Maharashtra Board Class 12 Solutions for Biology Chapter 3 Inheritance and Variation

Multiple-Choice Questions

1. Phenotypic ratio of incomplete dominance in Mirabilis jalapa.

1. 2 : 1 : 1 

2.  1 : 2 : 1

3.  3 : 1 

4.  2 : 2

Ans: Correct option: (b) 1 : 2 : 1

Explanation: In incomplete dominance or intermediate inheritance genotypic and phenotypic ratios are similar in the F2 generation i.e., 1:2:1.

2. In the dihybrid cross, F2 generation offspring show four different phenotypes while the genotypes are ................

1.  six  

2.  nine

3.  eight 

4.  Sixteen

Ans: Correct option: b. Nine  

Explanation: In a dihybrid cross, F2 generation offspring show four different phenotypes while the genotypes are nine. The dihybrid phenotypic ratio is - 9:3:3:1 and the dihybrid genotypic ratio is 1:2:2:4:1:2:1:2:1.

3. A cross between an individual with an unknown genotype for a trait with a recessive plant for that trait is ..............

1.  back cross 

2.  reciprocal cross

3.  monohybrid cross 

4.  test cross

Ans: Correct option: d. Test cross

Explanation: A test cross is a back cross but a back cross is not necessarily a test cross because a back cross is the cross of F1 hybrid with either a dominant parent or a recessive parent. In a test cross the cross is between F1 hybrid and a recessive parent.   

4. When phenotypic and genotypic ratios are the same, then it is an example of ............

1.  incomplete dominance

2.  complete dominance

3.  Multiple alleles

4.  cytoplasmic inheritance

Ans: Correct option: a. Incomplete dominance

Explanation: The phenomenon where both the genes or alleles are not dominant over the other is called incomplete dominance or intermediate inheritance. Genotypic and phenotypic ratios are similar in the F2 generation.

5. If the centromere is near the end of the chromosome, the chromosome is called ................

1.  Metacentric  

2.  Acrocentric

3.  Sub-Metacentric 

4.  Telocentric

Ans: Correct option: b. Acrocentric 

Explanation: Acrocentric is when the chromosome's centre is not in the middle instead it is at the end of the chromosome.

6. Chromosomal theory of inheritance was proposed by ................

1.  Sutton and Boveri

2.  Watson and Crick

3.  Miller and Urey

4.  Oparin and Halden

Ans: Correct option: a. Sutton and Boveri

Explanation: In 1902, Sutton and Boveri postulated the chromosomal theory of heredity. 

7. If the genes are in a chromosome as p-q-r-s-t, which of the following gene pairs will have least probability of being inherited together?

1.  p and q  

2.  r and s

3.  s and t 

4.  p and s

Ans: Correct option: d. P and S

Explanation: The p and s genes on chromosomes are present far away from each other and that is why they have the least probability of being inherited together. 

8. Find the mismatched pair

1. Down’s syndrome = 44 + XY

2. Turner’s syndrome = 44 + XO

3. Klinefelter syndrome = 44 + XXY

4. Super female = 44 + XXX

Ans: Correct option: a. Down’s syndrome = 44 + XY

Explanation: Down's syndrome/ Mongolism/ 21st trisomy sometimes because of this nondisjunction during oogenesis a 21 pair of both chromosomes go into a single egg and the egg has 24 chromosomes instead of 23 chromosomes and offspring has 47 chromosomes (45+XY in male and 45+XX in female).

9. A colour-blind man marries a woman, who is homozygous for normal colour vision, the probability of their son being colorblind is –

1.  0% 

2.  25%

3.  50% 

4.  100%

Ans: Correct option: a. 0%

Explanation: Colour blindness is a recessive sex-linked trait. When the gametes of colorblind fathers are fused with normal colour vision women. Then daughters will be affected and sons will be normal. 

Very Short-Answer Questions

1. Explain the statements

a. Test cross is back cross but back cross is not necessarily a test cross.

Ans: A test cross is a back cross but a back cross is not necessarily a test cross because a back cross is the cross of F1 hybrid with either a dominant parent or a recessive parent. In a test cross the cross is between F1 hybrid and a recessive parent.   

b. Law of dominance is not universal.

Ans: In, Law of dominance only one parental character in the monohybrid cross F1 generation expressed and of both parental characters in F2. 

• Law of dominance is not universal because in specific cases like in incomplete dominance one allele shows incomplete dominance over the recessive allele. In codominance, both the alleles express.

2. Define the following terms

a. Dihybrid Cross 

Ans: A cross between parents having two different traits is called a dihybrid cross e.x. cross of pure tall, round seeded plant with dwarf, wrinkled seeded plant.

b. Homozygous

Ans: An individual with identical alleles for a particular trait is called homozygous or pure for that trait. Example: tall with TT and dwarf with tt.

c. Heterozygous 

Ans: An individual with opposing alleles for a particular trait is called heterozygous. Example: F1 generation hybrids (Tt).

d. Test cross

Ans: A test cross is between F1 hybrid and a recessive parent and a test cross is a back cross but a back cross is not necessarily a test cross.

3. What is allosome?

Ans: Allosome is a sex chromosome, the presence, absence or particular form determines the sex of an individual in organisms. Example: XX-XY, XX-XO, ZZ-ZW, ZZ-ZO.

4. What is crossing over?

Ans: Crossing over takes place during meiosis in which the chromosomal segments interchange and recombination of genes take place. There are types of crossing over based on the number of chiasmata that appear: single crossing-over, double crossing-over, and multiple crossing-over. 

5. Give one example of autosomal recessive disorder.

Ans: Autosomal disorders arise in an individual by the gene mutation of autosomal chromosomes. Some examples of autosomal disorders are Down's syndrome, cystic fibrosis, and sickle cell anemia.

6. What are X-linked genes?

Ans: The inheritance of X - linked and Y-linked genes from parents to offspring is called sex-linked inheritance. Sex-linked inheritance is of three types, X-linked, Y-linked and XY-linked.

7. What are holandric traits?

Ans: The genes that occur only on the Y chromosome and can produce their effects only in males are called holandric genes. Example: TDF-Testis determining factors, hairy pinna (hypertrichosis) in human male. 

8. Give an example of chromosomal disorder caused because of nondisjunction of autosomes.

Ans: An example of chromosomal disorder caused because of nondisjunction of autosomes is Down's syndrome/ Mongolism/ 21st trisomy. Sometimes because of this nondisjunction during oogenesis a 21 pair of both chromosomes go into a single egg and the egg has 24 chromosomes instead of 23 chromosomes and offspring has 47 chromosomes (45 + XY in male and 45 + XX in female). 

9. Give one example of complete sex linkage?

Ans: The most common example of complete sex-linked inheritance is haemophilia also known as bleeder's disease. Haemophilia A is because of the deficiency of AGH and Christmas disease or haemophilia B is because of the absence of plasma plastin. 

Short Answer Questions

1. Enlist seven traits of pea plant selected/ studied by Mendel.

Ans: Seven pairs of contrasting visible characters studied by Mendel in pea plant (Pisum sativum) are:

• Seed shape, seed colour, pod shape, pod colour, flower colour, flower location, and plant height.

2. Why is the law of segregation also called the law of purity of gametes?

Ans: The law of segregation is also called the law of purity and is the third law of inheritance given by Mendel. 

• According to the law of segregation, it separates the factors regulating characters during the formation of gametes and a particular gamete transmits only one pair. 

3. Write a note on pleiotropy.

Ans: Pleiotropy or pleiotropic genes have multiple effects because they influence several traits simultaneously. 

• Example: Marphan's syndrome and sickle cell anaemia. 

4. What are the reasons for Mendel success?

Ans: Mendel successfully explained the laws of heredity or inheritance because he chose a pure pea plant (Pisum sativum). 

• The other important reason for Mendel's success was that he took one character or trait at a time in his experiments of hybridization. Whereas other scientists also conducted cross-hybridization for many characters or traits which made their experiments complex and did not get success in results.

5. “Father is responsible for determination of sex of child and not the mother”. Justify.

Ans: Humans have 44 autosomes and one pair of sex chromosomes by which the sex of an individual is determined. Males have 44 autosomes + XY allosomes and females have 44 autosomes + XX allosomes. 

• In males, during spermatogenesis the diploid germ cell produces 50% sperms it has 22 autosomes + X chromosome and 50% sperms and in females during oogenesis the diploid germ cell produces only one egg which has 22 autosomes + X chromosome. 

• The following table shows the fusion of father (male)- XY and mother (female)- XX gametes, which results in 50% of the offspring will be female (daughters) and 50% of offspring will be male (sons). 

Hence, Father is responsible for determination of sex of the child and not the mother. 

6. What is linkage? How many linkage groups do occur in human beings?

Ans: In 1906, Bateson and Punnet experimented on sweet peas and noticed that some genes ‌stay together in hereditary transmission, which is termed linkage. 

• Morgan well defined linkage in 1910 by the experiment he showed on fruit fly (Drosophila melanogaster). 

• Linkage is of two types:

• Complete linkage

• Incomplete linkage

• The number of genes present on chromosomes include a linkage group. Humans have 22 autosomes and sex chromosome so females have 23 linkage groups and males have 24 linkage groups.

7. Write a note on–PKU.

Ans: Folling in 1934 defined an inborn, autosomal, recessive metabolic disorder called phenylketonuria (PKU). 

• In this condition, an individual lacks the enzyme phenylalanine hydroxylase which converts phenylalanine to tyrosine in the liver. The enzyme lacks in an individual because of the abnormal autosomal recessive gene on chromosome 12.  

• Symptoms of this disorder are:

• Mental retardation

• The brain cannot develop during infancy 

• Accumulation and excretion of phenylalanine and phenyl pyruvic acid. 

8. Compare - X-chromosome and Y-chromosome.

Ans: 

9. Explain the chromosomal theory of inheritance.

Ans: The presumption that chromosomes might be the key to understanding heredity led several scientists to examine Mendel’s publications and re-evaluate his model and the chromosomal behaviour during mitosis and meiosis. 

10. Observe the given pedigree chart and answer the following questions.

Pedigree Chart

a. Identify whether the trait is sex linked or autosomal.

Ans: The following pedigree chart shows the sex-linked trait. Cris-cross inheritance is a type of sex-linked inheritance. Sex-linked inheritance is of three types, X-linked, Y-linked and XY-linked. 

b. Give an example of a trait in human beings which shows such a pattern of inheritance.

Ans: The most important and common X-linked genes are:

• Colour or red-green colorblindness

• Haemophilia

Match the column-I with column-II and re-write the matching pairs

Ans:

Long Answer Type Questions

1. What is the dihybrid cross? Explain with a suitable example and checkerboard method.

Ans: A cross between two parents with different traits is called a dihybrid cross. The phenotypic ratio of the F2 generation of dihybrid cross is 9 : 3 : 3 : 1. 

• Example: the cross of pure tall (TT), round seeded (RR) plant with dwarf (tt), wrinkled seeded (rr) plant.

Following checker board shows the hybrid cross and the F1 generation and the gametes of the F2 generation.

Following checker board shows the dihybrid cross of the F2 generation.

So according to the checkerboard the phenotypic ratio of the cross of pure tall (TT), round seeded (RR) plant with dwarf (tt), wrinkled seeded (rr) plant is:

Phenotypic Ratio: Tall Round - 9; Tall wrinkled - 3; dwarf Round - 3; dwarf wrinkled - 1.

Therefore, the dihybrid phenotypic ratio is - 9:3:3:1.

2. Explain with suitable examples an independent assortment.

Ans: Mendel's law of independent assortment states that when there is the involvement of two or more pairs of factors, the members of one pair of factors segregate and assort independently of other pairs.

• This law is based on dihybrid cross. It shows how the genes segregate themselves during gamete formation and randomly merge in fertilisation. 

• Example: the cross of pure tall (TT), round seeded (RR) plant with dwarf (tt), wrinkled seeded (rr) plant.

Following checker board shows the hybrid cross and the F1 generation and the gametes of the F2 generation.

Following checker board shows the dihybrid cross of the F2 generation.

So according to the checkerboard the phenotypic ratio of the cross of pure tall (TT), round seeded (RR) plant with dwarf (tt), wrinkled seeded (rr) plant is:

Phenotypic Ratio: Tall Round - 9; Tall wrinkled - 3; dwarf Round - 3; dwarf wrinkled - 1 that is 9:3:3:1.

And the dihybrid genotypic ratio is - TTRR-1, TTRr-2, TtRR-2, TtRr-4, ttRR-1, ttRr-2, TTrr-1, Ttrr-2, ttrr-1 that is 1:2:2:4:1:2:1:2:1. 

3. Define test cross and explain its significance.

Ans: A test cross is between F1 hybrid and a recessive parent and a test cross is a back cross but a back cross is not necessarily a test cross. 

• We perform the test cross to see whether an individual is homozygous or heterozygous for a dominant character. 

• If the cross is homozygous dominant, the offspring will be 100% dominant. 

• And with hybrid or heterozygous, the offspring ratio will be 50% dominant and 50% recessive. 

4. What is parthenogenesis? Explain the haplo-diploid method of sex determination in Honey bee.

Ans: In parthenogenesis, the egg develops into an embryo without being fertilised by a sperm. It is also called virgin birth. 

• The sex determination in honey bees is by a haplo-diploidy method. Sex is determined by the number of chromosomes received by an individual. 

• The egg fused by sperm develops into a female. Female and worker bees have 32 chromosomes.

• The unfertilized egg develops into a male by parthenogenesis (drone) and males have 16 chromosomes.

5. In the answer for inheritance of X-linked genes, Madhav had shown carrier male. His answer was marked incorrect. Madhav was wondering why his marks were cut. Explain the reason.

Ans: Cris-cross inheritance is a type of sex-linked inheritance. Sex-linked inheritance is of three types, X-linked, Y-linked and XY-linked. These disorder characters or traits which are regulated by recessive sex-linked genes or alleles are more common in males than females as males are hemizygous and due to presence of only one X-chromosome. As Y-chromosomes do not have any disorder alleles or genes. 

6. With the help of neat labelled diagrams, describe the structure of chromosomes.

Ans: The chromosome is present inside the nucleus of animal and plant cells. It is made up of protein and DNA. The chromosomes have genes which carry hereditary information. 

A chromosome structure has seven parts:

• Pellicle: the outer sheath which is non-genetic.

• Matrix: it is also non-genetic and made up of RNA, proteins and lipids.

• Chromonema

• Centromere

• Satellite/Trabant

• Telomere

• Nuclear organising region

Chromosome

7. What is criss-cross inheritance? Explain with a suitable example.

Ans: Cris-cross inheritance is a type of sex-linked inheritance. Sex-linked inheritance is of three types, X-linked, Y-linked and XY-linked. 

• The most important and common X-linked genes are:

• Colour or red-green colorblindness

• Haemophilia

• In humans, the father passes the sex-linked genes to his grandson via his daughter.

• Mothers can pass sex-linked genes to sons and daughters.

• These disorder characters or traits which are regulated by recessive sex-linked genes or alleles are more common in males than females. 

Following table shows the inheritance of colour blindness. 

8. Describe the different chromosomes.

Ans: There are four types of chromosomes based on where the centromere is attached to the chromosome. 

• Metacentric: the centromere of the chromosome is in the centre and is V-shaped. 

• Sub-Metacentric: the centromere is located a little away from the centre of the chromosome and is L-shaped. 

• Acrocentric: the centromere is near the end of the chromosome and is J-shaped.

• Telocentric: the centromere is at the end of the chromosome and is I-shaped. 

Importance of Class 12 Biology Chapter 3 Inheritance and Variation

Every offspring acquires the genetic impressions from its previous generation. The parent generation will transfer the genetic material to the offspring. Whether it is asexual or sexual reproduction, the transfer of genetic material is mandatory.

Due to this, the offspring will develop certain physiological features. These features determine whether they will survive or not. The method of transmission of the genetic material from the parents to the offspring is called inheritance or heredity.

There are several models that explain how genetic material is transmitted from one generation to the other. It entirely depends on the gametes that fuse to form an embryo. The parent genes get fused and transmitted to form a new life. The progeny will develop characteristics that are a hybrid of that of its parents.

On the other hand, progenies of asexual reproduction carry the exact copies of the genetic information transmitted from the parent. It means that the asexually produced offspring is similar to the parent genetically.

All these new concepts will be taught in this chapter. It will also offer exercises where you can test your preparation skills. To make it easier, check the Inheritance and Variation exercise solutions framed by the Vedantu experts.

Benefits of Maharashtra Board Class 12 Biology Solutions Chapter 3 Inheritance and Variation

• These solutions are designed following the Maharashtra Board standards. it will be easier for the board aspirants to correlate with the syllabus and the answering formats recommended by the education board.

• You can easily resolve your queries by following the chapter Inheritance and Variation Class 12 solutions delivered with the solutions. The easier format will help you understand and memorize the fundamental concepts.

• Remembering and answering the questions related to this chapter will become a lot easier when you use the solutions to prepare it.

• You can download the solution file without any hassle and refer to it whenever you want. Make your study sessions more productive with these solutions.

Download Maharashtra Board Class 12 Biology Chapter 3 Inheritance and Variation Solutions PDF

Why wait then? Perform the Inheritance and Variation exercise PDF download and complete your study material for this chapter. Develop exclusive knowledge and strengthen your foundation with the solutions to all the exercise questions. Learn how to answer them accurately and score well in the board exams.