Genetics is the blueprint of life, explaining how traits are inherited from one generation to the next. Chapter 1 of the SSLC Biology syllabus, Genetics of Life, introduces students to the fascinating world of DNA, chromosomes, genes, and inheritance patterns. This chapter builds a strong foundation in understanding how living organisms store, transmit, and express genetic information.
In this study guide, you’ll find well-structured questions and answers covering all major topics such as:
- DNA structure and its organization within cells
- The role of chromosomes and genes
- RNA and how proteins are synthesized
- Mendel’s laws of inheritance
- Non-Mendelian inheritance patterns like incomplete dominance and co-dominance
- Causes of genetic variation including mutation and crossing over
Whether you're preparing for your SSLC board exams or just want to strengthen your grasp of genetics, this guide provides clear explanations, real-life examples, and important ratios and facts to help you score high and understand better. Let's dive into the genetic code that shapes every living being!
Part 1: DNA Structure and Organisation
Q1. What is DNA, and where is it located?
Answer: DNA (Deoxyribonucleic acid) is the genetic material that contains all hereditary information. It is located in the nucleus of cells, organised within chromosomes.
Q2. Who discovered the structure of DNA and when?
Answer: In 1953, James Watson and Francis Crick presented the double helical model of DNA based on X-ray diffraction studies by Rosalind Franklin and Maurice Wilkins. The crucial evidence came from "Photo 51" taken by Rosalind Franklin. Watson, Crick, and Wilkins received the Nobel Prize in Medicine in 1962.
Q3. What are the components of a nucleotide?
Answer: Each nucleotide consists of three components:
- Deoxyribose sugar (5-carbon sugar)
- Phosphate group
- Nitrogen base (Adenine, Thymine, Guanine, or Cytosine)
Q4. Describe the structure of DNA.
Answer: DNA has a double helix structure with:
- Two strands running in opposite directions
- Strands made of sugar-phosphate backbone
- Rungs formed by nitrogen base pairs (A-T and G-C)
- Complementary base pairing (A pairs with T, G pairs with C)
Q5. How does such large DNA fit into tiny cells?
Answer: DNA is packed efficiently through a hierarchical organization:
- DNA wraps around histone proteins to form nucleosomes
- Nucleosomes coil and pack together to form chromatids
- Two chromatids join at the centromere to form a chromosome
- This packaging allows 6 feet of DNA to fit into microscopic cells
Part 2: Chromosomes and Genes
Q6. How many chromosomes do humans have?
Answer: Humans have 46 chromosomes (23 pairs):
- 22 pairs of autosomes (somatic chromosomes)
- 1 pair of sex chromosomes (XX for females, XY for males)
Q7. What are homologous chromosomes?
Answer: Homologous chromosomes are pairs of identical chromosomes that control the same traits. One chromosome from each pair is inherited from the mother and the other from the father.
Q8. What determines sex in humans?
Answer: Sex is determined by sex chromosomes:
- Females: 44 + XX
- Males: 44 + XY The SRY gene on the Y chromosome is responsible for male development.
Q9. What are some chromosomal abnormalities?
Answer:
- Turner syndrome (44+X0): Females with one X chromosome
- Triple-X syndrome (44+XXX): Females with three X chromosomes
- Klinefelter syndrome (44+XXY): Males with XXY chromosomes
- XYY syndrome (44+XYY): Males with XYY chromosomes
Q10. What is a gene?
Answer: A gene is a specific sequence of nucleotides in DNA that provides instructions for protein synthesis. Genes are responsible for characteristic features and control metabolic activities.
Part 3: RNA and Protein Synthesis
Q11. How does RNA differ from DNA?
Answer:
DNA characteristics:
- Double-stranded structure
- Contains deoxyribose sugar
- Nitrogen bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
- Function: Stores genetic information
RNA characteristics:
- Single-stranded structure
- Contains ribose sugar
- Nitrogen bases: Adenine (A), Uracil (U), Guanine (G), Cytosine (C)
- Function: Protein synthesis
Q12. What are the stages of protein synthesis?
Answer:
- Transcription: mRNA is formed from DNA in the nucleus using a specific gene sequence
- Translation: tRNA carries amino acids to ribosomes based on mRNA instructions, and rRNA helps combine amino acids to form proteins
Q13. What are the types of RNA and their functions?
Answer:
- mRNA (messenger RNA): Carries genetic information from DNA to ribosomes
- tRNA (transfer RNA): Transports specific amino acids to ribosomes
- rRNA (ribosomal RNA): Component of ribosomes that helps form bonds between amino acids
Part 4: Mendel's Laws of Inheritance
Q14. Who is known as the father of genetics and why?
Answer: Gregor Johann Mendel (1822-1884) is called the father of genetics. He conducted hybridization experiments on pea plants and established the fundamental laws of inheritance.
Q15. What is a monohybrid cross?
Answer: A monohybrid cross is a breeding experiment between parents that differ in a single trait. Mendel's monohybrid cross with tall and dwarf pea plants showed a 3:1 ratio in the F2 generation.
Q16. What are the key terms in genetics?
Answer:
- Alleles: Different forms of the same gene
- Phenotype: Observable characteristics of an organism
- Genotype: Genetic constitution responsible for characteristics
- Dominant trait: Trait expressed in F1 generation
- Recessive trait: Trait hidden in F1 but appears in F2 generation
Q17. What are Mendel's postulates from monohybrid cross?
Answer:
- A trait is controlled by two factors (alleles)
- In hybridization, only one trait is expressed in F1 (dominant), while the other remains hidden (recessive)
- During gamete formation, factors separate without mixing
- The ratio of dominant to recessive traits in F2 is 3:1
Q18. What is a dihybrid cross?
Answer: A dihybrid cross studies the inheritance of two pairs of contrasting traits simultaneously. Mendel's dihybrid cross showed that different traits are inherited independently without influencing each other.
Part 5: Non-Mendelian Inheritance
Q19. What is incomplete dominance?
Answer: Incomplete dominance occurs when neither allele is completely dominant over the other, resulting in a blended phenotype. Example: Red × White flowers = Pink flowers in four o'clock plants.
Q20. What is co-dominance?
Answer: Co-dominance occurs when both alleles are expressed simultaneously in the phenotype. Example: Roan coat pattern in cattle where both red and white hairs are present.
Q21. What is multiple allelism?
Answer: Multiple allelism occurs when a gene has more than two alleles. Example: ABO blood group system has three alleles (IA, IB, i) determining four blood types (A, B, AB, O).
Q22. What is polygenic inheritance?
Answer: Polygenic inheritance occurs when multiple genes control a single trait. Example: Human skin color is controlled by several genes affecting melanin production.
Part 6: Variations and Genetic Processes
Q23. What causes genetic variations?
Answer: Genetic variations are caused by:
- Crossing over: Exchange of genetic material between homologous chromosomes during meiosis
- Mutation: Sudden heritable changes in genetic constitution
- Recombination during fertilization: Random combination of alleles
Q24. What is crossing over?
Answer: Crossing over is the exchange of genetic material between paired homologous chromosomes during meiosis. It occurs at points called chiasmata and leads to new combinations of alleles, creating genetic diversity.
Q25. What is mutation?
Answer: Mutation is a sudden, heritable change in the genetic constitution of an organism. It can be caused by:
- Errors during DNA replication
- Exposure to chemicals
- Radiation exposure Mutations are important for evolution and genetic diversity.
Important Formulas and Ratios
Monohybrid Cross:
- F1 generation: All dominant phenotype
- F2 generation: 3:1 (dominant:recessive)
- Genotypic ratio: 1:2:1 (TT:Tt:tt)
Dihybrid Cross:
- F2 phenotypic ratio: 9:3:3:1
- Independent assortment of genes
Key Facts to Remember
- Human DNA length: If stretched, DNA from one cell would be 6 feet long
- Total human DNA: If all DNA from the human body were connected, it would wrap around Earth 2 million times
- Chromosome number varies by species, but each species has a fixed number
- Melanin determines skin color and varies based on geographical ancestry
- Environmental factors also influence gene expression
- Genetics has applications in medicine, agriculture, forensics, and research
Study Tips
- Practice drawing DNA structure and labeling components
- Work through Punnett squares for monohybrid and dihybrid crosses
- Memorize the differences between DNA and RNA
- Understand the steps of protein synthesis
- Practice identifying inheritance patterns (Mendelian vs. Non-Mendelian)
- Review chromosome abnormalities and their effects
- Connect genetic concepts to real-world examples