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

Key Genetic Terminology

Gene

A unit of inheritance, a specific sequence of DNA or RNA that codes for a functional product (like a protein or RNA molecule).

Allele

Alternative forms of a gene, located at the same locus on homologous chromosomes (e.g., 'T' for tall, 't' for dwarf).

Genotype

The genetic constitution of an individual, the set of alleles an organism possesses (e.g., TT, Tt, tt).

Phenotype

The observable physical or biochemical characteristics of an organism, resulting from the interaction of its genotype and environment (e.g., Tall, Dwarf).

Homozygous

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

Heterozygous

Having two different alleles for a particular gene (e.g., Tt).

Dominant Allele

An allele that expresses its phenotypic effect even when heterozygous with a recessive allele (e.g., 'T' in Tt leads to Tall phenotype).

Recessive Allele

An allele whose phenotypic effect is expressed only when homozygous (e.g., 't' in tt leads to Dwarf phenotype), masked by a dominant allele in heterozygotes.

Monohybrid Cross

A genetic cross between parents that differ in alleles for a single gene (e.g., cross between pure tall and pure dwarf plants).

Dihybrid Cross

A genetic cross between parents that differ in alleles for two different genes (e.g., cross between plants with round yellow seeds and wrinkled green seeds).

Mendel's Laws of Inheritance

Gregor Mendel, known as the 'Father of Genetics', conducted breeding experiments on **garden peas ( Pisum sativum) and formulated fundamental laws of inheritance. He chose pea plants due to their distinct contrasting characters, short life cycle, and ease of cross-pollination. Mendel studied seven pairs of contrasting traits** including stem height, flower colour, seed colour, seed shape, pod colour, pod shape, and flower position.

Law of Dominance

This law states that in a cross between two pure parents differing in one pair of contrasting characters, only one character (dominant) is expressed in the F1 generation, while the other (recessive) character remains hidden. In the F2 generation, the recessive character reappears. For example, when a tall pea plant (TT) is crossed with a dwarf pea plant (tt), all F1 progeny are tall (Tt). This law explains the expression of only one parental trait in a monohybrid cross in the F1 generation and the 3:1 phenotypic ratio in the F2 generation.

Law of Segregation (Law of Purity of Gametes)

This law states that during gamete formation, the two alleles for a trait (e.g., T and t) separate or segregate from each other such that each gamete receives only one allele. These alleles do not blend, and both characters are recovered in the F2 generation. Since gametes are pure for a trait, this is also called the Law of Purity of Gametes. For example, a heterozygous individual (Tt) produces two types of gametes (T and t) in equal proportions, each carrying only one allele.

Law of Independent Assortment

This law states that when two pairs of traits are combined in a hybrid (dihybrid cross), segregation of one pair of characters is independent of the other pair of characters. In simpler terms, alleles for different traits are sorted into gametes independently of one another. This applies to genes located on different chromosomes or far apart on the same chromosome. This law is best illustrated by a dihybrid cross, where the F2 phenotypic ratio is 9:3:3:1, indicating that the inheritance of one character (e.g., seed shape) does not influence the inheritance of another character (e.g., seed colour).

Deviations from Mendelian Inheritance

Chromosomal Theory of Inheritance & Linkage

1. Sutton and Boveri's Postulates — 1. Chromosomes occur in homologous pairs. 2. Homologous chromosomes segregate during meiosis. 3. Chromosomes carry genes.
2. Linkage — The physical association of genes on the same chromosome. Linked genes tend to be inherited together and do not assort independently. The strength of linkage is inversely proportional to the distance between genes.
3. Recombination — The generation of non-parental gene combinations due to crossing over between homologous chromosomes during meiosis. Recombination frequency is directly proportional to the distance between genes on a chromosome and is used to construct genetic maps.
4. Morgan's Experiments — Studied crosses in Drosophila involving linked genes (e.g., body colour and eye colour). He observed that genes on the same chromosome showed less recombination when closely linked, supporting the concept of linkage and providing the basis for genetic mapping.

Worked Example: Test Cross

• {"title":"Example: Unknown Tall Pea Plant (T_)","bodyMarkdown":"Cross a tall pea plant with unknown genotype (T_) with a dwarf pea plant (tt).\n\nCase 1: If the tall plant is homozygous dominant (TT)\nTT x tt → All Tt (Tall). All offspring will be tall.\n\nCase 2: If the tall plant is heterozygous (Tt)\nTt x tt → 1 Tt : 1 tt (Tall : Dwarf). Offspring will show a 1:1 ratio of tall to dwarf plants.\n\nConclusion: If any dwarf offspring appear, the unknown tall plant must be heterozygous (Tt). If all offspring are tall, the unknown plant is likely homozygous dominant (TT)."}

Exam Tip: Punnett Squares and Ratios

Always draw clear Punnett squares for genetic crosses. Label parents (P), F1, F2 generations, and genotypes/phenotypes. Remember to state both genotypic and phenotypic ratios clearly, especially for crosses like incomplete dominance where they differ from standard Mendelian ratios. For pedigree analysis, practice recognising patterns for autosomal dominant, autosomal recessive, X-linked dominant, and X-linked recessive traits. Look for 'skipping generations' (recessive) or 'affected offspring from unaffected parents' (autosomal recessive usually, or new mutation). For X-linked, look for male-specific patterns or affected father-daughter transmission.

Key Points to Remember

• Mendel's Laws (Dominance, Segregation, Independent Assortment) are the foundation of classical genetics.
• Monohybrid cross F2 phenotypic ratio is 3:1; genotypic ratio is 1:2:1.
• Dihybrid cross F2 phenotypic ratio is 9:3:3:1.
• Deviations like Incomplete Dominance and Co-dominance show different F2 phenotypic ratios (e.g., 1:2:1 for incomplete dominance).
• Chromosomal theory of inheritance links Mendelian factors to chromosome behaviour.
• Linkage reduces recombination, while crossing over increases genetic variation through recombination.
• Sex determination mechanisms vary (XY in humans, ZW in birds, XO in insects, haplo-diploidy in honeybees).
• Pedigree analysis uses standard symbols to trace inheritance patterns of traits/diseases across generations.
• Genetic disorders are broadly classified into Mendelian (e.g., haemophilia, sickle cell anemia) and Chromosomal (e.g., Down's, Klinefelter's, Turner's syndromes).

Practice Questions with Solutions

• Q: What is the main difference between incomplete dominance and co-dominance? A: In incomplete dominance, the heterozygote shows an intermediate phenotype (blending), while in co-dominance, both alleles are fully expressed without blending.
• Q: Why did Morgan choose Drosophila melanogaster for his genetic experiments? A: Drosophila has a short life cycle, can be bred in large numbers, has distinct phenotypic traits, and its chromosomes are visible under a microscope.
• Q: If a normal-vision woman (whose father was colorblind) marries a colorblind man, what is the probability their son will be colorblind? A: Color blindness is an X-linked recessive trait. The woman is a carrier (X^CX^c). The man is colorblind (X^cY). Their sons receive Y from the father and X from the mother. The probability of their son being colorblind (X^cY) is 50%.
• Q: Name one Mendelian disorder and one chromosomal disorder. A: Mendelian Disorder: Sickle Cell Anemia (or Hemophilia, Phenylketonuria). Chromosomal Disorder: Down's Syndrome (or Klinefelter's, Turner's Syndrome).

Frequently Asked Questions