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Principles of Inheritance and Variations Class 12 Important Questions And Answers MCQ PDF

Principles of Inheritance and Variations Class 12

Principles of Inheritance and Variations Class 12 – Introduction

Inheritance is  transfer of the genes from the parents to offsprings. The principles of inheritance and variation were explained by Gregor Mendel in his experiments on the  pea plant. He stated the three laws of inheritance on the  basis of his observations with  pea plant:

1.Law of Dominance

2.Law of Segregation

3.Law of Independent Assortment

Chapter 5 Principles of Inheritance and Variations Class 12

Principles of Inheritance and Variations Class 12
Principles of Inheritance and Variations Class 12

Principles of Inheritance and Variations Class 12 – Very Short Answer Types Questions

Q.1. What is  cross known as when the progeny of F1 and a homozygous recessive plant is crossed with each other? Write its advantage.

A.1. The cross is called as a  test cross. The advantage of test cross is  to determine genotype of parent plant.

Q.2. Write  the criteria for selecting the organisms to perform the  crosses to study  inheritance of a few traits?

A.2. Following criterias  are adopted for selecting the  organisms:

  • Traits should be easily visible.
  • Organisms should have the different traits.
  • They should have  short life span.
  • They must be having true breeds.
  • Pollination procedure should be simple.
  • Traits can be manipulated easily.
  • Random mating of the gametes should take place.

Q.3. Following pedigree shows a particular trait which is absent in  parents but found in subsequent generation irrespective of gender. Analyze pedigree chart and draw a conclusion.


A.3. Pedigree shows an autosomal recessive disorder. The parents are  carrier of the disease so  disease will be visible in only few offsprings. Other offsprings will be either a carrier or a  non-carrier.

Q.4. Explain why  Mendel self-pollinated the tall F1 plants to get  F2 generation and crossed a pure breeding tall plant with a pure breeding dwarf plant to obtain  F1 generation?

A.4. Genotype of 50% of  offspring will resemble one of the parent and the rest 50% will resemble the other parent. The F1 generation obtained from cross is heterozygous. So selfing F1 generation is sufficient to obtain  F2 generation. 

       It would also help to understand inheritance of the selected traits over generations.

Q.5. How are alleles of a gene different from other Allele? Write its importance?

A.5. Alleles are alternative forms of same gene.

       For example a gene for height comprises of the two alleles, one for tall (T) and the other for  dwarf (t). They differs  in their nucleotide sequence which results in different phenotypes.

Importance of the alleles in gene are –

  • They are essential in studying inheritance and the behaviour patterns.
  • They show variations in population due to try the contrasting phenotypes of the character.

Q.6. How far are  genes and the environment responsible for expression of a particular trait?

A.6. Genes remains active throughout our lives, switching on and off their expression in response to environment. External factors such as the light, temperature, nutrition, etc. are responsible for gene expression exhibiting changes in phenotype. Genes provides the  potentiality while environment provides an opportunity for expression of traits.

Principles of Inheritance and Variations Class 12 Quiz

Q.7. Write the  genetic basis of  wrinkled phenotype of the pea seed?

A.7. Single gene determines shape of the seed. The (R) is for  round shape, which is dominant over (r) for wrinkled seed. If homozygous alleles control seed shape, it will depict  phenotype of the same alleles, for example, RR (round), rr (wrinkled). If  alleles are heterozygous, phenotype of dominant allele will be expressed, for example Rr (round).

Q.8. Why  an individual have only two alleles even if a character shows the  multiple alleleism?

A.8. Multiple forms of an allele that occurs on  same gene locus are called as multiple alleles. But an individual carries only the two alleles. This happens because a zygote is formed by  fusion of the haploid sperm and the egg. They have only one allele for each of the trait. When  zygote becomes diploid, it has two alleles for each of the trait.

Q.9. How   mutation is  induced by mutagen? Explain your answer by giving examples.

A.9. Mutagen changes  base sequence by the insertion, deletion or substitution and induces mutation in the organism.

Q.10. Write difference between dominance, co-dominance and incomplete dominance.

A.10. Dominance is a phenomenon in which one variant of a gene masks effect of a different variant of  same gene.

Co-dominance is defined as the relationship between the two alleles of a gene. In this none of  alleles are recessive and phenotype of all  alleles are expressed.

Incomplete dominance is defined as  a form of intermediate inheritance in which one of allele of a gene for a specific trait is not expressed completely over its paired allele.

Q11.Write about the chromosomal theory of inheritance?

A11.Chromosomal theory of inheritance is defined as  fundamental theory of the genetics, which recognizes chromosomes as  carriers of the genetic material.

Principles of Inheritance and Variations Class 12

Q12. What do you mean by Linkage?

A12. In genetics,  linkage is defined as tendency of the genes to remain combined together during  inheritance. This phenomenon was first observed and reported by William Bateson and R.C. Punnet in  early  1900 century.

Principles of Inheritance and Variations Class 12 – Short Answer Types Questions

Q.1. How is it possible for th the child to have a blood group O if both the parents have the blood groups A and B?

A.1. Case I-

 If  father is IA and mother is IB, the child will have the blood groups AB, A, B, O. 

Case II- 

If father is IA and mother is IB,  child will have the same blood groups as in  the case I, i.e., AB, A, B, and O. Thus if  parents have t the heterozygous alleles,  child will have the blood group O.

Q.2. Write in brief about Down’s syndrome.

A.2. Down’s syndrome is an autosomal genetic disorder which is caused by the trisomy at the chromosome number 21, i.e., there is an extra copy of the chromosome 21. This condition affects both physically and mentally. Children born with the Down’s syndrome has a flat nose and small ears. They faces problems in thinking, understanding and reasoning throughout their lives. They might have trouble in  hearing and in seeing. They are often sometimes be dwarf.

Q.3. Why a women exceeding 40 years of age have more chances of having the child with Down’s syndrome?

A.3. Women exceeding 40 years of age have more chances of having a child with the Down’s syndrome because increased age affects  meiosis of chromosomes adversely. The meiosis remains incomplete until the fertilization. It remains arrested at prophase-I and chromosome is unpaired. If fertilisation occurs after a very long gap, chromosomes will have to remain unpaired for a longer time. The longer the time of unpairing,  greater are chances of its non-disjunction, and hence conditions like the trisomy arises.

Q.4. How you can say that  genes are located on the chromosomes?

A.4. Chromosomal theory of inheritance is proposed by Bovine and Sutton stating  that  genes are present on specific locations on a chromosome. Later, Thomas Morgan observed mutation in  eye colour of fruit flies and based on  inheritance patter concluded that  gene responsible for  eye colour is located on  X-chromosome.

Q.5. Plant with  yellow flowers are  crossed with a plant with red flowers.  F1 progeny obtained had orange flowers. What is the inheritance pattern?

A.5. Inheritance is incomplete dominance. In this, a new intermediate phenotype between  two original phenotypes is obtained. One allele for a specific trait is not completely expressed over other allele for same trait.

Principles of Inheritance and Variations Class 12 Study Materials

Q.6. Write the characteristics of true-breeding line.

A.6. The  characteristics of a true breeding plant is as follows:

  • It undergoes the process of self-pollination.
  • It depicts the stability in inheritance for several generations.
  •  It provide gametes with the similar traits, hence used as the parents for the artificial hybridization.
  • Homozygous recessive plants are used to identify the genotype through a test cross.

Q.7. Who proposed  The Chromosomal theory of inheritance?

A.7. Theodor Boveri and Walter Sutton are  two scientists who were credited with developing Chromosomal Theory of Inheritance during  early 1900s.

Principles of Inheritance and Variations Class 12

Q.8. Define  Recombination? Write its applications with reference to the genetic engineering.

A.8. Recombination is defined as the process of producing a new combination of the genes by crossing over during the meiosis.

Applications of Recombination are-

  • It’s a  means of introducing the new traits.
  • Variability is increased, which is necessary for the natural selection.
  • It is used for the preprations of  linkage chromosome maps.
  • Desired recombinants produced as a result of the crossing over are selected by  plant breeders to produce the new crop varieties.

Principles of Inheritance and Variations Class 12

Q.9. Explain why  sickle-cell anaemia persist in human population when it is believed that harmful alleles get eliminated from population after a certain time?

A.9. Sickle cell anaemia is an autosomal recessive disease where red blood cells ,RBC’s become sickle-shaped, inhibiting oxygen-carrying capacity of  blood. Despite of this, it protects carrier from the malaria. Individuals with the heterozygotes HbAS survive more than homozygotes HbSS because they are not exposed to same severity of risks.

Q.10. What do you mean by artificial selection. Does it affects process of natural selection?

A.10. Artificial selection is  intentional breeding of the plants and animals where breeders select desired traits and make them breed to produce offsprings with required characteristics. It is an ancient method of the genetic engineering. It surely affects process of natural selection. The organisms  cannot evolve on their own. The process is a threat to the biodiversity. The traits are not selected considering fitness of  organism.

Q11.What do u mean by Sex chromosomes?

A11. Sex chromosomes are defined as a pair of the chromosomes, which determines whether an individual is the male or the female. In all mammals, including the humans beings, have sex chromosomes X and Y  in their cells. Females have the two X chromosomes(XX), and males have an X and a Y chromosome (XY).

Q12.What do u mean by  chromosomes and who discovered the chromosomes?

A12. Chromosomes are the thread-like structures present within  nucleus of a cell. Each species has a unique number of the chromosomes and it varies from one organism to the  another. Humans have 23 pairs of chromosomes and the Humans have 23 pairs of chromosomes.  

      Carl Wilhelm von Nageli, a Swiss botanist, discovered the chromosomes. He was first person to observe the chromosomes in the plant cells in  year 1842.

Principles of Inheritance and Variations Class 12 – Long Answer Types Questions

Q.1. What do u mean by  aneuploidy? Write the differences  between the aneuploidy and the polyploidy.

A.1. Aneuploidy is defined as the chromosomal abnormality in which one or more chromosomes are gained or lost during the meiosis due to non-disjunction of the chromosomes.

Differences between the aneuploidy and the polyploidy are as follows –

Polyploidy is a type of the chromosomal aberration containing an entire extra set of the chromosome. It may be the triploid or the tetraploid. This phenomenon is common in case of the plants. It is, however lethal in case of thr6animals.

Q.2. Describe  individuals with  following chromosomal abnormalities:

  1. Trisomy at the  chromosome number 21
  2. XXY
  3. XO


1) Trisomy– Trisomy results in an autosomal linked the genetic disorder known as the Down’s syndrome. The individuals exhibit following characteristics:

  • Protruding tongue
  • Round head
  • Slanting eyes
  • Short height
  • Open mouth
  • Short neck
  • Mental retardation
  • Under developed genitals and gonads.

2) XXY– The presence of an additional copy of an X-chromosome results in the Kleinfelter’s syndrome. The patient exhibits  following characteristics:

  • The male individual possesses feminine characteristics.
  • Development of the breasts in the males
  • Male is sterile
  • Poor beard growth
  • Feminine voice

3) XO– Loss of the X-chromosome results in the Turner’s syndrome. Characteristics are as follows –

  • The female is sterile.
  • The ovaries are immature.
  • Webbed neck
  • Thorax is shield-shaped
  • Under-developed breasts.
  • Puffy fingers
  • Short height
  • Uterus is small.

Q.3. Colour-blind father has a daughter with the normal vision. The daughter marries a man with a normal vision. What is probability of her children to be colour blind? Explain in details with  help of a pedigree chart.


50% of the daughters are carrier while 50% have the normal vision. 50% of the  sons are diseased while  other 50% will have the normal vision.

Q.4.Tall plant with the red flowers (dominant) is crossed with a dwarf plant with the white flowers (recessive). Work out a dihybrid cross and state dihybrid ratio. What will be the effect on dihybrid ratio if two genes are interacting with each other?


dihybrid cross
Principles of Inheritance and Variations Class 12

Standard dihybrid ratio observed is 9:3:3:1. If  two genes interact with values will deviate. This is because when genes are linked they do not exhibit independent assortment and remain together in gametes and offsprings. Dihybrid ratio thus obtained is 3:1.

Q.5. Why Drosophila is used extensively for the genetic studies?

A.5. Drosophila is extensively for the genetic studies because it has following characteristics:

  • They have a life span of the two weeks.
  • They can be grown in lab on simple synthetic medium.
  • Large number of the progenies are produced by a single mating.
  • Male and female  Drosophila can be differentiated very easily.
  • It has many variations easily visible under the simple microscope.

Principles of Inheritance and Variations Class 12 toppers notes

Q6. Write the characteristics of Chromosome Theory of Inheritance.

A6. The important characteristics of Chromosome Theory of Inheritance are:

  1. Fertilization restores the diploid condition.
  2. Chromosomes segregates and assort independently.
  3. Homologous chromosomes separate at  time of meiosis.
  4. Both the chromosomes, as well as the genes, exist in pairs within diploid cells.
  5. Gamete only contains one chromosome of a particular type and only one of the two alleles of a character.

Q7.Write about the following terms-  autosome, hemizygous, homozygous, and heterozygous?


Autosome– All the chromosomes apart from  sex chromosomes are known as the Autosomes. The number of autosomes differs from one organism to the another. Humans have 44 number or 22 pairs of the autosomes.

Hemizygous– It is a condition in which an organism has only one copy of a gene or DNA sequence present in the diploid cells.

Homozygous— Here  an organism has two similar alleles of the  given gene (XX).

Heterozygous–It is a condition in which an organism has the two different alleles of the given gene (XY).

Q8.What do you mean by Sex-linkage?

A8. Sex linkage is  defined as  phenotypic expression of an allele, which is dependent on  individual’s gender. It describes the presentation of  chromosome and sex-specific patterns of the inheritance. Sex linkage is directly tied to  sex chromosomes– homogametic sex and heterogametic sex. In the  mammals, the homogametic sex  (XX) is the  female and the heterogametic sex (XY) is the male. Thus, the sex-linked genes are carried on X chromosome.

Q9.Explain why  colour blindness is  more prominent in case of males than in  females?

A9. Colour blindness is a sex-linked disorder and genes responsible are present on X-chromosome. To become affected by disease,  female should possess alleles for colour blindness on both X-chromosomes. If the allele is present on only one of the chromosome,  female becomes a carrier of  disease. Since the  males have only one X-chromosome, it carries  the allele renders them affected. That is why the males are more prone to the colour blindness disorder.

Q10.Why did the scientists select the fruit flies for their  genetics experiments?

A10. Drosophila melanogaster is a small common fly species, which belongs to family Drosophilidae. This species is generally called vinegar fly or  the  fruit fly.  

          In year 1830, Drosophila melanogaster was established as a key model organism for the biomedical science and it is due to considerable biological similarity to the mammals and an abundance of the available genetic tools.

Like the humans beings,these fruit flies species have a similar distribution of the chromosomes. An individual with a pair of X chromosomes is the female fruit fly and an individual with one X and one Y chromosome is the male.

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