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CBSE Class 12 · Biology

Principles of Inheritance and Variation

Official NCERT chapter from Biology (book code lebo1). ExamMaster notes are original teaching at CBSE Class 12 depth.

This lesson follows the official chapter “Principles of Inheritance and Variation” in Biology. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 12
  • Medium level
  • 8 concepts

1Mendel’s Laws of Inheritance

Mendel’s laws: segregation and independent assortment as taught — a 3:1 and a 9:3:3:1 as the school ratios when the gates sit. A law is a ratio-plus-reason, not a garden-story. One monohybrid is enough to start.

A 1:1 as “Mendel’s only ratio” is a miss if the cross was F2.

Figure. Dominance is the F1 looking like one parent. Segregation is the recessive returning in F2. Independent assortment is a later claim about two genes, not this one-trait chain.

How it works

  1. Name the law (segregate or assort)The write.
  2. Keep the school ratio as evidenceThe look.
  3. Refuse a story as the lawA ratio-reason.

2Inheritance of One Gene

One gene: a monohybrid as taught — two alleles, dominance if named, the 3:1 F2. One gene is a single-character cross, not a two-gene dump.

A 9:3:3:1 labelled one-gene is a miss.

Figure. Each cell is one equally likely fertilisation. Count cells for the look (3 tall, 1 dwarf) and again for the genotype (1 TT : 2 Tt : 1 tt). TT and Tt look the same.

How it works

  1. Name one character and two allelesThe cross.
  2. Read 3:1 in F2 if taughtThe ratio.
  3. Keep two-gene for the next headingA later word.

F2 count

A 3:1 look on 12 seeds. About how many show the dominant look?

  • 3/4 of 129
  • Recessive lookabout 3
  • Reada 3:1 evidence

Pro tip. A ratio is a count-pattern.

3Inheritance of Two Genes

Two genes: a dihybrid as taught — 9:3:3:1 when independent. Two genes is a two-character cross. Linkage as a leftover if the lesson named it (not 9:3:3:1).

A 3:1 labelled two-gene is a miss.

Figure. A dihybrid F2 is four phenotype classes, not a 3:1. The 9 : 3 : 3 : 1 lengths are the count of equally likely gamete pairs when two genes assort independently.

How it works

  1. Name two charactersThe cross.
  2. Read 9:3:3:1 if independent as taughtThe ratio.
  3. Keep linkage as the leftover if askedHonest extra.

4Sex Determination

Sex determination: XX/XY as taught (or the lesson’s other animal). Sex-chromosome is a pair-look, not a rumour. One human-school sentence is enough.

A “boys from the mother only” as the leftover this heading refuses if the lesson used XY.

Figure. The mother contributes only X. The father's gamete is X or Y, each half the time, so XX and XY are equally likely. The Y, not the mother, is what makes a son.

How it works

  1. Name the taught chromosome-lookThe determination.
  2. Keep it a pair-mapThe science.
  3. Refuse a rumourThis class.

5Mutation

Mutation: a change in DNA or chromosome as taught. Mutation is a change-word, not “always bad”. One named type (point, or a chromosome-look) is enough.

A habit labelled mutation is a miss.

Figure. A mutation here is a letter change in a sequence, not a new organism appearing. Only the third box changes: C to T. The rest of the row stays put so the change is the whole picture.

How it works

  1. Name a DNA or chromosome changeMutation.
  2. Keep one taught typeThe extra.
  3. Refuse always-bad as the testA change-word.

6Genetic Disorders

Genetic disorders: a named Mendelian or chromosomal look as taught (haemophilia, Down as framed) — name-plus-what-fails. Disorder is a school case, not a scare and not a family-blame.

A skull-sticker with no named look is a mute scare.

Figure. A Mendelian disorder is a change in one gene. A chromosomal disorder is a wrong chromosome count. Naming the scale — gene versus whole chromosome — is the first move, before any pedigree.

How it works

  1. Name the taught disorderThe word.
  2. Say gene or chromosome as the lookThe claim.
  3. Refuse a scare or a blameSchool-size.

7A ratio is evidence, not a decoration

A ratio is evidence, not a decoration: 3:1 or 9:3:3:1 must come from a count. A pretty Punnett with no count is a mute picture.

A 3:1 sticker on a dihybrid is the leftover.

Figure. 5474 to 1850 is about 2.96 to 1, which is why a 3:1 is claimed. A garden of eight plants cannot carry that claim. The ratio is evidence only after the count is large.

How it works

  1. Write the count or the PunnettThe evidence.
  2. Read the ratio as a test of the lawThe use.
  3. Refuse a decoration-ratioA number-test.

8A miss: treating one pedigree as every family

A miss: treating one pedigree as every family. A pedigree is a case; the law is the ratio-reason. One family does not rewrite Mendel.

A single tree labelled “all families” is the miss.

Figure. Four children in one house can sit 2 and 2 and still be a 3:1 process. The law is a claim about many families. Reading one pedigree as every family is the miss.

How it works

  1. Keep the pedigree as a caseOne family.
  2. Keep the law as the ratio-reasonThe science.
  3. Refuse one tree as every familyThe miss named.
A monohybrid F2 school ratio is
  1. 3:1 as taught
  2. 9:3:3:1
  3. 1:1 always

One gene.

Notes

  • Mapped to the official NCERT chapter “Principles of Inheritance and Variation”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • monohybrid F2: 3:1 as taught
  • dihybrid F2: 9:3:3:1 when independent

Recap

Hold these pegs from the official chapter “Principles of Inheritance and Variation”. The wording is ExamMaster’s teaching, not a textbook recap.

Mendel’s Laws of Inheritance
Mendel’s laws: segregation and independent assortment as taught — a 3:1 and a 9:3:3:1 as the school ratios when the gates sit.
Inheritance of One Gene
One gene: a monohybrid as taught — two alleles, dominance if named, the 3:1 F2.
Inheritance of Two Genes
Two genes: a dihybrid as taught — 9:3:3:1 when independent.
Sex Determination
Sex determination: XX/XY as taught (or the lesson’s other animal).
Mutation
Mutation: a change in DNA or chromosome as taught.
Genetic Disorders
Genetic disorders: a named Mendelian or chromosomal look as taught (haemophilia, Down as framed) — name-plus-what-fails.

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