NCERT Solution for class 12 Principles of Inheritance and variation Chapter 5

Principles of Inheritance and variation class 12 Biology NCERT Solution Chapter 5

Principles of Inheritance and variation taken from NCERT Book Biology Class 12 Important Chapter 5 There is all the topics are covered related to this Chapter

Principles of Inheritance and variation class 12 Introduction

The exploration of “Principles of Inheritance and Variation” unravels the fundamental laws governing the transmission of genetic traits across generations. Gregor Mendel’s pioneering work serves as the cornerstone, introducing key concepts such as alleles, dominant and recessive traits, and the laws of segregation and independent assortment. This chapter delves into the intricacies of genetic inheritance, including linkage, crossing over, sex determination, and the role of mutations in generating variations.

Understanding these principles not only unveils the mechanisms shaping individual traits but also forms the basis for comprehending broader concepts like population genetics and the evolutionary significance of genetic diversity. As students embark on this journey, they gain insights into the hereditary mechanisms that underpin the diversity of life forms.

Principles of Inheritance and variation class 12 ,Important Notes”
  1. Mendelian Laws:
    • Understand Mendel’s Laws of Inheritance, including the Law of Segregation and the Law of Independent Assortment.
    • Comprehend how these laws explain the inheritance of single and multiple traits.
  2. Genetic Terminology:
    • Define and differentiate between terms such as genes, alleles, genotype, phenotype, dominant, and recessive traits.
  3. Punnett Squares:
    • Learn to use Punnett squares to predict the outcomes of monohybrid and dihybrid crosses.
    • Apply the principles of probability to genetic crosses.
  4. Chromosomal Theory of Inheritance:
    • Explore how Mendelian principles align with the chromosomal theory of inheritance.
    • Understand the role of chromosomes in the transmission of genetic information.
  5. Linkage and Crossing Over:
    • Learn about genetic linkage and the phenomenon of crossing over during meiosis.
    • Understand how these events contribute to genetic variation.
  6. Sex Determination and Sex-Linked Inheritance:
    • Explore the mechanisms of sex determination and inheritance patterns of genes on sex chromosomes.
    • Understand sex-linked traits and their inheritance.
  7. Mutation and Genetic Disorders:
    • Recognize the types and causes of mutations.
    • Understand how mutations contribute to genetic diversity and the development of genetic disorders.
  8. Population Genetics:
    • Study the principles of population genetics, including gene frequencies, genetic drift, gene flow, and natural selection.
    • Explore how these factors influence the genetic makeup of populations over time.
  9. Evolutionary Significance:
    • Understand how the principles of inheritance and genetic variation contribute to the process of evolution.
    • Recognize the role of natural selection in shaping populations.
  10. Applications in Biotechnology:
    • Explore real-world applications of genetic principles in fields such as agriculture, medicine, and biotechnology.
    • Understand genetic engineering techniques and their implications.
Principles of Inheritance and variation class 12 Definition

The “Principles of Inheritance and Variation” in Class 12 biology encompass the fundamental laws and mechanisms governing the transmission of genetic traits from one generation to the next in living organisms. This branch of biology, rooted in Gregor Mendel’s pioneering work, explores the principles of heredity, genetic variation, and their role in shaping the characteristics of individuals and populations

It involves understanding key concepts such as genes, alleles, dominant and recessive traits, and the principles of genetic inheritance, including Mendel’s Laws of Segregation and Independent Assortment. The study also extends to the chromosomal basis of inheritance, sex determination, linkage, crossing over, mutations, and their implications in the broader contexts of population genetics and evolution

Principles of Inheritance and variation class 12 Explanation

Have you ever wondered why an elephant always gives birth only to a baby elephant and not some other animal? Or why a mango seed forms only a mango plant and not any other plant? Given that they do, are the offspring identical to their parents? Or do they show differences in some of their characteristics? Have you ever wondered why siblings sometimes look so similar to each other? Or sometimes even so different?

These and several related questions are dealt with scientifically in a branch of biology known as Genetics. This subject deals with the inheritance, as well as the variation of characters from parents to offspring. Inheritance is the process by which characters are passed on from parent to progeny; it is the basis of heredity. Variation is the degree by which progeny differ from their parents.

Principles of Inheritance and variation class 12

Humans knew from as early as 8000-1000 B.C. that one of the causes of variation was hidden in sexual reproduction. They exploited the variations that were naturally present in the wild populations of plants and animals to selectively breed and select for organisms that possessed desirable characters. For example, through artificial selection and domestication from ancestral species, humans have been able to influence the traits of plants and animals for their benefit.

Wild cows, we have well-known Indian breeds, e.g., Sahiwal cows in Punjab. We must, however, recognize that though our ancestors knew about the inheritance of characters and variation, they had very little idea about the scientific basis of these phenomena.


Contrasting Traits studied by Mendel In Pea

S.NoCharacters Contrasting Traits
1Stem height tall/dawarf
2Flower colour violet/white
3Flower position Axial/terminal
4pod shape Inflated /constricted
5Pod colour Green /yellow
6seed shape Round/ wrinkled
7seed colour yellow/green

It was during the mid-nineteenth century that headway was made in the understanding of inheritance. Gregor Mendel conducted hybridization experiments on garden peas for seven years (1856-1863) and proposed the laws of inheritance in living organisms. During Mendel’s investigations into inheritance patterns, it was for the first time that statistical analysis and mathematical logic were applied to problems in biology.

His experiments had a large sampling size, which gave greater credibility to the data that he collected. Also, the confirmation of his inferences from experiments on successive generations of his test plants proved that his results pointed to general rules of inheritance rather than being unsubstantiated ideas.

Principles of Inheritance and variation class 12

Mendel investigated characters in the garden pea plant that were manifested as two opposing traits, e.g., tall or dwarf plants, yellow or green seeds. This allowed him to set up a basic framework of rules governing inheritance, which was expanded on by later scientists to account for all the diverse natural observations and the complexity inherent in them.

Mendel conducted such artificial pollination/cross-pollination experiments using several true-breeding pea lines. A true-breeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations. Mendel selected 14 true-breeding pea plant varieties,

as pairs which were similar except for one character with contrasting traits. Some of the contrasting traits selected were smooth or wrinkled seeds, yellow or green seeds, inflated (full) or constricted green or yellow pods and tall or dwarf plants (Figure 5.1, Table 5.1).


Let us take the example of one such hybridization experiment carried out by Mendel where he crossed tall and dwarf pea plants to study the inheritance of one gene (Figure 5.2). He collected the seeds produced as a result of this cross and grew them to generate plants of the first hybrid generation. This generation is also called the Filial1 progeny or the F1. Mendel observed that all the F1 progeny plants were tall, like one of its parents; none were dwarf (Figure 5.3).

He made similar observations for the other pairs of traits – he found that the F1 always resembled either one of the parents, and that the trait of the other parent was not seen in them.

Mendel then self-pollinated the tall F1 plants and to his surprise found that in the Filial2 generation some of the offspring were ‘dwarf’; the character that was not seen in the F1 generation was now expressed. The proportion of plants that were dwarf were 1/4th of the F2 plants while 3/4th of the F2 plants were tall. The tall and dwarf traits were identical to their parental type and did not show any blending; that is, all the offspring were either tall or dwarf, none were of in-between height (Figure 5.3).

Similar results were obtained with the other traits that he studied: only one of the parental traits was expressed in the F1 generation while at the F2 stage both the traits were expressed in the proportion 3:1. The contrasting traits did not show any blending at either F1 or F2 stage.

Based on these observations, Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as ‘factors’.

Now we call them as genes. Genes, therefore, are the units of inheritance. They contain the information that is required to express a particular trait in an organism. Genes which code for a pair of contrasting traits are known as alleles, i.e., they are slightly different forms of the same gene.

If we use alphabetical symbols for each gene, then the capital letter is used for the trait expressed at the F1 stage and the small alphabet for the other trait. For example, in the case of the character of height, T is used for the Tall trait and t for the ‘dwarf’, and T and t are alleles of each other. Hence,

in plants the pair of alleles for height would be TT, Tt, or tt. Mendel also proposed that in a true breeding, tall or dwarf pea variety, the allelic pair of genes for height are identical or homozygous, TT and tt, respectively. TT and tt are called the genotype of the plant while the descriptive terms tall and dwarf are the phenotype. What then would be the phenotype of a plant that had a genotype Tt?

As Mendel found the phenotype of the F1 heterozygote Tt to be exactly like the TT parent in appearance, he proposed that in a pair of dissimilar factors, one dominates the other (as in the F1) and hence is called the dominant factor while the other factor is recessive. In this case, T (for tallness) is dominant over t (for dwarfness),

that is recessive. He observed identical behavior for all the other characters/trait-pairs that he studied. It is convenient (and logical) to use the capital and lower case of an alphabetical symbol to remember this concept of dominance and recessiveness.

(Do not use T for tall and d for dwarf because you will find it difficult to remember whether T and d are alleles of the same gene/character or not). Alleles can be similar as in the case of homozygotes TT and tt or can be dissimilar as in the case of the heterozygote Tt. Since

the genotype t. As a result, the F2 generation shows a phenotypic ratio of 3 tall (TT or Tt) to 1 dwarf (tt), and a genotypic ratio of 1 TT : 2 Tt : 1 tt.

From the observation that the recessive parental trait is expressed without any blending in the F2 generation, we can infer that, when the tall and dwarf plant produce gametes, by the process of meiosis, the alleles of the parental pair separate or segregate from each other and only one allele is transmitted to a gamete.

This segregation of alleles is a random process, and so there is a 50 per cent chance of a gamete containing either allele, as has been verified by the results of the crossings. In this way, the gametes of the tall TT plants have the allele T and the gametes of the dwarf tt plants have the allele t. During fertilization, the two alleles,

T from one parent (say, through the pollen), and t from the other parent (then through the egg), are united to produce zygotes that have one T allele and one t allele. In other words, the hybrids have Tt. Since these hybrids contain alleles that express contrasting traits, the plants are heterozygous. The production of gametes by the parents,

The formation of the zygotes, the F1 and F2 plants can be understood from a diagram called Punnett Square as shown in Figure 5.4. It was developed by a British geneticist, Reginald C. Punnett. It is a graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross. The possible gametes are written on two sides,

usually the top row and left columns. All possible combinations are represented in boxes below in the squares, which generates a square output form.

The Punnett Square shows the parental tall TT (male) and dwarf tt (female) plants, the gametes produced by them, and the F1 Tt progeny. The F1 plants of genotype Tt are self-pollinated. The symbols & and % are used to denote the female (eggs) and male (pollen) of the F1 generation, respectively. The F1 plant of the genotype Tt,

when self-pollinated, produces gametes of the genotype T and t in equal proportion. When fertilization takes place, the pollen grains of genotype T have a 50 per cent chance to pollinate eggs of the genotype T, as well as of genotype t. Also, pollen grains of genotype t have a 50 per cent chance of pollinating eggs of genotype T, as well as of genotype

Principles of Inheritance and variation class 12

From the Punnett square, it is easily seen that 1/4th of the random fertilizations lead to TT, 1/2 lead to Tt, and 1/4th lead to tt. Though the F1 have a genotype of Tt, the phenotypic character seen is ‘tall’. At F2, 3/4th of the plants are tall,

where some of them are TT, while others are Tt. Externally, it is not possible to distinguish between the plants with the genotypes TT and Tt. Hence, within the genotypic pair Tt, only one character ‘T ’ tall is expressed. Hence the character T or ‘tall’ is said to dominate over the other allele t or ‘dwarf’ character

It is thus due to this dominance of one character over the other that all the F1 are tall (though the genotype is Tt), and in the F2, 3/4th of the plants are tall (though genotypically 1/2 are Tt and only 1/4th are TT). This leads to a phenotypic ratio of 3/4th tall : (1/4 TT + 1/2 Tt) and 1/4th tt, i.e., a 3:1 ratio, but a genotypic ratio of 1:2:1.

The 1/4 : 1/2 : 1/4 ratio of TT: Tt: tt is mathematically condensable to the form of the binomial expression (ax + by)2, that has the gametes bearing genes T or t in equal frequency of ½. The expression is expanded as given below:


Mendel self-pollinated the F2 plants and found that dwarf F2 plants continued to generate dwarf plants in F3 and F4 generations. He concluded that the genotype of the dwarfs was homozygous – tt. If he self-pollinated a tall F2 plant, he would have gotten either Tt or TT genotype in the progeny.

From the preceding paragraphs, it is clear that though the genotypic ratios can be calculated using mathematical probability, by simply looking at the phenotype of a dominant trait, it is not possible to know the genotypic composition. That is

for example, whether a tall plant from F1 or F2 has TT or Tt composition cannot be predicted. Therefore, to determine the genotype of a tall plant at F2, Mendel crossed the tall plant from F2 with a dwarf plant.

This he called a test cross. In a typical test cross, an organism (pea plants here) showing a dominant phenotype (and whose genotype is to be determined) is crossed with the recessive parent instead of self-crossing. The progenies of such a cross can easily be analyzed to predict the genotype of the test organism. Figure 5.5 shows the results of a typical test cross where violet color flower (W) is dominant over white color flower (w).

Based on his observations on monohybrid crosses, Mendel proposed two general rules to consolidate his understanding of inheritance in monohybrid crosses. Today these rules are called the Principles or Laws of Inheritance: the First Law or Law of Dominance and the Second Law or Law of Segregation.

.2.1 Law of Dominance

(i) Characters are controlled by discrete units called factors.

(ii) Factors occur in pairs.

(iii) In a dissimilar pair of factors, one member of the pair dominates (dominant) the other (recessive).

The law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 and the expression of both in the F2. It also explains the proportion of 3:1 obtained at the F2.

5.2.2 Law of Segregation This law is based on the fact that the alleles do not show any blending and that both the characters are recovered as such in the F2 generation, though one of these is not seen at the F1 stage. Though the parents contain two alleles during gamete formation,

the factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors. Of course, a homozygous parent produces all gametes that are similar, while a heterozygous one produces two kinds of gametes, each having one allele with an equal proportion.

Principles of Inheritance and variation class 12 summary

Genetics, a branch of biology, delves into the principles of inheritance and their practical applications. The phenomenon of offspring mirroring their parents in both morphological and physiological features has been a focal point for numerous biologists.

Gregor Mendel, the pioneer in systematically studying this phenomenon, laid the groundwork for the field by proposing what is now known as “Mendel’s Laws of Inheritance.”

In his meticulous examination of pea plants with distinct traits, Mendel postulated that the regulating elements, later termed genes, exist in pairs known as alleles. His observations revealed a consistent pattern in the expression of traits across different generations—first generation (F1), second generation (F2), and so forth.

Notably, some traits exhibit dominance over others. When factors are in a heterozygous condition, dominant characters are expressed, establishing the Law of Dominance. Conversely, recessive characters manifest only in homozygous conditions.

Mendel’s discernment extended to the non-blending nature of characters in heterozygous conditions. He proposed that during gamete formation, the factors governing traits segregate, leading to the Law of Segregation. This segregation ensures that recessive characters, dormant in heterozygous conditions,

may re-emerge when the organism becomes homozygous for that trait. In essence, Mendel’s laws provided a structured framework for understanding the patterns and mechanisms underlying the inheritance of traits in organisms.

Principles of Inheritance and variation class 12 Question and Answer

Question:1 Differentiate between the following

  1. Dominance and Recessive
    (b) Homozygous and Heterozygous
    (c) Monohybrid and Dihybrid.


In the presence or absence of a recessive trait, a dominant factor or allele expresses itself.
A recessive trait expresses itself only in the absence of a dominant trait.

Example: In a pea plant, round seeds and violet flowers are dominant characters.
Example: In a pea plant, white flower, dwarf plant, etc., are recessive characters

(b) Homozygous and heterozygous


For a particular trait, homozygous contains two similar alleles.
For a particular trait, heterozygous contains two different alleles.

Only one type of gamete is produced.
It produces more than one type of gamete – two different types of gametes, to be precise.

(c) Monohybrid and dihybrid

It is a cross between parents differing in only one pair of contrasting characters.
It is a cross between parents differing in two pairs of contrasting characters.

Example: A cross between a dwarf and a tall pea plant
Example: A cross between a yellow wrinkled seed and a green rounded seed

Question:2 A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?

Answer: For a diploid organism, which is heterozygous for 4 loci, then 24 i.e. 2 x 2 x 2 x 2 = 16 types of gametes can be produced if the genes are not linked because for each heterozygous pair of genes there are two possibilities. So, for 4 pair the number of combination will be 16 gametes.

Question:3 What is pedigree analysis? Suggest how such an analysis, can be useful.

Answer. Pedigree analysis is study of pedigree for the transmission of particular trait and finding the possibility of absence or presence of that trait in homozygous or heterozygous state in a particular individual. Pedigree analysis helps-
(i) in analysis of transmission of character in family over generation.
(ii) in genetic counselling of disease like haemophilia.
(iii) to identify whether a particular genetic disease is due to recessive gene or a dominant gene.
(iv) to identify the possible origin of the defective gene in the family or in a population.

Question:4  How is sex determined in human beings?

Answer. Sex determination refers to the mechanisms employed by organisms to produce offsprings that are of two different sexes. The sex of an individual is determined by the genetic information present in the individual’s sex chromosomes. Sex determination in human is done by XY type chromosome. In humans, females have two XX chromosomes and males have two different chromosomes (XY).

Question:5 What is point mutation? Give one example.

Answer.Mutations arising due to change in single base pair of DNA is called point mutation. Eg., sickle cell anaemia, haemophilia.

Principles of Inheritance and variation class 12 MCQ Question and Answer

Question:1 What is the basic unit of inheritance?

  1. a) Gene
  2. b) Chromosome
  3. c) Allele
  4. d) Nucleotide

Question:2 Mendel’s law of segregation states that:

  1. a) Genes on the same chromosome segregate independently
  2. b) Alleles segregate during gamete formation
  3. c) Homologous chromosomes segregate during meiosis
  4. d) Genes on different chromosomes segregate independently

Question:3 In a dihybrid cross, the ratio of phenotypes in the F2 generation according to Mendel’s law of independent assortment is:

  1. a) 9:3:3:1
  2. b) 3:1
  3. c) 1:2:1
  4. d) 1:1:1:1

Question:4 In incomplete dominance, the phenotype of heterozygotes is:

  1. a) Intermediate between the phenotypes of the homozygotes
  2. b) Identical to one of the homozygotes
  3. c) A blend of the phenotypes of the homozygotes
  4. d) Determined by codominance

Question:5 Which of the following is an example of codominance?

  1. a) ABO blood group system
  2. b) Mendelian inheritance
  3. c) Incomplete dominance
  4. d) Polygenic inheritance

Question:6 The sex chromosomes in a human male are:

  1. a) XX
  2. b) XY
  3. c) YY
  4. d) XXY

Question:7 What is the probability of having a carrier offspring if both parents are carriers of a recessive genetic disorder?

  1. a) 0%
  2. b) 25%
  3. c) 50%
  4. d) 75%

Question:8 Which of the following is a sex-linked disorder?

  1. a) Cystic fibrosis
  2. b) Hemophilia
  3. c) Sickle cell anemia
  4. d) Huntington’s disease

Question:9 A test cross is used to determine:

  1. a) Whether a trait is dominant or recessive
  2. b) The genotype of an individual with a dominant phenotype
  3. c) The phenotype of an individual with a known genotype
  4. d) The ratio of phenotypes in the F2 generation

Question:10 Polygenic inheritance involves:

  1. a) One gene influencing multiple traits
  2. b) Multiple genes influencing one trait
  3. c) A single gene determining a single trait
  4. d) A single gene influencing multiple traits

MCQ Answers

Question NoAnswer Question NoAnswer


Mendel’s Laws of Inheritance, including the Law of Dominance and the Law of Segregation, form the cornerstone of the Class 12 study on Principles of Inheritance and Variation, elucidating the patterns and mechanisms governing the transmission of traits from generation to generation.

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