CBSE class 12 biology chapter 5 NCERT solutions

CBSE Class 12 Biology Chapter 5 - NCERT Solutions of Principles of Inheritance and Variation solutions

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Important points to remember 

  1. Genetics is a branch of biology which deals with principles of inheritance and its practices. Progeny resembling the parents in morphological and physiological features. 
  2. Mendel was the first to study this phenomenon systematically. While studying the pattern of inheritance in pea plants of contrasting characters, 
  3. Mendel proposed the principles of inheritance, which are today referred to as ‘Mendel’s Laws of Inheritance’. He proposed that the ‘factors’ (later named as genes) regulating the characters are found in pairs known as alleles.
  4. The dominant characters are expressed when factors are in heterozygous condition (Law of Dominance). The recessive characters are only expressed in homozygous conditions.
  5. Different combinations of gametes are theoretically represented in a square tabular form known as ‘Punnett Square’. The factors (now known as gene) on chromosomes regulating the characters are called the genotype and the physical expression of the chraracters is called phenotype.
  6. genes are located on the chromosomes. Many genes were linked to sexes also, and called as sex-linked genes. The two sexes (male and female) were found to have a set of chromosomes which were common, and another set which was different. 
  7. The chromosomes which were different in two sexes were named as sex chromosomes. The remaining set was named as autosomes. 
  8. In humans, a normal female has 22 pairs of autosomes and a pair of sex chromosomes (XX). A male has 22 pairs of autosomes and a pair of sex chromosome as XY. 
  9. In chicken, sex chromosomes in male are ZZ, and in females are ZW.
  10. Mutation is defined as change in the genetic material. A point mutation is a change of a single base pair in DNA. Sickle-cell anemia iscaused due to change of one base in the gene coding for beta-chain of hemoglobin.

1. Mention the advantages of selecting pea plant for experiment by Mendel.
Answer : Mendel is known as the father of genetics because of his ground-breaking work on inheritance in pea plants.
Mendel studied inheritance on pea (Pisum sativum) plant. He chose pea plant because they had been used for similar studies, are easy to grow and can be sown each year. Pea flowers contain both male and female parts, called stamen and stigma, and usually self-pollinate. Self-pollination happens before the flowers open, so progeny are produced from a single plant.
These plants have a short life span wherein they produce plenty of seeds in one generation alone.

Peas can also be cross-pollinated by hand, simply by opening the flower buds to remove their pollen-producing stamen (and prevent self-pollination) and dusting pollen from one plant onto the stigma of another.

2. Differentiate between the following –
(a) Dominance and Recessive
Answer : Dominant : It is an allele that expresses its trait  even in the presence of an alternative allele i.e.  in heterozygous condition only. Alternatively,
the allele that expresses in F1 is called
dominant. (It is an allele of a pair that masks  the expression of other allele in F1
 generation.) Recessive : This allele is not expressed in the presence of an alternative allele (in heterozygous condition). It expresses only in the presence of another identical allele. It is an allele that does not express in F1hybrid.

(b) Homozygous and Heterozygous
Answer : Homozygous (pure) :An individual possessing identical allels for a particular trait, is called  homozygous or pure for that trait.  Homozygous breeds true to the trait and  produces only one type of gametes e.g., tall  with TT and dwarf with tt.

Heterozygous : An individual possessing
contrasting alleles for a particular trait, is
called heterozygous. Heterozygous does not breed true for that trait and produces two types of gametes e.g. F1 generation hybrids (Tt). Heterozygous individual is also called hybrid.

(c) Monohybrid and Dihybrid.
Answer :
Monohybrid cross is a cross between parents differing in only one pair of contrasting characters Dihybrid cross is a cross between parents differing in two pairs of contrasting characters

Example of  monohybrid cross– a cross between a dwarf and a tall pea plant. Example of dihybridd cross – a cross between yellow wrinkled seed and a green rounded seed

3. A diploid organism is heterozygous for 4 loci, how many types of gametes
can be produced?
Answer : A locus is a fixed point on a chromosome that is occupied by one or more genes. For an allelic pair, heterozygous entities contain different alleles. Thus, a diploid entity which is heterozygous at four loci has four different contrasting characters at four different loci.

Explanation :  Types of gametes produced by organism= 2n where as n= number of loci for which the organism is heterozygous.

The given diploid organism is heterozygous for 4 loci, types of gametes produced =  2x2x2x2
 = 16.

4. Explain the Law of Dominance using a monohybrid cross.
Answer : The law of dominance is used to explain  the expression of only one of the parental  characters of a monohybrid cross in F1 and the expression of both in F2.
Statement of Law of Dominance : “When
two homozygous individuals with one or more sets of contrasting characters are crossed, the alleles (characters) that appear in F1 are dominant and those which do not appear in F1 are recessive”.

Example- When a monohybrid cross between two pea plants having Tall plant (TT) and Dwarf plant (tt) was carried out, all the seeds in F1 generation were observed to be Tall (Tt). If the round seeds were self-fertilized both the characters – Tall and dwarf plant appeared in F2 generation in 3:1 ratio. Therefore, in F1 generation, the character that is dominant i.e., the Tall plant surfaced and the recessive character i.e., the dwarf plant got suppressed that resurfaced in the F2 generation.

5. Define and design a test-cross.
Answer : Test cross : The cross of F1
 hybrid with the homozygous recessive parent is known as a test cross. It is used to test whether an individual is homozygous (pure) or heterozygous(hybrid).  Test cross is easy, simple, repeatable and predictable.
Test cross can be used to find out genotype
of any plant with dominant expression. But it is not known whether it is homozygous (pure) or heterozygous for that trait.

For example, A pea
plant having violet (purple) flowers is crossed with a pea plant with white flowers. If all flowers produced are violet, we can conclude that plant is pure or homozygous and if we get violet and white flowers in 1:1 ratio, we can conclude that plant is heterozygous. Test cross is also used to introduce useful recessive traits
in the hybrids of self pollinated plants during rapid crop improvement programs.
Following is the graphic representation of
test cross. Recessive parent is crossed to find out unknown genotype.

6. Using a Punnett Square, workout 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 : In guinea pigs, heterozygous male with black coat colour (Bb) is crossed with the female having white coat colour (bb). The male will produce two types of gametes, B and b, while the female will produce only one kind of gamete, b. The genotypic and phenotypic ratio in the progenies of F1 generation will be same i.e., 1:1.

7. When a cross in made between tall plant with yellow seeds (TtYy) and
tall plant with green seed (Ttyy), what proportions of phenotype in the
offspring could be expected to be
(a) tall and green.
(b) dwarf and green.
Answer : When a cross between a tall plant with yellow seeds and a tall plant with green seeds is carried out, the following is produced:
Three tall green plants
One dwarf green plant.

8. Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in F1 generation for a dibybrid cross?
Answer : Linkage is defined as the coexistence of two or more genes in the same chromosome. If the genes are situated on the same chromosome and lie close to each other, then they are inherited together and are said to be linked genes.

For example, a cross between yellow body and white eyes and wild type parent in a Drosophila will produce wild type and yellow white progenies. It is because yellow bodied and white eyed genes are linked. Therefore, they are inherited together in progenies.

9. Briefly mention the contribution of T.H. Morgan in genetics.
Answer : Thomas Hunt Morgan was an American biologist. He used fruit fly (Drosophila melanogaster) in genetic
reserch and established the chromosomal theory of heredity. He also discovered
the principle of linkage, sex linkage and crossing over. Margan’s work played key role in the field of genetics. He was awarded a Nobel Prize in 1933, in Physiology and Medicine.

The contributions of T.H. Morgan in the field of genetics is as follows:

  1. Thomas Hunt Morgan proposed and established that genes are positioned on the chromosomes
  2. He discovered the basis for variations as a result of sexual reproduction
  3. Thomas Hunt Morgan discovered the concept of linkage and discriminated linked and unlinked genes
  4. He stated the chromosomal theory of linkage
  5. T.H Morgan carried out a study on sex-linked inheritance

10. What is pedigree analysis? Suggest how such an analysis, can be useful.
Answer : analysis of traits in a several of generations of a family is called the pedigree analysis. In the pedigree analysis the inheritance of a particular trait is represented in the family tree over generations. In human genetics, pedigree study provides a strong tool, which is
utilised to trace the inheritance of a specific trait, abnormality or disease.

11. How is sex determined in human beings?
Answer :

  1. The chromosomal mechanism of sex determination in human beings is XX-XY type.
  2. In human beings, the nucleus of each somatic cell contains 46 chromosomes or 23 pairs of chromosomes. Out of these, 22 pairs are autosomes and one pair of sex chromosomes.
  3. Human female has a pair of XX, homomorphic sex chromosomes while male has XY, heteromorphic sex chromosomes.

Thus genotype of :
Female = 44 Autosomes + XX
Male = 44 Autosomes + XY

  1. During gamete formation in male, the diploid germ cells in testis undergo spermatogenesis to produce two types of haplaid sperms, 50% sperms contain 22 autosomes and X chromosome while, 50% sperms contain 22 autosomes and Y chromosome.
  2. In Female, the diploid germ cells in ovaries undergo oogenesis to produce only one type of egg. All eggs contain 22 autosomes and X chromosome. Thus human male is heterogametic and female is homogametic.
  3. If sperm containing X chromosome fertilizes egg (ovum), then diploid zygote is formed, that grows into a female child. If sperm containing Y chromosome fertilizes the egg, then diploid zygote is formed that grows into a male child.

12. A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.
Answer : The blood group characteristic in humans is controlled by three set of alleles, namely, IA, IB, and i. The alleles, IA and IB, are equally dominant whereas allele, i, is recessive to the other alleles. The individuals with genotype, IA IA and IA i, have blood group A whereas the individuals with genotype, IB IB and IB i, have blood group B. The persons with genotype IA IB have blood group AB while those with blood group O have genotype ii.

Hence, if the father has blood group A and mother has blood group B, then the possible genotype of the parents will be Father Mother
IA IA or IA i IB IB or IB i

A cross between homozygous parents will produce progeny with AB blood group. progeny with AB blood group

A cross between heterozygous parents will produce progenies with AB blood group (IAIB) and O blood group (ii).

13. Explain the following terms with example
(a) Co-dominance
Answer : In co-dominance, both the alleles (genes) of an allelomorphic pair express themselves equally in F1 hybrids. Such alleles which are able to express themselves equally independently in hybrids, are called co-dominant alleles. Thus in co-dominance both alleles are expressed.

Classic example of co-dominance is coat
colour in cattle. There are two types one with red coat (with red colour hair) and other with white coat (with white hair). When red cattles (RR) are crossed with white cattles (WW), F1 hybrids (RW) are roan.

Roans have the mixture of red and white
colour hair. Thus both the traits are expressed equally. In F2 generation red (RR), roans (RW) and white (WW) are produced in the ratio 1:2:1. Thus in Co-dominance, the genotypic and phenotypic ratios are identical

(b) Incomplete dominance.
Answer :  Incomplete dominance:
In the incomplete dominance, both the
alleles (genes) of an allelomorphic pair express themselves partially. One allele (gene) cannot supress the expression of the other allele (gene) completely. In such case, there is an intermediate expression in the F1 hybrid. A well-known example is the flower colour of Mirabilis jalapa. If a red-flowered (RR) plant is crossed with a white-flowered (rr) plant, then F1 offsprings have pink (Rr) flowers.

14. What is point mutation? Give one example.
Answer : A point mutation is a mutation that only affects a single nucleotide of nucleic acid. Point mutations most commonly involve the substitution of one base for another (which changes the complementary base as well in DNA). The term point mutationt also includes insertions or deletions of a single base pair.

15. Who had proposed the chromosomal theory of the inheritance?
Answer : In 1902, the chromosomal theory of inheritance was independently put forward by Theodore Boveri and Walter Sutton.

This theory states that the chromosomes
are present in pairs in somatic cells. During
gamete formation homologous chromosomes pair, segregate and assort independently during meiosis.

16. Mention any two autosomal genetic disorders with their symptoms
Answer :
1. Thalassemia : Thalassemia is an autosomal, inherited recessive disease. Haemoglobin molecule is made of four polypeptide chains- 2 alpha (a) and 2 beta (b) chains. The synthesis of alpha chains are controlled by two closely linked genes (HBA1 and HBA2) on chromosome 16 while the synthesis of beta chain is controlled
by a single gene (HBB) on chromosome 11.

Symptoms :
In Thalassemia, person shows symptoms like anaemia, pale yellow skin, change in size and shape of RBCs, slow growth and development, dark urine, etc. Massive blood transfusion is needed to these patients.

2. Down’s Syndrome (21st trisomy) :
Down’s syndrome is named after the
physician John Langdon Down who first
described this autosomal chromosomal disorder in 1866.

This Syndrome is caused due to an extra
copy of chromosome number 21st. It shows
presence of three copies of 21st chromosome instead of homologous pair. These individuals will have 47 chromosomes instead of the normal number 46. 21st Trisomy occurs due
to non-disjuction or failure of separation of
chromosomes (autosomes) during gamete
formation. The incidence of non-disjunction
is distinctly higher in mothers who are over 45 years old.

Symptoms :
These patients have mild or moderate
mental retardation and skeletal development is poor. Distinct facial features like small head, ears and mouth, face is typically flat and rounded with flat nose, open mouth and protruding tongue, eyes slant up and out with internal epicanthal folds, flat hands and stubby fingers and palm is broad with single palmer crease.

Chapter 1 : cbse class 12 biology chapter 1 ncert solutions Reproduction in Organisms

Chapter 3 : cbse class 12 biology chapter 3 ncert solutions human reproduction

Chapter 6 : cbse class 12 biology chapter 6 Molecular Basis of Inheritance