Principles of Inheritance and Variation: Class 12 Biology NCERT

Essential Genetic Terminology

Gene

The basic physical and functional unit of heredity. Genes are segments of DNA that code for a specific trait or protein.

Allele

Alternate forms of a gene that are found at the same place (locus) on a chromosome. For example, the gene for flower color can have a purple allele and a white allele.

Genotype

The genetic constitution of an individual organism. It is represented by letters, such as TT, Tt, or tt.

Phenotype

The set of observable physical characteristics of an individual resulting from the interaction of its genotype with the environment. For example, tallness or dwarfness in a pea plant.

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).

Mendel's Laws of Inheritance

Gregor Mendel, through his work on pea plants, deduced the fundamental principles of heredity, which are now known as Mendel's Laws of Inheritance. These laws form the foundation of classical genetics.

1. Law of Dominance: This law states that in a heterozygous individual (with two different alleles for a trait), only one allele is expressed phenotypically. This expressed allele is called the 'dominant' allele, while the one that is not expressed is the 'recessive' allele. For example, when a pure-bred tall pea plant (TT) is crossed with a pure-bred dwarf pea plant (tt), all offspring in the first generation (F1) are tall (Tt). The allele for tallness (T) is dominant over the allele for dwarfness (t).

1. Law of Segregation: This law states that during the formation of gametes (sperm or egg cells), the two alleles for a heritable character separate (or segregate) from each other so that each gamete ends up with only one allele for that character. This is why it's also known as the law of purity of gametes. For the F1 generation (Tt), 50% of gametes will carry the 'T' allele and 50% will carry the 't' allele.

1. Law of Independent Assortment: This law states that alleles for different traits are inherited independently of one another. The inheritance of an allele for one trait (like seed color) does not affect the inheritance of an allele for another trait (like seed shape), provided the genes are on different chromosomes. This leads to new combinations of traits in the offspring.

Worked Example: A Monohybrid Cross

• Let's trace a monohybrid cross between a homozygous tall pea plant (TT) and a homozygous dwarf pea plant (tt). Step 1: Parental (P) Generation Genotypes: TT (Tall) × tt (dwarf) Phenotypes: Tall × Dwarf Step 2: Gametes from P Generation The TT parent produces only one type of gamete: T. The tt parent produces only one type of gamete: t. Step 3: First Filial (F1) Generation When the gametes fuse, all offspring will have the genotype Tt. Phenotype: According to the Law of Dominance, since 'T' is dominant, all F1 plants will be Tall. Step 4: Selfing the F1 Generation (Tt × Tt) The F1 plants are now crossed with themselves. Each Tt parent produces two types of gametes in equal proportion: T and t. Step 5: Second Filial (F2) Generation (using a Punnett Square) We can predict the outcome using a Punnett Square: | | T | t | |---|---|---| | T | TT | Tt | | t | Tt | tt | Genotypic Ratio: From the square, we get 1 TT : 2 Tt : 1 tt. * Phenotypic Ratio: The plants with TT and Tt genotypes will be Tall, and plants with tt will be dwarf. So, the ratio is 3 Tall : 1 Dwarf.

Exam Traps and Key Distinctions

In board exams, questions often test your understanding of the nuances between similar-sounding concepts. Pay close attention to these:

• Codominance vs. Incomplete Dominance: Don't confuse them! In incomplete dominance, the heterozygote shows a phenotype that is an intermediate blend of the two parental phenotypes (e.g., red and white flowers producing pink flowers). In codominance, both alleles are fully and equally expressed in the heterozygote (e.g., human AB blood group, where both A and B antigens are present).

• Test Cross vs. Back Cross: A test cross is a specific type of back cross where an organism with a dominant phenotype but unknown genotype is crossed with a homozygous recessive parent to determine its genotype. A back cross is any cross of a hybrid with one of its parents. So, every test cross is a back cross, but not every back cross is a test cross.

• Independent Assortment and Linkage: Mendel's Law of Independent Assortment only holds true for genes located on different chromosomes or very far apart on the same chromosome. When genes are close together on the same chromosome, they are 'linked' and tend to be inherited together, which is a deviation from Mendel's principles.

Practice Questions with Solutions

• Q: In pea plants, yellow seeds (Y) are dominant over green seeds (y), and round seeds (R) are dominant over wrinkled seeds (r). What are the expected phenotypic ratios in the offspring of a cross between RrYy and RrYy? A: Step 1: Identify the cross. This is a dihybrid cross between two heterozygous parents for both traits. Step 2: Recall the standard dihybrid cross phenotypic ratio. For a cross involving two heterozygous individuals for two independently assorting traits, the expected phenotypic ratio is 9 (dominant for both) : 3 (dominant for first, recessive for second) : 3 (recessive for first, dominant for second) : 1 (recessive for both). Step 3: Apply the ratio to the given traits. - 9/16 will be Round and Yellow (R_Y_) - 3/16 will be Round and green (R_yy) - 3/16 will be wrinkled and Yellow (rrY_) - 1/16 will be wrinkled and green (rryy) Final answer: The expected phenotypic ratio is 9 Round Yellow : 3 Round green : 3 wrinkled Yellow : 1 wrinkled green.
• Q: A man with blood group A marries a woman with blood group B. Their first child has blood group O. What are the genotypes of the parents and the child? A: Step 1: Analyze the child's blood group. Blood group O has the genotype i i. This means the child must have inherited one 'i' allele from each parent. Step 2: Determine the parents' genotypes. Since the man has blood group A and must carry the 'i' allele, his genotype is I^A i. Step 3: Similarly, since the woman has blood group B and must also carry the 'i' allele, her genotype is I^B i. Final answer: The genotype of the man is I^A i, the woman is I^B i, and the child is i i.
• Q: What is a test cross? Explain its significance with an example. A: Step 1: Define a test cross. A test cross is a genetic cross between an organism exhibiting a dominant phenotype (whose genotype is unknown) and another organism that is homozygous recessive for the same trait. Step 2: Explain its significance. The purpose of a test cross is to determine the genotype of the dominant parent. If it's homozygous dominant (e.g., TT) or heterozygous (e.g., Tt). Step 3: Provide an example. To determine the genotype of a tall pea plant (could be TT or Tt), we cross it with a dwarf pea plant (tt). If all offspring are tall, the parent was homozygous dominant (TT). If half the offspring are tall and half are dwarf, the parent was heterozygous (Tt). Final answer: A test cross involves crossing a dominant phenotype individual with a homozygous recessive individual to determine the former's genotype by observing the phenotypic ratio in the offspring.
• Q: Explain why sickle-cell anaemia is an example of pleiotropy. A: Step 1: Define pleiotropy. Pleiotropy is a phenomenon where a single gene influences multiple, seemingly unrelated phenotypic traits. Step 2: Explain the genetic basis of sickle-cell anaemia. The disease is caused by a mutation in a single gene (HbS) that codes for the beta-globin chain of haemoglobin. This is the single gene effect. Step 3: List the multiple phenotypic effects. This single gene mutation leads to the production of abnormal haemoglobin, which causes red blood cells to become sickle-shaped. This, in turn, leads to a cascade of symptoms like chronic anaemia, pain, organ damage, and increased resistance to malaria. Because one gene affects multiple traits (RBC shape, oxygen-carrying capacity, malaria resistance, etc.), it is a classic example of pleiotropy. Final answer: Sickle-cell anaemia is an example of pleiotropy because the single gene responsible for it (HbS) produces multiple phenotypic effects, including changes in red blood cell shape, anaemia, and resistance to malaria.

Frequently Asked Questions