CBSE Class 12 Biology (2026–27)
Chapter 4: Principles of Inheritance and Variation
20 Important Questions and Answers
The chapter covers Mendel’s laws, inheritance patterns, chromosomal theory, sex determination, mutations, and genetic disorders. These are among the most important topics for CBSE Board examinations.
1. Why did Mendel choose pea plants for his experiments?
Answer:
Gregor Mendel selected garden pea (Pisum sativum) because it possessed several advantages for genetic studies. It had many distinct contrasting traits such as tall/dwarf and yellow/green seeds. The plant was easy to cultivate, had a short life cycle, and produced numerous offspring. Pea flowers are naturally self-pollinating, allowing the maintenance of pure lines, but artificial cross-pollination could also be performed easily. These features enabled Mendel to conduct controlled experiments and obtain reliable statistical data. His observations on pea plants helped establish the fundamental laws of inheritance that form the basis of modern genetics.
2. State and explain the Law of Dominance.
Answer:
The Law of Dominance states that when two contrasting alleles are present together in a heterozygous organism, only one allele expresses itself, while the other remains hidden. The expressed allele is called dominant, and the masked allele is called recessive. Mendel observed this in pea plants when he crossed tall plants with dwarf plants. All F₁ offspring were tall, showing that the allele for tallness dominated over the allele for dwarfness. This law explains why recessive traits may disappear in one generation and reappear in later generations. It also introduced the concept of dominant and recessive genes in inheritance.
3. Explain the Law of Segregation.
Answer:
The Law of Segregation, also known as the Law of Purity of Gametes, states that the two alleles of a gene separate during gamete formation. As a result, each gamete receives only one allele of a gene pair. During fertilization, these alleles unite randomly to restore the pair. Mendel proposed this law based on his monohybrid cross experiments. In a heterozygous tall pea plant (Tt), gametes carry either T or t allele. This segregation ensures that genetic traits are passed accurately from parents to offspring. The law explains the reappearance of recessive traits in the F₂ generation.
4. What is a monohybrid cross? Mention its significance.
Answer:
A monohybrid cross is a genetic cross involving a single pair of contrasting characters. Mendel crossed pure tall pea plants (TT) with pure dwarf plants (tt). The F₁ generation consisted entirely of tall plants (Tt). Upon self-pollination, the F₂ generation showed a phenotypic ratio of 3 tall : 1 dwarf and a genotypic ratio of 1 TT : 2 Tt : 1 tt. This experiment demonstrated the Law of Dominance and the Law of Segregation. The monohybrid cross provided the first scientific evidence explaining how traits are inherited and became the foundation of classical genetics.
5. Explain the Law of Independent Assortment.
Answer:
The Law of Independent Assortment states that alleles of different genes segregate independently during gamete formation. Mendel demonstrated this principle through dihybrid crosses involving two pairs of contrasting traits. He crossed round-yellow seeds with wrinkled-green seeds and observed a phenotypic ratio of 9:3:3:1 in the F₂ generation. This ratio showed that inheritance of one trait did not influence the inheritance of another. The law applies when genes are located on different chromosomes or are far apart on the same chromosome. It contributes significantly to genetic variation among offspring and increases diversity in populations.
6. Differentiate between genotype and phenotype.
Answer:
Genotype refers to the genetic constitution or allele combination of an organism, while phenotype refers to the observable characteristics expressed by the organism. For example, TT and Tt are different genotypes for tallness in pea plants, but both produce the same phenotype, tall plants. The genotype remains constant throughout life, whereas phenotype may be influenced by environmental factors. Genotype determines the hereditary potential of an organism, while phenotype represents the actual expression of that genetic information. Understanding the difference between genotype and phenotype is essential for studying inheritance patterns and predicting traits in offspring.
7. What is incomplete dominance? Give an example.
Answer:
Incomplete dominance is a pattern of inheritance in which neither allele completely dominates the other. As a result, the heterozygous individual exhibits an intermediate phenotype. A classic example is the flower colour in snapdragon plants. When a red-flowered plant (RR) is crossed with a white-flowered plant (rr), all F₁ offspring have pink flowers (Rr). In the F₂ generation, the phenotypic and genotypic ratio becomes 1 red : 2 pink : 1 white. This pattern differs from Mendel’s complete dominance because the dominant allele does not fully mask the recessive allele, resulting in blended expression.
8. What is codominance? Explain with ABO blood groups.
Answer:
Codominance is a condition in which both alleles of a gene express themselves equally in a heterozygous individual. The ABO blood group system in humans is a common example. The alleles IA and IB are codominant. When both are present together in genotype IAIB, they produce blood group AB. In this condition, both A and B antigens are expressed on the surface of red blood cells. Neither allele suppresses the other. This differs from incomplete dominance because both traits appear fully rather than showing an intermediate condition. Codominance demonstrates an important exception to Mendel’s principle of dominance.
9. What are multiple alleles? Explain with an example.
Answer:
Multiple alleles refer to the presence of more than two alternative forms of a gene in a population, although an individual possesses only two alleles at a time. The ABO blood group system illustrates this concept. It is controlled by three alleles: IA, IB, and i. IA and IB are codominant, while both are dominant over i. Different combinations of these alleles produce four blood groups—A, B, AB, and O. Multiple alleles increase genetic diversity and create several possible phenotypes. Such inheritance patterns cannot be explained by simple dominant-recessive relationships alone and represent a significant extension of Mendelian genetics.
10. State the Chromosomal Theory of Inheritance.
Answer:
The Chromosomal Theory of Inheritance was proposed by Sutton and Boveri. It states that genes are located on chromosomes and that chromosomes behave during meiosis in a manner similar to Mendel’s hereditary factors. Chromosomes occur in pairs, segregate during gamete formation, and reunite during fertilization. This theory provided a cytological basis for Mendel’s laws of inheritance. It established a direct relationship between chromosomes and heredity, explaining how genetic information is transmitted from one generation to the next. The theory became a major milestone in genetics and helped integrate Mendelian principles with cell biology.
11. What is linkage?
Answer:
Linkage refers to the tendency of genes located on the same chromosome to be inherited together. Such genes do not assort independently because they are physically connected on the chromosome. Thomas Hunt Morgan discovered linkage while studying fruit flies (Drosophila melanogaster). The closer two genes are on a chromosome, the stronger the linkage between them. Linkage reduces the number of recombinant offspring and influences inheritance patterns. However, crossing over during meiosis can separate linked genes and produce new combinations. The concept of linkage helped explain deviations from Mendel’s Law of Independent Assortment in certain genetic crosses.
12. What is recombination?
Answer:
Recombination is the process by which new combinations of genes are formed due to the exchange of genetic material between homologous chromosomes during meiosis. This exchange occurs through crossing over in prophase-I of meiosis. Recombination increases genetic variation among offspring by creating unique combinations of parental genes. It plays a crucial role in evolution and adaptation because it introduces diversity within populations. The frequency of recombination is used to estimate the distance between genes on chromosomes. Thus, recombination contributes significantly to heredity, variation, and the generation of new genetic combinations in organisms.
13. Explain sex determination in humans.
Answer:
In humans, sex is determined by the XY system. Females possess two X chromosomes (XX), whereas males possess one X and one Y chromosome (XY). During gamete formation, females produce eggs containing only X chromosomes. Males produce two types of sperms: half carry X chromosomes and half carry Y chromosomes. Fertilization of an egg by an X-bearing sperm produces a female child (XX), while fertilization by a Y-bearing sperm produces a male child (XY). Therefore, the father determines the sex of the offspring. This mechanism ensures an approximately equal probability of male and female births in human populations.
14. What is haemophilia?
Answer:
Haemophilia is a sex-linked recessive genetic disorder characterized by impaired blood clotting. It is caused by the absence or deficiency of specific clotting factors in the blood. The defective gene is located on the X chromosome. Males are more frequently affected because they possess only one X chromosome. Females generally act as carriers and may transmit the disorder to their offspring. Individuals with haemophilia experience prolonged bleeding even from minor injuries. Since the disease follows an X-linked inheritance pattern, it is commonly used to explain sex-linked inheritance in humans. Early diagnosis and treatment can help manage the condition effectively.
15. What is colour blindness?
Answer:
Colour blindness is an X-linked recessive genetic disorder in which a person cannot distinguish certain colours properly, especially red and green. The responsible gene is located on the X chromosome. Males are more commonly affected because they possess only one X chromosome, while females usually remain carriers. A colour-blind father cannot pass the disorder directly to his sons but can transmit the defective gene to his daughters. The condition does not affect overall vision but alters colour perception. Colour blindness is one of the most frequently cited examples of sex-linked inheritance in human genetics.
16. What is sickle-cell anaemia?
Answer:
Sickle-cell anaemia is an inherited genetic disorder caused by a mutation in the gene responsible for haemoglobin synthesis. The mutation leads to the production of abnormal haemoglobin called HbS. Under low oxygen conditions, red blood cells become sickle-shaped instead of remaining biconcave. These distorted cells can block blood vessels and reduce oxygen transport. The disorder follows an autosomal recessive inheritance pattern, meaning an individual must inherit two defective alleles to express the disease. Symptoms include anaemia, fatigue, pain, and reduced oxygen supply to tissues. It is a well-known example of a Mendelian genetic disorder in humans.
17. What is Down syndrome?
Answer:
Down syndrome is a chromosomal disorder caused by the presence of an extra copy of chromosome 21, a condition known as trisomy 21. Affected individuals possess 47 chromosomes instead of the normal 46. The disorder results from nondisjunction during meiosis, leading to abnormal chromosome numbers in gametes. Common characteristics include intellectual disability, short stature, flattened facial features, and developmental delays. The risk of Down syndrome increases with maternal age. It is one of the most common chromosomal abnormalities in humans and serves as an important example of how chromosomal changes can affect development and health.
18. What is Turner syndrome?
Answer:
Turner syndrome is a chromosomal disorder found in females due to the absence of one X chromosome. Individuals have a chromosome complement of 45, XO instead of the normal 46, XX. This condition results from nondisjunction during gamete formation. Affected females are usually short in stature and have underdeveloped ovaries, leading to infertility. They may also exhibit delayed sexual development and certain physical abnormalities. Despite these features, intelligence is generally normal. Turner syndrome demonstrates the importance of sex chromosomes in normal growth and development and is a classic example of a chromosomal disorder studied in genetics.
19. What is Klinefelter syndrome?
Answer:
Klinefelter syndrome is a chromosomal disorder that occurs in males due to the presence of an extra X chromosome. The affected individual possesses 47 chromosomes with the genotype XXY. This condition results from nondisjunction during meiosis. Individuals often have tall stature, reduced fertility, underdeveloped testes, and some female secondary sexual characteristics. Learning difficulties may also occur in certain cases. Since an additional sex chromosome is present, normal male reproductive development is affected. Klinefelter syndrome highlights the role of sex chromosomes in determining physical and reproductive characteristics and is an important example of chromosomal variation in humans.
20. What is mutation? Mention its significance.
Answer:
A mutation is a sudden and heritable change in the genetic material of an organism. Mutations may occur in genes or chromosomes and can arise spontaneously or due to environmental factors such as radiation and chemicals. Most mutations are neutral or harmful, but some create beneficial variations that contribute to evolution. Mutations generate new alleles and increase genetic diversity within populations. They play an important role in adaptation, natural selection, and the origin of new species. Many inherited disorders also result from mutations. Therefore, mutations are considered a major source of genetic variation in living organisms.