NCERT Solutions for Class 12 Biology Chapter 4 Principles of Inheritance and Variation

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Students can download the PDF from the link given below to make a quick and easy revision before exams. The Class 12 Biology Chapter 4 Principles of Inheritance and Variation question answers are explained clearly to help understand key topics. These solutions are based on the latest syllabus and are well-explained in the NCERT Solutions for Class 12.

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Class 12 Biology Chapter 4 Principles of Inheritance and Variation Solutions (Exercise Questions)

This chapter explains different principles of inheritance and variation in an easy way. To make learning easier, Class 12 Biology Chapter 4 Principles of Inheritance and Variation question answers are prepared according to the latest NCERT curriculum. Solving these questions regularly builds a strong base and boosts confidence for exams.

Q1. Mention the advantages of selecting a pea plant for the experiment by Mendel.

Answer:

The merits of choosing a garden pea (Pisum sativum) for experimentation by Mendel are:

1. Pea possesses numerous visible contrasting characters.

2. The duration of the life of pea plants is short, and they yield numerous seeds in a single generation.

3. Pea flowers are bisexual and exhibit self-pollination, with reproductive whorls being covered by the corolla.

4. It is not difficult to artificially cross-pollinate the pea flowers. The hybrids thus obtained were fertile.

Q2. Differentiate between the following

(a) Dominant and Recessive

Answer:

(b) Homozygous and Heterozygous

Answer:

(c) Monohybrid and Dihybrid

Answer:

Q3. A diploid organism is heterozygous for 4 loci. How many types of gametes can be produced?

Answer:

If a diploid organism is heterozygous at 4 loci, it implies it has four genes where it possesses two distinct alleles at each of the four loci.

We calculate the number of various gametes that can be formed by the formula 2ⁿ, where n represents the number of heterozygous loci.

Here, n = 4, thus the number of gametes = 2⁴ = 16.

Thus, 16 different varieties of gametes can be formed.

Q4. Explain the Law of Dominance using a monohybrid cross.

Answer:

The Law of Dominance, as suggested by Mendel, is:

• Traits are determined by factors (genes) that come in pairs (alleles).

• In a heterozygous organism, one allele (the dominant allele) suppresses the expression of the other allele (the recessive allele).

• The recessive allele is expressed only when it is in a homozygous state.

• A monohybrid cross illustrates this law well. For instance, cross-pollinating a homozygous tall pea plant (TT) with a homozygous dwarf pea plant (tt):

• P Generation: Tall (TT) x Dwarf (tt)

• F₁ Generation: All the offspring are heterozygous (Tt) and tall. The tall allele (T) is dominant to the dwarf allele (t), and therefore all plants have the tall phenotype, suppressing the dwarf trait.

• F₂ Generation: If the F₁ generation (Tt) is self-pollinated, the F₂ generation exhibits a 3:1 phenotypic ratio (3 tall: 1 dwarf). The dwarf trait re-emerges because some plants are now homozygous recessive (tt).

• This re-emergence of the recessive trait in the F₂ generation proves that the recessive allele existed in the F₁ generation, but its expression was suppressed by the dominant allele.

In this cross, it can be seen that in the F₁ generation, only tall plants were seen; no plant was a dwarf. However, in the F₂ generation, the F₁ progeny was self-crossed, three genotypes were observed, among these, the hybrids were showing the dominant trait.

Q5. Define and design a test cross.

Answer:

A test cross is a technique used to determine whether a person exhibiting a dominant phenotype is homozygous dominant or heterozygous for a particular character.

Definition: The cross between an F₁ hybrid and its homozygous recessive parent is called a test cross.

Design:

• Choose a person showing the dominant phenotype but having an unknown genotype (e.g., a tall plant might be TT or Tt).

• Cross this person with a homozygous recessive person (e.g., a dwarf plant with the genotype tt).

Examine the offspring:

• If all offspring have the dominant phenotype, the mystery parent was most likely homozygous dominant (TT).

• If the offspring have a 1:1 ratio of dominant and recessive phenotypes, the mystery parent was heterozygous (Tt).

Q6. Using a Punnett Square, work out the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.

Answer:

Let's take one locus with alleles B (black coat colour, dominant) and b (white coat colour, recessive) in guinea pigs. A cross between a homozygous recessive female (bb) and a heterozygous male (Bb):

• Genotypes: 50% Bb (heterozygous), 50% bb (homozygous recessive)

• Phenotypes: 50% black coat colour, 50% white coat colour

• Ratio: The F₁ progeny will show a 1:1 ratio of black coat colour to white coat colour.

Q7. When a cross is made between a tall plant with yellow seeds (TtYy) and a tall plant with green seeds (Ttyy), what proportions of phenotype in the offspring could be expected to be

(a) tall and green

Answer:

Let's perform the cross: TtYy x Ttyy

From the Punnett square, we can determine the offspring phenotypes:

• Tall and Yellow (T_Y_): TTYy, TtYy, TtYy (3)

• Tall and green (T_yy): TTyy, Ttyy, Ttyy (3)

• Dwarf and Yellow (ttYy): ttYy (1)

• Dwarf and Green (ttyy): ttyy (1)

• The genotypic ratio is 3:3:1:1

Hence, there will be plants showing three tall and green seed traits.

(b) dwarf and green

Answer:

(a) tall and green.

The proportion of tall and green offspring is 3/8 or 37.5%

(b) dwarf and green.

The proportion of dwarf and green offspring is 1/8 or 12.5%

Q8. Two heterozygous parents are crossed. If the two loci are linked, what would be the distribution of phenotypic features in the F₁ generation for a dihybrid cross?

Answer:

When two heterozygotes are crossed (e.g., AaBb x AaBb) and the two loci (A/a and B/b) are linked, the segregation of the phenotypic traits in the F₁ generation of a dihybrid cross is not in the usual 9:3:3:1 ratio.

1. Linkage: This is the physical linkage of genes on the same chromosome. Linked genes are inherited together.
2. If the genes are linked (no crossing over), only the parental phenotypes will be seen in the offspring.
3. If the genes are partially linked (crossing over has taken place), both parental and recombinant phenotypes will be seen, but the parental phenotypes will occur more often than the recombinant phenotypes.
4. For instance, a Yellow body, white eyes, and a wild-type parent cross in Drosophila. Hence, they are co-inherited in progenies. Parental types and recombinant types are 98.7% and 1.3%, respectively.

Q9. Briefly mention the contribution of T.H. Morgan to genetics.

Answer:

Contribution of T.H. Morgan to the discipline of genetics:

• Chromosomal Theory of Inheritance: Morgan, along with his co-worker, presented better evidence of genes on chromosomes rather than the chromosomal theory of inheritance.

• Sex-Linked Inheritance: Morgan showed sex-linked inheritance by studying Drosophila melanogaster and exhibiting that there are certain characteristics that are linked with sex chromosomes.

• Linkage and Recombination: Morgan described the phenomenon of linkage (genes on the same chromosome are inherited together) and recombination (crossing over produces new combinations of alleles).

• Mapping of Genes: Morgan's group established techniques to map the relative positions of genes on chromosomes in terms of recombination frequencies.

Q10. What is pedigree analysis? Suggest how such an analysis can be useful.

Answer:

Pedigree Analysis:

Definition: Pedigree analysis refers to the examination of the pattern of inheritance of a certain trait in a family line. It entails mapping a family pedigree and following the presence of the trait in generations.

Usefulness:

• Determining Inheritance Pattern: Pedigree analysis determines if a trait is dominant, recessive, sex-linked, or autosomal.

• Risk Prediction: It is possible to use it to estimate the likelihood of future offspring inheriting a given genetic condition.

• Genetic Counselling: Genetic counsellors employ pedigree analysis to counsel families regarding their risk of having children with genetic disorders.

• Identification of Carriers: Pedigrees can be used to identify carriers of recessive genetic disorders.

Q11. How is sex determined in human beings?

Answer:

Sex determination in human beings is on a chromosomal basis:

1. Human beings possess 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes.
2. Females possess two X chromosomes (XX).
3. Males possess one X and one Y chromosome (XY).
4. In meiosis, females give rise to eggs that all carry an X chromosome.
5. Males give rise to sperm that are either X-bearing or Y-bearing.
6. If a sperm carrying X fertilises the egg, the child will be female (XX). If a Y-bearing sperm fertilises the egg, the child will be male (XY).

Thus, the offspring's sex is determined by the sperm.

Q12. A child has blood group O. If the father has blood group A and the mother's blood group B, work out the genotypes of the parents and the possible genotypes of the other offspring.

Answer:

If the child has an O blood group, then it is evident that the parents are heterozygous. Thus, the genotype of the father will be I^Ai, and that of the mother will be I^Bi. The child having blood group O will have genotype ii. The other children may have genotypes I^Ai (A blood group), I^Bi (B blood group), and I^AI^B (AB blood group).

Q13 . Explain the following terms with an example

(a) Co-dominance

Answer:

Codominance-

Codominance refers to the phenomenon of two alleles lacking a dominant-recessive relationship and both expressing themselves in the heterozygous condition. In human beings, ABO blood grouping is controlled by gene I. The gene has three alleles I A, I B and i. One person may have any two of these three alleles. Among these alleles, I A, and I B are dominant over i. Alleles IA and IB. When I A and I B are present together, both express themselves because of co-dominance.

(b) Incomplete dominance

Answer:

Incomplete dominance-

Incomplete dominance is the phenomenon of neither of the two alleles being dominant, so that expression in the hybrid is a fine mixture or intermediate between the expressions of the two alleles. In snapdragon ( Mirabilis jalapa), there are two types of pure breeding plants, red-flowered and white-flowered. On crossing the two, F₁ plants possess pink flowers. On selfing them, the F₂ generation has 1red: 2pink: 1white. The pink flower is due to incomplete dominance

Q14. What is a point mutation? Give one example.

Answer:

• Point Mutation: A mutation that is a change at one point or location in the DNA sequence. It is a single-base pair or a few-base pair change.

• Example: Sickle cell anaemia. The disease is caused by a single base substitution in the gene for the beta-globin chain of haemoglobin. The GAG to GUG substitution at the sixth codon causes glutamic acid to be replaced by valine, leading to abnormal haemoglobin that causes red blood cells to develop sickle shapes.

Q15. Who proposed the chromosomal theory of inheritance?

Answer:

Sutton and Boveri in 1902 proposed the chromosomal theory of inheritance.

Q16. Mention any two autosomal genetic disorders with their symptoms.

Answer:

Sickle Cell Anaemia:

• Genetic Basis: Autosomal recessive disease due to a mutation of the gene for beta-globin on chromosome 11.

• Symptoms: Chronic anaemia, pain crises, fatigue, susceptibility to infection, retarded growth, and organ damage.

Down Syndrome:

• Genetic Basis: Autosomal disease due to trisomy 21 (an extra copy of chromosome 21).

• Symptoms: Intellectual disability, characteristic face (e.g., flat nose, slanted eyes), heart malformation, and other medical disorders.

Also, check the NCERT Books and the NCERT Syllabus here:

• NCERT Books Class 12 Biology
• NCERT Syllabus Class 12 Biology
• NCERT Books Class 12
• NCERT Syllabus Class 12

Approach to Solve Principles of Inheritance and Variation Class 12 Question Answer

To solve the questions effectively, one needs to understand the concepts first. The chapter has certain key terms, such as Mendel's laws, monohybrid and dihybrid crosses, mutation, and genetic disorders. Making notes of the chapter is important; students can do it by highlighting all the important points and terms. The Class 12 Biology Chapter 4 Principles of Inheritance and Variation question answer are written in simple language and include all necessary information. Students must practice the Class 12 Biology Chapter 4 Principles of Inheritance and Variation Solutions to have an overall idea of the chapter and build confidence before the exams.

NCERT Solutions for Class 12- Subject-wise

• NCERT Solutions for Class 12 Maths
• NCERT Solutions for Class 12 Chemistry
• NCERT Solutions for Class 12 Physics

Important Questions of NCERT Class 12 Biology Chapter 4

Given below are some of the practice questions from this chapter. To gain clarity and improve their knowledge, students need to practice different types of questions to get familiar with the question pattern. For this purpose, students can depend on the NCERT Solutions for Class 12 Biology Chapter 4 Principles of Inheritance and Variation.

Q1. Occasionally, a single gene may express more than one effect. The phenomenon is called:

Options:

1. Multiple allelism

2. Mosaicism

3. Pleiotropy

4. Polygeny

Answer:

The phenomenon known as pleiotropy refers to the expression of more than one effect by a single gene. This occurs when a gene is implicated in various biological pathways or processes, thereby influencing multiple, seemingly disparate traits or phenotypic attributes within an organism. A classic illustration of this is Marfan syndrome, a human genetic disorder resulting from a single gene mutation impacting connective tissue, which manifests in a spectrum of symptoms such as elongated limbs, cardiac issues, and ocular abnormalities.

Hence, the correct answer is option 3) Pleiotropy.

Q2. Mendel’s Law of independent assortment holds good for genes situated on the:

Options:

1. non-homologous chromosomes

2. homologous chromosomes

3. extranuclear genetic element

4. same chromosome

Answer:

Mendel's Law of Independent Assortment states that genes located on non-homologous chromosomes (i.e., chromosomes that are not paired or do not have corresponding counterparts) assort independently during gamete formation. This means that the inheritance of one gene does not affect the inheritance of another gene located on a different chromosome. This law applies to genes that are far apart on the same chromosome or to genes on different chromosomes, as they segregate independently during meiosis, resulting in a variety of genetic combinations in the offspring.

Hence, the correct answer is option 1) non-homologous chromosomes.

Q3. The inheritance pattern of a gene over generations among humans is studied by pedigree analysis. The character studied in the pedigree analysis is equivalent to:

Options:

1. quantitative trait

2. Mendelian trait

3. polygenic trait

4. maternal trait

Answer:

The character examined in pedigree analysis corresponds to a heritable trait, also known as a phenotype, which is passed down from one generation to another among humans. This analytical approach is predominantly employed to monitor the genetic transmission of disorders or characteristics (for instance, eye colour, blood type, or vulnerability to illnesses such as cystic fibrosis and hemophilia).

Hence, the correct answer is option 2) Mendelian trait

Also, Read the NCERT Syllabus for Class 12 Other Subjects

• NCERT Syllabus for Class 12 Maths
• NCERT syllabus for class 12 Chemistry
• NCERT syllabus for class 12 Physics
• NCERT Syllabus for Class 12 English

What Students Learn from Principles of Inheritance and Variation NCERT Solutions

This chapter helps students explore the fundamentals of genetics and understand how traits are passed from one generation to another.

1. Students learn about Mendel’s experiments with pea plants, including monohybrid and dihybrid crosses.

2. The role of the chromosomal theory of inheritance and Morgan’s work on linkage and recombination is explained in detail through the Principles of Inheritance and Variation NCERT Solutions.

3. Learners understand variations in inheritance such as incomplete dominance, codominance, multiple alleles, pleiotropy, and polygenic inheritance. These topics are commonly asked in the board exams and medical entrance exams, such as NEET.

4. Different mechanisms of sex determination in various organisms are clearly explained with examples in the Principles of Inheritance and Variation Class 12 question answer.

5. The chapter also provide knowledge about pedigree analysis and human genetic disorders like haemophilia, colour blindness, and sickle-cell anaemia.

What Extra Should Students Study Beyond the NCERT for NEET?

Studying concepts beyond the NCERT will help in gaining conceptual clarity. Some extra concepts that can be studied are given below. Along with this, students need to practice diagrams, flow charts, and definitions, which are given in the NCERT Solutions for Class 12 Biology Chapter 4 Principles of Inheritance and Variation.

NCERT Solutions for Class 12 Biology: Chapter-Wise