proteins found inside the nucleus of a cell.
• Each chromosome consists of two sister chromatids held together by a centromere.
• Humans have 23 pairs of chromosomes, totaling 46 chromosomes (except for sperm and egg cells).

Types of Chromosomes

• Autosomes: The first 22 pairs of chromosomes, which are the same in males and females.
• Sex chromosomes: The 23rd pair of chromosomes, determining an individual's sex.
  • Females have two X chromosomes (XX).
  • Males have one X and one Y chromosome (XY).

Karyotyping

• Karyotyping is a technique used to analyze an individual's chromosomes.
• It involves staining and photographing the chromosomes to examine their number, size, and structure.
• Karyotyping can help diagnose chromosomal abnormalities and genetic disorders.

2. Genes

Definition and Structure

• Genes are segments of DNA that contain instructions for building proteins.
• Each gene is located at a specific position on a chromosome called a locus.
• Genes determine various traits, such as eye color, height, and susceptibility to diseases.

Gene Expression

• Gene expression involves the process of turning a gene's instructions into a functional protein.
• It includes transcription (DNA to RNA) and translation (RNA to protein).
• Regulation of gene expression ensures that specific genes are activated or silenced in different cell types.

Inheritance Patterns

• Mendelian inheritance patterns describe how genes are passed from parents to offspring.
• Autosomal dominant: A single copy of the mutated gene from one parent is sufficient to cause the disorder.
• Autosomal recessive: Two copies of the mutated gene (one from each parent) are needed to manifest the disorder.
• X-linked: Genes located on the X chromosome exhibit different inheritance patterns in males and females.

3. Chromosomal Abnormalities

Numerical Abnormalities

• Aneuploidy: Abnormal number of chromosomes due to errors in chromosome segregation during meiosis.
  • Examples: Down syndrome (Trisomy 21), Turner syndrome (Monosomy X).

Structural Abnormalities

• Deletions: Loss of a portion of a chromosome.
• Duplications: Extra copies of a chromosome segment.
• Translocations: Chromosome segments exchange places.
• Inversions: Reversal of a chromosome segment.
• These abnormalities can lead to various genetic disorders.

4. Genetic Disorders

Autosomal Dominant Disorders

• Huntington's disease: Neurological disorder causing progressive degeneration of brain cells.
• Marfan syndrome: Connective tissue disorder affecting the skeletal, cardiovascular, and ocular systems.

Autosomal Recessive Disorders

• Cystic fibrosis: Affects the lungs, digestive system, and other organs, causing breathing and digestion difficulties.
• Phenylketonuria (PKU): Inability to break down the amino acid phenylalanine, leading to intellectual disability if untreated.

X-Linked Disorders

• Hemophilia: Impaired blood clotting due to a deficiency in clotting factors.
• Duchenne muscular dystrophy: Progressive muscle weakness and degeneration.

Remember to dive deeper into each disorder, associated symptoms, diagnostic methods, and available treatments while preparing for the USMLE. Understanding the mechanisms underlying these disorders will help you with clinical decision-making and patient management.

Good luck with your USMLE preparation!