Class 12 Biology - CBSE
Principles of Inheritance and Variation
Principles of Inheritance and Variation forms the core of genetics in Class 12 Biology CBSE syllabus. It explores how traits are passed from parents to offspring, unraveling Gregor Mendel's laws of inheritance and deviations like incomplete dominance and codominance. The chapter also covers chromosomal theory of inheritance, linkage, crossing over, and human genetic disorders such as pedigree analysis, hemophilia, sickle-cell anemia, and chromosomal aberrations like Down's and Turner's syndromes. Mastery of this chapter is vital for board exams as it consistently features heavy-weight numerical problems, reasoning questions, and diagram-based problems crucial for scoring high.
Start Learning FreeKey Concepts
Mendel's Laws of Inheritance
Mendel proposed the Law of Dominance, Law of Segregation (pure gametes), and Law of Independent Assortment based on his pea plant hybridization experiments.
Incomplete Dominance and Codominance
Incomplete dominance shows a blended phenotype (e.g., pink snapdragons), while codominance expresses both alleles equally and independently (e.g., AB blood group).
Chromosomal Theory of Inheritance
Proposed by Sutton and Boveri, it states that chromosomes are the vehicles of genetic heredity and behave parallelly to Mendelian factors.
Linkage and Recombination
Linked genes tend to stay together during inheritance exceptions to independent assortment, whereas recombination occurs due to crossing over during meiosis.
Pedigree Analysis
A study of family history used to trace the inheritance of specific traits or genetic disorders across generations through standardized charts.
Chromosomal Disorders
Disorders caused by absence, excess, or abnormal arrangement of one or more chromosomes, leading to conditions like Down syndrome, Klinefelter syndrome, and Turner syndrome.
Important Formulas
Board Exam Info
In the CBSE Class 12 Biology examination, this chapter along with its sequential chapter (Molecular Basis of Inheritance) under the Unit 'Genetics and Evolution' carries a high weightage of approximately 15-18 marks. Expect direct definitions, crosses to work out (monohybrid/dihybrid), pedigree analysis problems, and distinguishing questions (e.g., hemophilia vs. sickle-cell anemia).
Frequently Asked Questions
How do I solve dihybrid cross word problems quickly in the exam?
Break down the dihybrid cross into two independent monohybrid crosses (using the product rule of probability) rather than drawing a massive 16-square Punnett square.
What is the difference between incomplete dominance and codominance?
In incomplete dominance, neither allele is completely dominant, resulting in a blended intermediate phenotype (red + white = pink). In codominance, both alleles are fully expressed side-by-side without blending (red + white = roth/spotted).
Are pedigree analysis questions guaranteed in the board paper?
Yes, almost every CBSE paper features at least one pedigree chart where you must identify whether a given disorder is autosomal dominant, autosomal recessive, or X-linked.
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