Principles of Inheritance and Variation: Class 12 Biology Chapter 5 Notes

Key Genetic Terminology

Allele

Alternative forms of a gene that are located on the same position (locus) on homologous chromosomes. For example, 'T' (for tall) and 't' (for dwarf) are alleles for the gene controlling height.

Genotype

The genetic makeup of an individual organism, representing the combination of alleles it possesses (e.g., TT, Tt, or tt).

Phenotype

The observable physical or biochemical characteristics of an organism, determined by its genotype and environmental factors (e.g., tall plant, purple flower).

Homozygous

An individual having two identical alleles for a particular gene (e.g., TT or tt).

Heterozygous

An individual having two different alleles for a particular gene (e.g., Tt).

Dominant Allele

An allele that expresses its phenotypic effect even when present in a heterozygous condition. It masks the effect of the recessive allele.

Recessive Allele

An allele that expresses its phenotypic effect only when present in a homozygous condition. Its effect is masked by a dominant allele.

Punnett Square

A graphical representation used to predict the possible genotypes of offspring from a particular cross or breeding experiment.

Test Cross

A cross between an organism with a dominant phenotype (but unknown genotype) and a homozygous recessive organism to determine the genotype of the dominant parent.

Mendel's Laws of Inheritance

Gregor Mendel, through his work on pea plants (Pisum sativum), established the fundamental principles of heredity. His findings, which were initially overlooked, now form the basis of classical genetics. His three primary laws describe how traits are transmitted from parents to offspring.

1. Law of Dominance: This law states that when two homozygous individuals with one or more sets of contrasting characters are crossed, the character that appears in the F1 generation is dominant, and the one which is suppressed is recessive. The character is controlled by discrete units called factors (now known as genes), which occur in pairs. In a dissimilar pair of factors, one member of the pair dominates the other.

2. Law of Segregation: This law is based on the fact that alleles do not blend and both characters are recovered as such in the F2 generation, though one of these is not seen at the F1 stage. During gamete formation, the two alleles of a gene segregate (separate) from each other such that each gamete receives only one of the two factors. This is why it is also known as the law of purity of gametes. A monohybrid cross (a cross involving one pair of contrasting traits) between a tall (TT) and a dwarf (tt) plant produces all tall (Tt) plants in F1, but in F2, it produces tall and dwarf plants in a 3:1 phenotypic ratio.

3. Law of Independent Assortment: This law states that when two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair of characters. This law was formulated based on the results of a dihybrid cross (a cross involving two pairs of contrasting traits). For example, in a cross between a plant with round yellow seeds (RRYY) and wrinkled green seeds (rryy), the F2 generation shows a phenotypic ratio of 9:3:3:1. This demonstrates that the alleles for seed shape (R/r) assorted independently of the alleles for seed color (Y/y).

Must-Remember Concepts for Exams

• Mendel chose Pisum sativum (pea plant) due to its short life cycle, bisexual flowers, and easily identifiable contrasting traits.
• The phenotypic ratio of a Mendelian monohybrid cross is 3:1; the genotypic ratio is 1:2:1 (TT:Tt:tt).
• The phenotypic ratio of a Mendelian dihybrid cross is 9:3:3:1.
• A test cross result of 1:1 indicates the dominant parent was heterozygous.
• Linkage and Recombination are exceptions to the Law of Independent Assortment. Linked genes are located on the same chromosome and tend to be inherited together.
• Chromosomal Theory of Inheritance, proposed by Sutton and Boveri, states that genes are located on chromosomes and that chromosome behavior during meiosis explains Mendel's laws.
• Incomplete Dominance (e.g., flower color in Snapdragon) results in a blending of traits, with a 1:2:1 phenotypic ratio in F2.
• Codominance (e.g., ABO blood groups) involves both alleles expressing themselves fully and independently in the heterozygote.
• Pleiotropy is when a single gene controls multiple phenotypic traits. Example: Phenylketonuria.
• Pedigree analysis is a tool to trace the inheritance of a specific trait, abnormality, or disease in humans.

Mendelian vs. Post-Mendelian Inheritance

Worked Mini-Examples

• {"title":"Monohybrid Cross (Height in Pea Plant)","content":"A cross is made between a homozygous tall plant (TT) and a homozygous dwarf plant (tt).\n- Parental (P) Generation: TT x tt\n- Gametes from P: T and t\n- F1 Generation: All offspring are Tt (heterozygous tall) by the Law of Dominance.\n- F2 Generation (Selfing F1: Tt x Tt):\n - Genotypic Ratio: 1 TT : 2 Tt : 1 tt\n - Phenotypic Ratio: 3 Tall : 1 Dwarf"}
• {"title":"Codominance (ABO Blood Groups)","content":"A cross is made between a man with blood group A (heterozygous: I^Ai) and a woman with blood group B (heterozygous: I^Bi).\n- Parental Genotypes: I^Ai x I^Bi\n- Gametes: (I^A, i) from the man; (I^B, i) from the woman.\n- Possible Offspring Genotypes (using Punnett Square): I^AI^B, I^Ai, I^Bi, ii\n- Possible Offspring Phenotypes (Blood Groups): AB, A, B, O in a 1:1:1:1 ratio."}

How to Analyze a Pedigree Chart

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Exam Traps & Scoring Tips

Board Exam Alert:

• Crosses: Always draw the Punnett square neatly. Marks are often allotted for showing the correct gametes, the square itself, and the final genotypic and phenotypic ratios. Don't just write the final answer.
• Codominance vs. Incomplete Dominance: This is a favourite question. In codominance, both alleles express (e.g., AB blood type). In incomplete dominance, a blend or intermediate phenotype appears (e.g., pink flowers from red and white parents).
• Disorders: For questions on genetic disorders, you must specify if it's autosomal/sex-linked and dominant/recessive. For example, 'Sickle-cell anaemia is an autosomal recessive trait,' and 'Haemophilia is an X-linked recessive trait.'
• Pedigree Analysis: Don't jump to conclusions. First, rule out possibilities. For instance, if parents are unaffected but have an affected child, it's definitely recessive. Start from there.

Quick Revision Check

• What is a test cross and why is it performed? A test cross is a cross between an organism with a dominant phenotype and an organism with a homozygous recessive genotype. It is performed to determine whether the dominant organism is homozygous (e.g., TT) or heterozygous (e.g., Tt).
• State the Chromosomal Theory of Inheritance. Proposed by Sutton and Boveri, it states that genes are located on chromosomes, and the segregation and independent assortment of chromosomes during meiosis are the physical basis for Mendel's Laws of Inheritance.
• What is the key difference between multiple allelism and polygenic inheritance? In multiple allelism, a single gene has more than two alleles in a population (e.g., ABO blood group gene). In polygenic inheritance, a single trait is controlled by multiple genes (e.g., skin color).
• Name one X-linked recessive disorder and one autosomal recessive disorder. X-linked recessive disorder: Haemophilia or Colour blindness. Autosomal recessive disorder: Sickle-cell anaemia or Phenylketonuria.

Frequently Asked Questions

Frequently Asked Questions