Class 12 Biology Principles of Inheritance and Variation Notes by PW

Genetics is one of the most scoring chapters in Class 12 Biology, yet many students lose marks on small details. They mix up genotype and phenotype, write the wrong ratio for a cross, or forget which disorder is X-linked. Most of these mistakes happen because the terms are memorised without understanding what they mean.

PW’s Class 12 Biology notes explain the chapter the way we would in class: one idea at a time, building from basic terms to Mendel’s laws and then to mutations and disorders. Use them along with NCERT diagrams, practice questions and the PW online batches to check your understanding after each topic.

Basic Genetic Terms

Inheritance is the transfer of characters from parents to offspring. Heredity is the presence and passing on of those characters. Variation is the difference between offspring and their parents, and between individuals of the same species.

A gene is the unit of inheritance and is a segment of DNA. The order of organisation is cell, nucleus, chromosome, DNA and gene. The position of a gene on a chromosome is its locus. Alternative forms of the same gene are called alleles, such as the alleles for tallness and dwarfness in pea plants. You can revise how genes are carried in DNA structure and the genetic material before moving on.

A dominant allele is expressed even when another allele is present, while a recessive allele is expressed only when the dominant one is absent. In a Tt plant, T is expressed, so the plant is tall.

• Genotype is the genetic makeup, such as TT, Tt or tt.

• Phenotype is the visible appearance, such as tall or dwarf.

• Homozygous means identical alleles (TT or tt), and heterozygous means different alleles (Tt).

A tall plant can be either TT or Tt, so phenotype alone does not always reveal genotype.

The number of gamete types an individual forms is 2^n, where n is the number of heterozygous gene pairs. A genotype like YyRr has two heterozygous pairs, so it forms 2^2 = 4 types of gametes.

Memory tip: Count the heterozygous pairs first, then apply 2^n.

Mendel’s Experiments on Pea Plants

Gregor Mendel, known as the Father of Genetics, worked on pea plants from 1856 to 1863. He chose them because they are easy to grow, have a short life cycle, produce many seeds, have bisexual flowers that allow controlled pollination, and show clearly contrasting traits. He used 14 true-breeding varieties, which gave him seven pairs of contrasting characters.

To control pollination, he removed the anthers from a flower (emasculation) and covered it to keep out unwanted pollen (bagging). The seven characters he studied were:

• Seed shape: round (dominant) and wrinkled (recessive)

• Seed colour: yellow (dominant) and green (recessive)

• Pod shape: inflated (dominant) and constricted (recessive)

• Pod colour: green (dominant) and yellow (recessive)

• Flower colour: violet (dominant) and white (recessive)

• Flower position: axial (dominant) and terminal (recessive)

• Stem height: tall (dominant) and dwarf (recessive)

Monohybrid Cross and Mendel’s Laws

A monohybrid cross studies one character at a time. When a pure tall plant (TT) is crossed with a pure dwarf plant (tt), all F1 plants are Tt and tall. When the F1 plants are self-pollinated (Tt × Tt), the F2 generation shows:

• Genotypic ratio: 1 TT : 2 Tt : 1 tt

• Phenotypic ratio: 3 tall : 1 dwarf

The dwarf trait disappears in F1 and reappears in F2. This shows that alleles do not blend or get destroyed; they stay separate.

A Punnett square is a simple grid for predicting the genotypes and ratios of offspring. A test cross finds the unknown genotype of a plant showing the dominant trait by crossing it with a homozygous recessive plant. If the offspring are all tall, the plant was TT. If tall and dwarf appear in a 1:1 ratio, it was Tt.

Mendel’s three laws follow from these results. You can see how they connect in Mendel’s laws of inheritance.

1. Law of Dominance: When two different alleles are present, the dominant one is expressed and the recessive one stays hidden in the F1 generation.

2. Law of Segregation: The two alleles of a gene separate during gamete formation, so each gamete carries only one. The pair is restored at fertilisation.

3. Law of Independent Assortment: Alleles of different genes assort independently during gamete formation, provided the genes are not linked.

Incomplete Dominance, Codominance and Pleiotropy

In incomplete dominance, neither allele fully masks the other, so the heterozygote looks intermediate. When red-flowered snapdragon (Antirrhinum majus) is crossed with white, all F1 plants are pink. The F2 ratio is 1 red : 2 pink : 1 white, so the phenotypic and genotypic ratios are the same (1:2:1).

In codominance, both alleles express themselves fully in the heterozygote. The ABO blood groups show this clearly. The alleles IA and IB are codominant, so a person with IAIB has blood group AB. Both alleles dominate over i. This gives four blood groups and six genotypes: IAIA and IAi give group A, IBIB and IBi give group B, IAIB gives group AB, and ii gives group O. The ABO system also shows multiple allelism, because the population has three alleles although each person carries only two.

Pleiotropy means one gene affects more than one character. Phenylketonuria is a good example, because a single gene defect affects several features of the body.

Dihybrid Cross, Linkage and Recombination

A dihybrid cross follows two characters together, such as seed colour and seed shape. A YyRr plant forms four gamete types: YR, Yr, yR and yr. The F2 phenotypic ratio is 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green, and the genotypic ratio is 1:2:1:2:4:2:1:2:1.

Memory tip: A dihybrid ratio is two monohybrid ratios multiplied: (3:1) × (3:1) = 9:3:3:1.

Genes on the same chromosome do not assort independently. This is called linkage, and linked genes tend to be inherited together. Crossing over can break this association and produce non-parental combinations, which is called recombination. Genes that lie close together show strong linkage and low recombination. Genes that lie far apart show weak linkage and high recombination. Alfred Sturtevant used recombination frequency to map genes, and 1% recombination is about one map unit, or one centimorgan.

Sex Determination

In humans, females are XX and males are XY. Females form only X-bearing eggs, so they are homogametic. Males form both X-bearing and Y-bearing sperm, so they are heterogametic. The sperm therefore decides the sex of the child. 

• XX–XO: Females are XX and males are XO, as in grasshoppers.

• ZZ–ZW: Males are ZZ and females are ZW, as in birds.

• Honeybees: Unfertilised eggs develop into haploid drones, and fertilised eggs develop into diploid females. This is called haplodiploid sex determination.

Memory tip: Unfertilised egg means drone; fertilised egg means female.

Mutations and Genetic Disorders

A mutation is a sudden, heritable change in genetic material. It can be caused by physical, chemical or biological mutagens. Mutations are of three kinds: genomic (a change in chromosome number), chromosomal aberration (a change in chromosome structure) and gene mutation (a change in DNA sequence). Aneuploidy is the gain or loss of a single chromosome, so trisomy is 2n + 1 and monosomy is 2n − 1.

The main chromosomal disorders are:

• Down syndrome: trisomy of chromosome 21

• Klinefelter syndrome: XXY male

• Turner syndrome: XO female

Mendelian disorders arise from mutations in single genes. 

• Phenylketonuria: autosomal recessive, caused by a deficiency of the enzyme phenylalanine hydroxylase

• Sickle-cell anaemia: autosomal recessive, caused by a base substitution that produces abnormal haemoglobin

• Thalassaemia: a defect in the synthesis of globin chains

• Colour blindness: X-linked recessive

• Haemophilia: X-linked recessive blood clotting disorder

When two carrier parents have children, each child has a 25% chance of having sickle-cell anaemia. X-linked recessive disorders are more common in males because they have only one X chromosome.

Pedigree Analysis

A pedigree traces a trait through a family tree. A square stands for a male, a circle for a female and a shaded symbol for an affected person. Three simple rules help you read one:

• A trait appearing in every generation often points to dominant inheritance.

• A trait that skips generations often points to recessive inheritance.

• A trait passing from mother to son may point to sex linkage.

Pedigree analysis matters in humans because we cannot carry out controlled crosses as Mendel did with peas.

Once you understand how alleles separate and combine, ratios such as 3:1, 1:2:1 and 9:3:3:1 become easy to work out, and questions on linkage, sex determination and disorders become much simpler. PW’s Class 12 Biology notes help you revise these ideas in the right order, from basic terms to Mendel’s laws and then to mutations and pedigrees. Practise drawing Punnett squares and pedigrees regularly, and use PW’s practice questions and online batches to test your understanding.