Introduction to Genetics including Chromosomes

Introduction to Genetics including Chromosomes

1. Introduction to Genetics Sreenu Thalla Associate Professor Department of Pharmacology

2. Definition of Genetics Genetics is the branch of biological science that deals with the study of genes , genetic variation , and heredity in organisms. It explains how traits and characteristics are passed from one generation to the next through DNA. Genetics plays a crucial role in understanding life processes, inheritance patterns, genetic disorders, evolution, and even the development of new medical treatments.

3. Historical Background Gregor Johann Mendel (1822–1884), an Austrian monk, is known as the Father of Genetics . Through his experiments with pea plants ( Pisum sativum ), Mendel discovered the fundamental laws of inheritance in the mid-1800s. His work remained unrecognized until the early 20th century when it was rediscovered independently by Hugo de Vries , Carl Correns , and Erich von Tschermak . Later, the discovery of DNA (Deoxyribonucleic Acid) as the genetic material by James Watson and Francis Crick in 1953 marked a revolutionary step in modern genetics.

4. Basic Concepts in Genetics Gene A gene is a segment of DNA that contains the instructions for the synthesis of a specific protein. It is the basic unit of heredity. DNA (Deoxyribonucleic Acid) DNA is a double-helical molecule composed of nucleotides (adenine, thymine, cytosine, guanine). It carries the genetic instructions used in growth, development, functioning, and reproduction.

5. Chromosomes Chromosomes are long threads of DNA wound around proteins called histones. Humans have 23 pairs of chromosomes (46 in total), including 22 pairs of autosomes and 1 pair of sex chromosomes (XX or XY). Alleles Alleles are different versions of a gene found at the same locus on homologous chromosomes. An individual may inherit two identical alleles (homozygous) or two different alleles (heterozygous) for a trait.

6. Genotype and Phenotype Genotype refers to the genetic makeup of an organism (e.g., Aa, BB). Phenotype is the physical or observable expression of the genotype (e.g., brown eyes, tall plant).

7. Mendelian Principles of Inheritance Law of Segregation : Each individual has two alleles for each gene, and these alleles segregate during gamete formation, so each gamete carries only one allele. Law of Independent Assortment : Genes for different traits are inherited independently of one another, provided they are not linked on the same chromosome. Law of Dominance : In a heterozygote, one allele may mask the expression of another. The expressed allele is dominant; the masked one is recessive.

8. Branches of Genetics Classical Genetics : Focuses on Mendelian inheritance patterns and chromosomal behaviour during meiosis. Molecular Genetics : Studies the structure and function of genes at the molecular level, including DNA replication, transcription, and translation. Population Genetics : Explores gene frequency changes in populations under the influence of evolutionary forces like selection, mutation, and genetic drift. Quantitative Genetics : Deals with traits that are controlled by multiple genes (polygenic traits), such as height and intelligence. Genomics : Involves the study of entire genomes, including mapping, sequencing, and analysing DNA content.

9. Applications of Genetics Medical Genetics : Understanding genetic diseases (e.g., cystic fibrosis, thalassemia), gene therapy, and pharmacogenomics. Agriculture : Breeding high-yield or disease-resistant crops and livestock. Forensic Science : DNA fingerprinting for crime investigation and paternity testing. Biotechnology : Genetic engineering, recombinant DNA technology, and CRISPR gene editing. Evolutionary Biology : Studying genetic variation to understand evolution and speciation.

10. Modern Advances in Genetics Human Genome Project (HGP) : Completed in 2003, it successfully mapped the entire human genome, identifying all the genes and their locations. CRISPR-Cas9 Technology : A revolutionary tool that allows for precise genome editing. Gene Therapy : Techniques to treat or prevent disease by inserting genes into a patient’s cells.

11. Chromosomes Chromosomes are thread-like structures located within the nucleus of animal and plant cells. They are made of DNA (deoxyribonucleic acid) and proteins , and carry genetic information that is passed from parents to offspring. Chromosomes play a key role in cell division , genetic inheritance , and cellular function .

12. Structure of Chromosomes Chromosomes consist of the following components: DNA (Deoxyribonucleic Acid) The core component of a chromosome. DNA is a long, double-stranded molecule composed of nucleotides (adenine, thymine, cytosine, guanine). Carries genes—the basic units of heredity. Histone Proteins DNA wraps around histone proteins to form nucleosomes , which help package the DNA into a compact structure. These proteins also play a role in gene regulation

13. Chromatin When the cell is not dividing, chromosomes exist as chromatin , a loosely packed form of DNA and proteins. During cell division, chromatin condenses to form visible chromosomes. Centromere The constricted region of a chromosome. It divides the chromosome into two arms: short arm (p) and long arm (q) . Essential for proper separation of chromosomes during mitosis and meiosis. Telomeres Repetitive nucleotide sequences at the ends of chromosomes. Protect the chromosome from deterioration or fusion with neighbouring chromosomes. Shorten with each cell division, linked to aging and cancer.

14. Type Description Metacentric Centromere is in the middle; arms are equal in length. Submetacentric Centromere is slightly off-center; arms are unequal. Acrocentric Centromere is close to one end; one very short arm. Telocentric Centromere is at the very end (not found in humans). Types of Chromosomes Chromosomes are classified based on the location of the centromere

15. Number of Chromosomes in Humans Humans have 46 chromosomes , arranged in 23 pairs : 22 pairs of autosomes (non-sex chromosomes). 1 pair of sex chromosomes : Females: XX Males: XY Each parent contributes one chromosome to each pair.

16. Functions of Chromosomes Genetic Information Storage Carry genes that determine inherited traits such as eye color , height, and disease susceptibility. Regulation of Gene Expression Chromosomal structure helps control which genes are turned on or off. Transmission of Hereditary Information During reproduction, chromosomes pass genetic material to the next generation. Cell Division and Growth Chromosomes ensure DNA is accurately replicated and distributed in mitosis (body cell division) and meiosis (gamete formation).

17. Chromosomal Disorders Abnormalities in chromosome number or structure can lead to genetic disorders: Numerical Abnormalities Down Syndrome (Trisomy 21) – Extra copy of chromosome 21. Turner Syndrome (XO) – Missing one sex chromosome in females. Klinefelter Syndrome (XXY) – Extra X chromosome in males. Structural Abnormalities Deletion – Loss of a chromosome segment. Duplication – Repetition of a segment. Inversion – Reversed segment. Translocation – Segment moves to a different chromosome.

18. Chromosomes in Cell Division Mitosis Ensures each daughter cell receives an exact copy of the parent cell's chromosomes. Meiosis Reduces the chromosome number by half to form gametes (sperm and egg). Introduces genetic variation through crossing-over and independent assortment. Chromosomes in Other Organisms Bacteria have a single, circular chromosome in the cytoplasm (not in a nucleus). Plants and animals have multiple linear chromosomes in a membrane-bound nucleus. The number of chromosomes varies widely among species (e.g., fruit fly: 8, dog: 78, fern: over 1000).