Important Questions for CBSE Class 12 Biology Chapter 4 - Principles of Inheritance and Variation 2024-25

Very Short Questions and Answers (1 Marks Questions)

1. Give any two reasons for the selection of pea plants by Mendel for his experiments. 

Ans: The two reasons for the selection of pea plants by Mendel for his experiments are:

(i) Many varieties are present with contrasting forms of characters. 

(ii) They can be easily cross-pollinated as well as self-pollinated. 

2. Name any one plant that shows the phenomenon of incomplete dominance during the inheritance of its flower colour. 

Ans: Dog flower or Snapdragon (Antirrhinum sp.) is the plant that shows the phenomenon of incomplete dominance during the inheritance of its flower colour.

3. Name the base change and the amino acid change, responsible for sickle cell anaemia. 

Ans: The change in base GAG as GUG and substitution of Glutamic acid by valine is responsible for sickle cell anaemia. 

4. Name the disorder with the following chromosome complement. 

(i) 22 pairs of autosomes + X X Y 

Ans: The name of disorders are:

(i) Klinefelter's Syndrome with chromosome complement as 22 pairs of autosomes + X X Y.  

(ii) 22 pairs of autosomes + 21st chromosome + XY. 

Ans: Downs syndrome with chromosome complement as 22 pairs of autosomes + 21st chromosome + XY.

5. A haemophilic man marries a normal homozygous woman. What is the probability that their daughter will be haemophilic? 

Ans: When a haemophilic man marries a normal homozygous woman, is the probability of their daughter being haemophilic is $ 0 $%.

6. A test is performed to know whether the given plant is homozygous dominant or heterozygous. Name the test and phenotypic ratio of this test for a monohybrid cross. 

Ans: A test cross is performed to know whether the given plant is homozygous dominant or heterozygous and 1:1 is the phenotypic ratio of the test cross for a monohybrid cross.

7. Name the phenomena that occur when homologous chromosomes do not separate during meiosis. 

Ans: The name of the phenomenon is non – disjunction that occurs when homologous chromosomes do not separate during meiosis. 

8. Name one trait each in humans & in drosophila whose genes are located on the sex chromosome. 

Ans: The name of traits whose genes are located on the sex chromosome in humans is colourblindness and in drosophila is eye colour.

9. What is meant by aneuploidy? 

Ans: Aneuploidy is the phenomenon of the existing or loss of one or more chromosomes due to failure in the separation of members of a homologous pair of chromosomes during meiosis. 

10. What genetic principle could be derived from a monohybrid cross? 

Ans: The Law of dominance is the genetic principle derived from a monohybrid cross.

11. Which one change is the cause of sickle–cell anaemia? 

Ans: Sickle–cell anaemia is caused due to a point mutation at the 6th position in the B-chain of haemoglobin in which glutamic acid is replaced by valine. 

12. What is a test cross? 

Ans: A test cross is a cross in which offspring with a dominant phenotype is crossed with a homozygous recessive individual to determine the offspring’s genotype for the specific trait. 

13. What is mutagen? Give an example? 

Ans: The physical or chemical agents that cause mutations are called mutagen. For eg x-rays, CNBr etc. 

14. What was the total number of varieties of garden pea which Mendel had taken to start his experiment? 

Ans: The total number of fourteen varieties of a garden pea has been taken by Mendel to start his experiment. 

15. Name any one plant & its feature that shows the phenomena of incomplete dominance? 

Ans: Antirrhinum majus shows the phenomenon of incomplete dominance in flower colour. 

Short Questions and Answers (2 Marks Questions)

1. Identify the sex of the organism as male or female in which the sex chromosome is found as 

(i) ZW in birds 

Ans: ZW in birds: Female 

(ii) XY in Drosophila

Ans: XY in Drosophila: Male

(iii) ZZ in birds  

Ans: ZZ in birds: Female 

(iv) XO in grasshoppers

Ans: XO in grasshopper: Male 

2. Mention two differences between Turner’s syndrome and Klinefelter's syndrome.

Ans: The two differences between Turner’s syndrome and Klinefelter's syndrome is as follows:

3. The human male never passes on the gene for haemophilia to his son. Why is it so? 

Ans: The human male never passes on the gene for haemophilia to his son because the gene for haemophilia is present on the X chromosome. A male has only one X chromosome which he receives from his mother and the Y chromosome from his father. The human male passes the X chromosome to his daughters or female progeny and not to the sons or male progeny. 

4. Mention four reasons why Drosophila was chosen by Morgan for his experiments in genetics. 

Ans: The four reasons for which Morgan has chosen Drosophila for his experiments in genetics are as follows:

(i) Drosophila has a very short life cycle i.e. of 2-weeks.

(ii) It can be grown easily in the laboratory. 

(iii) In single mating it produces a large number of flies. 

(iv) Many hereditary variations have been shown by males and females.

(v) It has only 4 pairs of chromosomes that are distinct in size and shape. 

5. Differentiate between point mutation and frameshift mutations. 

Ans: The difference between point mutation and frameshift mutation is as follows:

6. Give any two similarities between the behaviour of genes (Mendel’s factor) during inheritance and chromosomes during cell division. 

Ans: The two similarities  between the behaviour of genes (Mendel’s factor) during inheritance and the behaviour of chromosomes during cell division are as follows:

(i) In diploid cells, the chromosomes are found in pairs like Mendelian factors. 

(ii) The chromosomes of different homologous pairs are assorted independently into gametes during meiosis. This is at random showing parallelism with mendelian factors. 

7. Which law of Mendel is universally accepted? State the law? 

Ans: Mendel’s law of segregation is universally accepted. The law states that – “The two alleles of a gene remain separate and do not contaminate each other in F1 or the hybrid. At the time of gamete formation, two alleles get separated and pair into different gametes”. 

8. How will you find out whether a given plant is homozygous or heterozygous? 

Ans: To test whether a plant is homozygous or heterozygous, a test cross is performed in which an individual is crossed with homozygous recessive for the trait. If a plant is heterozygous, the progeny of testcross consists of tall and dwarf plants in the ratio of 1:1.

If the plant is homozygous, the progeny of testcross will have all tall plants. 

9. Why do sons of a haemophilic father never suffer from this trait? 

Ans: Since haemophilic is a sex-linked character and the gene responsible for haemophilia is present on the X chromosome, it shows criss-cross inheritance i.e., the X chromosome is transferred from father to his daughter and not to his son. Therefore the son of a haemophilic father is never haemophilic. 

10. How is the child affected if it has grown from the zygote formed by an XX-egg fertilized by Y-carrying sperm? What do you call this abnormality? 

Ans: If a child has grown from the zygote formed by XX-egg fertilized by Y-sperm, the child will suffer from Klinefelter syndrome and he will have XXY genotype. The Klinefelter syndrome is characterized by the child’s development of prominent feminine characters such as tall stature with feminised physique, breast development, pubic hair pattern, poor beard growth, & sterility. 

11. The map distance between a certain organisms genes A and B is 4 units, between B and C is 5 units and between C and D is 8 units. Which one of the gene pairs will show more recombination frequency? Give reason.

Ans: Here, C and D will show maximum gene recombination because genes that are linked more closely are having the least frequency of recombination and vice versa. 

12. Give the chromosomal constitution and related sex in each of the following:- 

(i) Turner syndrome 

Ans: Turner syndrome – In Turner syndrome XO is the chromosomal constitution and is female containing 45 chromosomes and lacking in one X-Chromosome. 

(ii) Klinefelter syndrome 

Ans: Klinefelter syndrome – In Klinefelter syndrome XXY is the chromosomal constitution and is male containing 47 chromosomes and has one extra X-chromosome. 

13. What is pedigree analysis? How is it useful? 

Ans: The analysis of case history about the inheritance of a specific trait in several generations of a family is named pedigree analysis. It provides a robust tool that's utilized to trace the inheritance of specific traits or abnormalities or diseases.

14. What are multiple alleles? Give an example? 

Ans: The presence of more than two alleles of a trait is called multiple alleles. E.g. in human beings, four types of blood groups are recognized and their different alleles ${ I }^{ A }$, ${ I }^{ B }$, and ${ I }^{ O }$ of a gene determines the phenotype of four blood groups. 

Long Questions and Answers (3 Marks Questions)

1. A woman with an O blood group marries a man with an AB blood group 

(i) work out all the possible phenotypes and genotypes of the progeny. 

Ans: When a woman with an O blood group marries a man with an AB blood group then- 

All the possible phenotypes of the progeny include blood groups A and B and genotypes of the progeny will be${ I }^{ A }{ i }$and${ I }^{ B }{ i }$ because blood group AB has alleles ${ I }^{ A }{ I }^{ B }$  and blood group O has ${ i }{ i }$ which on the cross gives both types of blood groups A and B while the genotype of progeny will be ${ I }^{ A }{ i }$ and ${ I }^{ B }{ i }$. 

Father’s genes: ${ I }^{ A }{ I }^{ B }$ 

Mother’s genes: ${ i }{ i }$

Therefore, the cross will be

(ii) Discuss the kind of dominance in the parents and the progeny in this case. 

Ans: In the above-mentioned case, the kind of dominance in the parents and the progeny is that they ( ${ I }^{ A }$ and ${ I }^{ B }$) are equally dominant or co-dominant. In the case of multiple allelism, a gene I exists in three allelic forms, ${ I }^{ A }$, ${ I }^{ B }$ and ${ i }$. 

2. Explain the cause of Klinefelter​’s syndrome. Give any four symptoms shown by the sufferer of this syndrome. 

Ans: The presence of an extra chromosome in males i.e., XXY is the cause of Klinefelter​’s syndrome. The symptoms found in the sufferer of Klinefelter’s syndrome are the development of breast, female type pubic hair pattern, poor beard growth, underdeveloped testes, and tall stature with the feminized physique.

3. In Mendel's breeding experiment on garden peas, the offspring of the ${ F }_{ 2 }$ generation are obtained in the ratio of ${ 25 }$% pure yellow pod, ${ 50 }$% hybrid green pods and ${ 25 }$% green pods. State 

(i) which pod colour is dominant 

Ans: In Mendel's breeding experiment on garden peas, Green pod colour is dominant in the offspring of the  ${ F }_{ 2 }$ generation.

(ii) The Phenotypes of the individuals of the ${ F }_{ 1 }$ generation. 

Ans: Green pod colour is the phenotype of the individuals of the  ${ F }_{ 1 }$ generation. 

(iii) Workout the cross.

Ans: The cross will be shown as:

Phenotypic ratio 3: 1 

Genotypic ratio 1: 2: 1 

4. In Antirrhinum majus a plant with red flowers was crossed with a plant with white flowers. Work out all the possible genotypes and phenotypes of ${ F }_{ 1 }$ and  ${ F }_{ 2 }$ generations, comment on the pattern of inheritance in this case? 

Ans: When Antirrhinum majus, a plant with red flowers, was crossed with a plant with white flowers, the  ${ F }_{ 1 }$ hybrid was pink i.e. a middle colour between red and white will appear which indicates that both red and white are incompletely dominant. When ${ F }_{ 1 }$ individuals were self-pollinated, the  ${ F }_{ 2 }$ generation consists of red, pink and white flowers that appear in the ratio of 1:2:1 respectively. The pattern of inheritance of flower colour in snapdragon or Antirrhinum majus is an example of incomplete dominance.

5. A red-eyed male fruit fly is crossed with a white-eyed female fruit fly. Work out the possible genotype and phenotype of  ${ F }_{ 1 }$ and  ${ F }_{ 2 }$ generation. Comment on the pattern of inheritance in this cross? 

Ans: When a red-eyed male fruit fly is crossed with a white-eyed female fruit fly, the offspring will have both white-eyed male & red-eyed female in the ratio of 1:1  in the  ${ F }_{ 1 }$ generation. In the  ${ F }_{ 2 }$ generation, ${ 50 }$% females will be red-eyed and ${ 50 }$% will be white-eyed, similarly, in males ${ 50 }$% will be red-eyed and ${ 50 }$% will be white-eyed. This result indicates that in fruitflies males transmit their sex-linked characters to their grandson through their daughter; such type of inheritance is called criss-cross inheritance.

6. A man with AB blood group marries a woman with O group blood. 

(i) Work out all the possible phenotypes and genotypes of the progeny. 

Ans. When a man with AB blood group marries a woman with O group blood then, Half the progeny will have blood group A with genotype ${ I }^{ A }  { I }^{ O }$ & half the progeny will have blood group B with genotype  ${ I }^{ B }  { I }^{ O }$.

(ii) Discuss the kind of domination in parents and progeny in this case.

Ans: In this case both the genes ${ I }^{ A }$ and  ${ I }^{ B }$ are dominant over the  ${ I }^{ O }$ gene therefore the progeny shows either blood group A or B. Since in parents both the dominant genes are present together this phenomenon is called codominance. 

7. In a cross made between a hybrid tall and red plant (TtRr) with dwarf & white flower (ttrr). What will be the genotype of plants in the ${ F }_{ 1 }$ generation? 

Ans: When a cross is made between a hybrid tall and red plant (TtRr) with dwarf and white flower (ttrr) plants then TtRr, ttRr, ttRr, and ttRr will be the genotypes of the plants develop in the  ${ F }_{ 1 }$ generation.

8. How sex is determined in human beings? 

Ans: In human beings, all the females bear a pair of X-chromosomes while males have one X-chromosome and one Y-chromosome which is comparatively smaller in size. Thus, in a cross between males and females, there is an equal probability of males and females in progeny and sex is determined based on the presence of a Y-chromosome in the progeny. If a Y-chromosome is present it is male otherwise it is a female child.

9. A smooth seeded and red-flowered pea plant (SsRr) is crossed with a smooth seeded and white-flowered pea plant (Ssrr). Determine the phenotypic and genotypic ratio in  ${ F }_{ 1 }$ progeny? 

Ans: When a  cross is done between smooth seeded and red-flowered pea plant (SsRr) along with a smooth seeded and white-flowered pea plant (Ssrr) then 1:3:3:1 will be the phenotypic ratio in ${ F }_{ 1 }$ progeny i.e., rough seed and red flower =1, smooth seed and red flower =3, smooth seed and white flower =3, and rough seed and white flower =1. The genotypic ratio is  ${ F }_{ 1 }$ progeny will be 1:2:1:2:1:1 i.e., SSRr =1, SsRr =2, SSrr =1, Ssrr =2, ssRr =1, and ssrr =1.

Very Long Questions and Answers (5 Marks Questions)

1. A dihybrid heterozygous round, yellow seeded garden pea (Pisum sativum) was crossed with a double recessive plant. 

(i) What type of cross is this?

Ans: When a dihybrid heterozygous round- yellow seeded garden pea (Pisum sativum) was crossed with a double recessive plant then, this type of cross is known as a dihybrid test cross. 

(ii) Work out the genotype and phenotype of the progeny. 

Ans: Below given is the Work out the genotype and phenotype of the progeny- 

(iii) What principle of Mendel is illustrated through the result of this cross?

Ans: Through the result of the dihybrid test cross Mendel’s principle of independent assortment is illustrated. 

2. In dogs, barking trait is dominant over silent trait & erect ears are dominant over drooping ears. What is the expected phenotypic ratio of offspring when dogs heterozygous for both the traits are crossed? 

Ans: When dogs are heterozygous for both (barking/silent trait and erect ears/drooping ears) the traits are crossed then the expected phenotypic ratio of offspring will be  9 : 3 : 3: 1 where traits for barking & erect ears = 9,  barking & drooping ears =3,  silent & erect ears = 3, and silent & drooping ears =1. 

3. Differentiate between dominance, codominance and incomplete dominance with one example each. 

Ans: The differences between dominance, codominance and incomplete dominance with one example are as follows:

Dominance	Codominance	Incomplete dominance
When a cross is made between two characters in which one character is completely dominant over the other character in the progeny, this phenomenon is called dominance.	If the two equally dominant genes are present together, both of them will be equally expressed, this phenomenon is called codominance.	When a cross is made between two characters of which none of them is completely dominant then an intermediate character develops in the progeny, this phenomenon is called Incomplete dominance.
For eg. When the true-breeding tall pea plant is crossed with the true-breeding dwarf pea plant, all the plants in the  ${ F }_{ 1 }$ generation are tall; this shows that the tall character is dominant over dwarf in the progeny.	For eg. alleles of the blood group, ${ I }^{ A }$ and ${ I }^{ B }$ are dominant over ${ I }^{ O }$ but when both the alleles are present together, both of them will equally express and form a phenotype AB.	For eg. when a cross is made between a red flower & a white flower in a snapdragon flower an intermediate pink colour appears in the progeny.

4. A dihybrid heterozygous tall and yellow pea plant was crossed with the double recessive plant. 

(i) What type of cross is this? 

Ans: When a dihybrid heterozygous tall and yellow pea plant was crossed with the double recessive plant then, this type of cross is known as a test cross.

(ii) Work out the genotype & phenotype of progeny.

Ans: Below given is the work out the genotype & phenotype of progeny- 

(iii) What principle of Mendel is illustrated through the result of this cross?

Ans: Through the result of testcross Mendel’s Principle of Independent Assortment is illustrated. According to the principle, in the inheritance of contrasting characters, the factors of each pair of characters segregate independently of the factors of the other pair of characters. 

Vedantu’s Important Questions of Chapter 4 Biology Class 12

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• The PDF provides a simplified explanation of major topics, making it easier for students to understand and clear their doubts.

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• The questions in the PDF cover all the major parts and points of the chapter, serving as a summary for students.

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Class 12 Biology Chapter 4 Important Questions - Summary

• Heredity can be defined as a mechanism of transfer heritable traits and genes from parents to their progeny.

• Genetics is the branch of biology that discusses inheritance and variation concepts and mechanisms.

• Inheritance is the basis of inheritance and characteristics are passed down from the parents to the offspring through this process. The continuity of the gene pool is preserved by the inheritance mechanism.

• Genes are found on chromosomes and are the fundamental unit of inheritance.

• Among individuals of one species, variation exists. Variation can happen due to recombination, crossing over, environmental effects, and mutation on the expression of genes that are found on chromosomes.

Mendel's Law of Inheritance:

1. Law of Dominance States That:

• Characters are regulated by discrete units that are called factors.

• Factors often emerge in pairs.

• One member of a pair dominates the other in a distinct pair of factors.

2. Law of Segregation States That:

Each of the alleles gets separated during the process of meiosis, at the time of gamete formation. In this process, there occurs no blending and also the characters are found to be passed to distinct gametes. The production of homozygotes can be done using only a single kind of gametes whereas heterozygotes are produced with the help of two distinct kinds of gametes.

3. Law of Independent Assortment States That:

The law specifies that 'when two pairs of characteristics are mixed in a hybrid, the segregation of one pair of characters is independent of the other pair of characters.' Two new combinations are produced in Dihybrid cross, round green & wrinkled yellow due to an independent assortment of seed color, i.e. yellow and green and seed shape characteristics, i.e. round, wrinkled.

Chromosomal Theory of Inheritance:

• Sutton and Boveri accepted the findings of Mendel and claimed that the carrier of genes is chromosomes.

• Chromosomes exist as a homologous pair and a gene's two alleles are found at the same location on the homologous pair of chromosomes.

• In the course of gamete formation, homologous chromosomes differentiate during meiosis.

• Chromosomes independently segregate and assort.

• Gametes pair up and produce the progenies with a diploid number of chromosomes, which are similar to the parents. The fusion of gametes occurs during fertilization.

• Morgan worked extensively on fruit flies, Drosophila melanogaster, and gave experimental arguments to confirm the chromosomal theory of inheritance.

Linkage and Recombination:

• The physical association of chromosome-located genes is known as linkage.

• In a dihybrid cross, If two of the genes are closely related or situated on the very same chromosome, then it is observed that the parental combination is way more common than recombinants or non-parental combinations.

• The recombination and linkage do vary on the distance present between the pair of genes. As the distance between the genes increases, the probability of recombination also increases.

Sex Determination:

• In various species, there are various sex determination mechanisms involved.

• First, Henking noticed the X chromosome and called it the X body.

• The chromosomes that determine sexual character formation are referred to as sex chromosomes and the remaining chromosomes are referred to as autosomes.

• If the male develops two distinct forms of gamete then It is classified as male heterogamety. Examples may include grasshoppers, humans, drosophila, etc.

• If the female develops two distinct forms of gamete then It is termed as female heterogamety. Examples may include birds.

Mutation:

• Any modification in the DNA sequence is considered a mutation. Viable mutations are observed to undergo inheritance from one generation to another.

• This process of alteration in the DNA sequence might affect both the genotype and an organism's phenotype.

• It is associated with different disorders, although not all mutations are harmful.

• The mutation results in modifications such as deletion, addition, replication, replacement, etc. The significant cause of cancer is mutation.

Genetic Disorders:

Genetic disorders can usually be divided into two groups, Mendelian and Chromosomal disorders.

1. Mendelian Disorders:

• Mendelian disorders occur due to single gene modification.

• As per Mendel's rule, it follows the same inheritance pattern.

• Pedigree Analysis can be used to know the pattern of inheritance and also decide whether the trait is dominant or recessive.

• Sickle cell anaemia, Muscular dystrophy, Colour blindness, Thalassemia,Cystic fibrosis, Phenylketonuria are few of the examples of mendelian disorders.

2. Chromosomal Disorders:

• Chromosomal diseases are diseases that occur due to chromosomes’ absence, excess, or irregular arrangement.

• There are two types of chromosomal disorders:

1. Aneuploidy - Gain or loss of a chromosome or more. It occurs due to the chromatid segregation failure during meiosis anaphase.

2. Polyploidy - It is commonly present in plants. This occurs in an increase in the number of chromosomes.

• Examples of chromosomal disorders may include Turner’s syndrome, Down's syndrome, and others.

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Conclusion

The principle of inheritance and variation is a fundamental concept in biology that explores how traits are passed from one generation to another and the factors that contribute to genetic diversity. Vedantu's collection of important questions on this topic provides a valuable resource for Class 12 biology students to enhance their understanding and preparation for exams. These questions cover a wide range of topics, including Mendelian genetics, sex determination, genetic disorders, DNA replication, and genetic engineering. By practicing these questions, students can reinforce their knowledge, improve their problem-solving skills, and gain confidence in tackling biology examinations. Vedantu's comprehensive question bank is a valuable tool for students aiming to excel in the principle of inheritance and variation.