# Principles of Inheritance and Variation Notes | SwaVid Principles of Inheritance and Variation, NCERT Class 12 Biology: notes with key ideas, key terms, common mistakes and 3 quick checks with worked answers. Canonical: https://www.swavid.com/cbse/class-12/biology/chapter/principles-of-inheritance-and-variation Source: https://www.swavid.com/cbse/class-12/biology/chapter/principles-of-inheritance-and-variation # Principles of Inheritance and Variation ## Key terms ## Key ideas ## Common mistakes ## Quick checks ## Keep going ## Questions parents ask ## Class 12 Biology on SwaVid ### Mendel's laws of inheritance ### Incomplete dominance and co-dominance ### Dihybrid cross and independent assortment ### Chromosomal theory and linkage ### Sex determination mechanisms ### Mutation and pedigree analysis ### Mendelian and chromosomal disorders ### Next chapter ### Related chapter ### Related chapter ### All chapters SwaVid › CBSE › Class 12 › Biology › Principles of Inheritance and Variation Notes written by SwaVid from the NCERT textbook. Class 12 Biology, Chapter 4. Key ideas, formulas and common mistakes. CBSE prints 20 marks for Unit VII, not for this chapter alone. Short answer. This chapter explains the biological principles of inheritance and variation formulated through Gregor Mendel's experiments on garden peas. It explores how alleles segregate and assort independently, along with chromosomal theory, sex determination, linkage, and genetic disorders. Source: NCERT Biology (Reprint 2026-27) , Chapter 4, Principles of Inheritance and Variation Gregor Mendel conducted hybridisation experiments on garden peas (Pisum sativum) using seven pairs of contrasting traits. Based on monohybrid crosses, he proposed the Law of Dominance and the Law of Segregation. Incomplete dominance shows an intermediate phenotype in heterozygotes, as seen in the flower colour of Antirrhinum majus. Co-dominance occurs when both alleles express themselves equally, exemplified by human ABO blood groups determined by the I gene. Crosses involving two pairs of contrasting traits give a phenotypic ratio of 9:3:3:1 in the F2 generation. This led to the Law of Independent Assortment, stating that segregation of one pair of traits is independent of another pair. Sutton and Boveri noted that chromosomes parallel the behaviour of genes during cell division. T.H. Morgan used Drosophila melanogaster to prove the chromosomal theory and showed that closely linked genes do not assort independently. Sex determination can involve male heterogamety, such as XX-XY in humans and XX-XO in grasshoppers, or female heterogamety, such as ZW-ZZ in birds. In honeybees, sex is determined through a haplodiploid mechanism. Mutation is a sudden inheritable change in the DNA sequence, leading to alterations in phenotype. Pedigree analysis traces the inheritance of specific traits or abnormalities across multiple generations in human families. Mendelian disorders are caused by single gene alterations and include haemophilia, sickle-cell anaemia, and phenylketonuria. Chromosomal disorders arise from an abnormal number or structure of chromosomes, such as Down's syndrome, Turner's syndrome, and Klinefelter's syndrome. Try each question first. Then tap it to see the answer. Cross is Rr × Rr showing incomplete dominance. Gametes produced by each parent are R and r with equal probability. Offspring genotypes are 1 RR : 2 Rr : 1 rr. Phenotypes correspond directly to genotypes as red, pink, and white. 1 Red : 2 Pink : 1 White The cross is a standard Mendelian dihybrid cross: VVRR × vvrr yielding F1 hybrid VvRr. Selfing VvRr gives the classic dihybrid phenotypic ratio of 9:3:3:1. The double recessive phenotype corresponds to 1 part out of 16 total parts. 1/16 (or 6.25%) Father's genotype is Iᴬi, mother's genotype is Iᴮi. Possible combinations of alleles are IᴬIᴮ, Iᴬi, Iᴮi, and ii. These genotypes yield blood groups AB, A, B, and O respectively. A, B, AB, and O Molecular Basis of Inheritance . Evolution . Human Reproduction . The 13 chapters of Class 12 Biology . CBSE does not give marks to one chapter. It prints 20 marks for Unit VII, Genetics and Evolution, which includes Principles of Inheritance and Variation. No. They are written by SwaVid from the NCERT textbook. The CBSE curriculum is the reference for what is assessed. Molecular Basis of Inheritance. SwaVid is an AI tutor for Indian students in Classes 6 to 12. It finds the earlier ideas a child missed, then teaches today’s chapter. Start free to see how it works. - Ch 4 NCERT book - 20 marks, Unit VII - 3 quick checks - Confusing incomplete dominance with co-dominance . Remember that incomplete dominance yields an intermediate blended phenotype (such as pink flowers from red and white parents), while co-dominance expresses both parental phenotypes fully and side-by-side (such as blood group AB). - Assuming sex determination in birds is the same as in humans . Humans show male heterogamety (males are XY, females are XX), whereas birds show female heterogamety (females are ZW, males are ZZ). - Treating Turner's syndrome as an autosomal disorder . Turner's syndrome is a sex chromosome disorder caused by the loss of one X chromosome, giving a karyotype of 45 with XO. ## Key Links - [CBSE](https://www.swavid.com/cbse) - [Class 12](https://www.swavid.com/cbse/class-12) - [Biology](https://www.swavid.com/cbse/class-12/biology/chapters) - [Start free →](https://www.swavid.com/signup) - [All 13 chapters](https://www.swavid.com/cbse/class-12/biology/chapters) - [NCERT Biology (Reprint 2026-27)](https://ncert.nic.in/textbook.php) - [Open notes](https://www.swavid.com/cbse/class-12/biology/chapter/molecular-basis-of-inheritance) - [Open notes](https://www.swavid.com/cbse/class-12/biology/chapter/evolution) - [Open notes](https://www.swavid.com/cbse/class-12/biology/chapter/human-reproduction) - [Class 12 Biology](https://www.swavid.com/cbse/class-12/biology/chapters) - [Start free →](https://www.swavid.com/signup)