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

Principles of Inheritance and Variation is an important chapter for 12th with the topic of inheritance of traits from parent to parent and diversification of characteristics within species. It is the most important chapter one should learn to understand genetics and heredity, and briefly discusses Mendel's Laws, sex determination, mutation, and hereditary diseases.

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1. Principles of Inheritance and Variation Class 12 NCERT Solutions- PDF Download
2. NCERT Solutions for Class 12 Biology Chapter 4: Questions and answers
3. Overview of Class 12 Biology Chapter 4
4. Important Topics in Class 12 Principles of Inheritance and Variation NCERT Solutions
5. Practice Questions for Class 12 Chapter 4 Principles of Inheritance and Variation
6. NCERT Solutions for Class 12 Biology: Chapter-wise

In this article, we will describe the major points that were described in this chapter and look at how NCERT solutions can help with exam preparation. For the CBSE exam preliminary students, NCERT Solutions for Class 12 is the most important asset. These answers include step-by-step reasoning and practice questions that enable students to understand intricate topics such as dominant and recessive genes, mono and dihybrid crosses, and Punnett squares.

Principles of Inheritance and Variation Class 12 NCERT Solutions- PDF Download

The downloadable PDF of the questions with detailed answers is given below :

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• NCERT Notes for class 12 biology chapter 4 Principles of Inheritance and Variation
• NCERT Exemplar for class 12 biology chapter 4 Principles of Inheritance and Variation

NCERT Solutions for Class 12 Biology Chapter 4: Questions and answers

Below are the detailed answers which can help you:

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 life of pea plants is short and they yield numerous seeds in a single generation.

3. Pea flowers are bisexual and exhibit self-pollination, reproductive whorls being covered by 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) Dominance 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 F1 generation only tall plants were seen, no plant was 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 to check if a person showing a dominant phenotype is homozygous dominant or heterozygous for the character.

Definition: A test cross consists of a cross between a person whose genotype is unknown (but shows the dominant phenotype) with one person who is homozygous recessive for the same character.

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 (0)

• Dwarf and Green (ttyy): ttyy (1)

• The genotypic ratio is 3:3:0:1

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 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: Yellow body, white eyes, and 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 in genetics.

Answer:

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

• Chromosomal Theory of Inheritance: Morgan along with his co-workers presented robust 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 which 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 to inherit a given genetic condition.

• Genetic Counseling: 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 fertilizes the egg, the child will be female (XX). If a Y-bearing sperm fertilizes 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 parents are heterozygous. Thus, the genotype of the father will be IAi and that of the mother will be IBi . The child having blood group O will have genotype ii. The other children may have genotypes IAi (A blood group), IBi (B blood group) and IAIB (AB blood group).

Q13 . Explain the following terms with an example

(a) Co-dominance

Answer:

Codominance-

It refers to the phenomenon of two alleles lacking a dominance-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 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, F2 generation has 1red: 2 pink: 1white. The pink flower is due to incomplete dominance

Q14. What is point mutation? Give one example.

Answer:

• Point Mutation: A mutation which 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 Baveri in 1902 proposed the chromosomal theory of inheritance.

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

Answer:

Sickle Cell Anemia:

• Genetic Basis: Autosomal recessive disease due to mutation of the gene for beta-globin in 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: Mental retardation, characteristic face (e.g., flat nose, slanted eyes), heart malformation, and other medical disorders.

NCERT Solutions for Class 12 - Subjectwise

Overview of Class 12 Biology Chapter 4

The chapter Principles of Inheritance and Variation covers basic concepts related to genetics and evolution. Heredity and variation are important components of the processes of evolution. The Principles of Inheritance and Variation NCERT solutions class 12 becomes essential for learning as well as for board examinations and competitive examinations like NEET.

Important Topics in Class 12 Principles of Inheritance and Variation NCERT Solutions

Some of the important topics of this chapter include:

Important subtopics of Class 12 Biology Principles of Inheritance and Variation NCERT Solutions

The important subtopics of this chapter include:

• Law of Dominance

• Law of Segregation

• Incomplete Dominance

• Co-dominance

• Law of Independent Assortment

• Chromosomal Theory of Inheritance

• Linkage and Recombination

• Sex Determination in Humans

• Sex Determination in the Honey Bee

• Pedigree Analysis

• Mendelian Disorders

• Chromosomal Disorders

Important diagrams in Chapter 4

Here are some important diagrams from Principles of Inheritance and Variation:

• Mendel’s Law of Segregation

• ---

  Law of Independent Assortment

• ---

  Law of Dominance

• ---

  Monohybrid Cross

• Mendel and his work

Also Read 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

How do you solve NCERT Class 12 Biology Chapter 4 questions effectively?

To solve the textbook questions effectively, one needs to understand the concepts clearly first. The chapter has certain key terms, such as Mendel's laws, monohybrid and dihybrid cross, mutation and genetic disorders. Making notes of the chapter is really important by highlighting all the important points and terms. The Principles of Inheritance and Variation NCERT Solutions contains the solved exercise questions with to-the-point answers and necessary information. Students must practice these solutions to have an overall idea of the chapter and build confidence before the exams.

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

Practice Questions for Class 12 Chapter 4 Principles of Inheritance and Variation

Below are a few solved practice questions:

Q1. Name the Scientists who explained Mendel’s laws after the chromosomes were discovered.

Answer:

Walter Sutton and Theodore Boveri explained Mendel’s laws after the chromosomes were discovered.

Q2. State Mendel’s law of dominance.

Answer:

Mendel's law of dominance: The offspring always exhibits a dominant trait from the two alleles received from parents, the only dominant allele is expressed and the other will be suppressed.

Q3. Explain Mendel’s “Law of segregation” in a typical monohybrid cross with the help of a suitable example.

Answer:

Mendel’s “Law of Segregation"

Q4. State what are Mendelian disorders. Both thalassemia and colour blindness are categorised as Mendelian disorders. Justify.

Answer:

Mendelian Disorder is a type of genetic disorder occurring due to alterations in one gene or as a result of anomalies in the genome.

Thalassemia:

It is an inherited autosomal recessive disorder which affects the blood. The individuals whose one of the parents is a carrier also become carriers of the disease. If both parents are carriers of the disease, the individual has a 25% chance of inheriting the disease.

Colour blindness:

It is a genetic disorder which is inherited from the mother. Most of the colour-blind people have red or green colour blindness. If the male child inherits this colour blindness from the concerned mother, he obtains an X chromosome from the mother which carries the gene for colour blindness. Hence, the majority of the male will be affected.

Q5. (a) Why did T.H. Morgan select Drosophila melanogaster for his experiments ?

(b) How did he disprove the Mendelian dihybrid F2 phenotypic ratio of 9:3:3:1? Explain giving reasons.

Answer:

a) Morgan worked with the tiny fruit flies, Drosophila melanogaster, which was found very suitable for his experiments because of the following reasons

1. They could be grown on a simple synthetic medium in the laboratory.
2. They complete their life cycle in about two weeks, and a single mating could produce a large number of progeny flies.
3. There was a clear differentiation of the sexes – the male and female flies are easily distinguishable.
4. It has many types of hereditary variations that can be seen with low-power microscopes.

b) Morgan observed that genes of the traits closely linked on the same chromosome show a very less recombinant percentage, whereas genes of the traits loosely linked on the same chromosome showed a higher recombinant percentage.

NCERT Solutions for Class 12 Biology: Chapter-wise

In short, the Class 12 Biology Chapter 4 NCERT solutions educate students on the process by which traits are passed from one generation to the next and why organisms vary. It informs them of important principles like Mendel's laws of inheritance and genetic variation types. Learning such subjects not only helps in students' examination knowledge but also informs them about how genetics influence life around us. This is relevant in the fields of science, medicine, agriculture, and the environment. In general, it makes the students enjoy the way life works and prepares them to solve real biological problems in the future. This chapter is important to prepare for CBSE board exams and other entrance exams.

NCERT Exemplar Class 12 Solutions