Principle and Inheritance of variation NCERT Solutions for Class 12 Biology Chapter 5 with Answers

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NCERT Solutions for Class 12 Biology


Page No. 93

Q1. Mention the advantages of selecting pea plant for experiment by Mendel.

Answer: Advantages of selecting pea plant as experimental material

1) Many varieties were available with observable and contrasting characters.
2) Peas normally self pollinate so pure lines could be obtained for experimental purpose
3) It was easily available.
4) The life span of pea is short and it produced large number of offsprings
5) They were easy to cultivate and did not require much care.

Q2. Differentiate between the following –

(a) Dominance and Recessive


1) When the allele expresses itself in both heterozygous as well as homozygous condition.
2) The allele is unmodified and functional.
1) When the allele expresses itself only in homozygous condition.
2) The allele is modified and non-functional.

(b) Homozygous and Hetrozygous


1) Condition where both the alleles of the gene are same.
2) The gametes formed from them are same.
eg. TT or tt
1) Condition where the alleles are different.
2) The gametes formed from them are different.
eg. Tt

(c) Monohybrid and Dihybrid


1) When an individual is heterozygous for genes controlling one character it is called monohybrid
2) In F2 generation it produces a phenotypic ratio of 3:1.
1) When an individual is heterologous for genes controlling two characters it is called dihybrid.
2) In F2 generation it produces a phenotypic ratio of 9:3:3:1

Q3. 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.

Example – if an entity is heterozygous at four loci with four different characters, such as Mm, Nn, Oo, Pp, then while meiotic division they split to form 8 different gametes.

If genes are not linked to each other, then the diploid entities will produce 16 different gametes. But, if genes are linked, gametes will decrease their number as the genes may be linked which inturn will be inherited together during the meiotic cell division.

Q4. Explain the Law of Dominance using a monohybrid cross.

Answer: The Law of dominance states that when a pair of alleles or allelomorphs are brought together in F1 hybrid, then only one of them expresses itself, masking the expression of the other completely. Monohybrid cross was made to study the simultaneous inheritance of a single pair of Mendelian factors. The cross in which only alternate forms of a single character are taken into consideration is called a monohybrid cross. The trait which appeared in the F1 generation was called dominant and the other which did not appear in the F1 population was called recessive.

Thus, when a pair of alleles are brought together in an F1 hybrid, then only one of them expresses itself masking the expression of the other completely. In the above example, in Tt – F1 hybrid (tall) only ‘T’ expresses itself so dominant, and ‘t’ is masked so recessively. Thus, this’ proves and explains the law of dominance.

Q5. Define and design a test – cross?

Answer: Test cross is a cross between an organism with unknown genotype and a recessive parent. It is used to determine whether the individual is homozygous or heterozygous for a trait.

If the progenies produced by a test cross show 50% dominant trait and 50% recessive trait, then the unknown individual is heterozygous for a trait. On the other hand, if the progeny produced shows dominant trait, then the unknown individual is homozygous for a trait.

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: 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, r. The genotypic and phenotypic ratio in the progenies of F1 generation will be same i.e., 1:1.

Q7. 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: A cross between tall plant with yellow seeds (TtYy) & tall plant with green seed (Ttyy) is given below.

Q8. Two heterozygous parents are crossed. If the two loci are linked, then what would be the distribution of phenotypic features in the F1generation for a dihybrid cross?

Answer: When two genes are inherited together, they are referred to as linked genes, and linkage is the term used to describe this process. There will be no segregation of alleles if two genes are connected, and the alleles will be inherited together as a single gene. If two genes are connected, the phenotype of the F1 generation will solely contain parental features and no recombinants. There may be few recombinants in the case of inadequate linkage, but the proportion of parental phenotypes will be higher.

Q9. Briefly mention the contribution of T.H. Morgan in genetics.

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

  • He stated the chromosomal theory of linkage
  • He carried out a study on sex-linked inheritance
  • Morgan stated chiasma type hypothesis demonstrating that the chiasma causes crossing over
  • He observed that the frequency of recombination between two linked genes is directly proportional to the distance between them both
  • Proposed the theory of inheritance
  • He put forward the methodology for chromosome mapping
  • He carried out a study on mutation

Q10. What is pedigree analysis? Suggest how such an analysis, can be useful.

Answer: Pedigree analysis is a record of occurrence of a trait in several generations of a family. It is based on the heritability of certain characteristic features in families.
Its usefulness are:

  • It shows the origin of a trait and flow of that trait in a family.
  • It is useful in knowing the possibility of expressive or recessive allele which may cause genetic disorder, such as colour blindness.
  • It predicts the harmful effects of marriage between close relatives.
  • It helps in genetic counselling to avoid disorders in children.

Q11. How is sex determined in human beings?

Answer: The chromosomal mechanism of determination of sex in human beings is of the genotype XX-XY. The nucleus of each cell consists of 23 pairs of chromosomes or 46 chromosomes out of which 22 pairs are autosomes and the last pair, the 23rd pair is the sex chromosome. Females are homomorphic, i.e., they possess two same sex chromosomes XX whereas males are heteromorphic, they possess two different sex chromosomes XY. Females are homogametic, producing only one type of egg (22+X) whereas males are heterogametic producing two types of sperms – (22+X) & (22+Y).

In the process of fertilization, if the sperm containing X chromosome fertilizes the egg, then the developing offspring would be a female (XX) and if the sperm containing Y chromosome fertilizes the egg, then the developing offspring would be a male (XY). Therefore, the sex ratio that is produced in the progeny is 1:1. Heterogamesis is the chromosomal mechanism of sex determination which can be male heterogamety or female heterogamety.

Q12. 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: If the father has blood group A i.e., IAIA (homozygous) and mother has blood group B i.e., IBIB (homozygous) then all the offsprings will have blood group AB (IAIB) and not blood group O.

Thus the genotypes of the parents of child with blood group O will be IAi and IBi There is the possibility of 3 other types of blood groups of offsprings besides O blood group offspring. They are IAi (blood group A). IBi (blood group B) and IAIB (blood group AB).

Q13. Explain the following terms with example

(a) Co-dominance
(b) Incomplete dominance

Answer: (a) Co-dominance

Co-dominance is the phenomenon in which both the alleles of a contrasting character are expressed in heterozygous condition. Both the alleles of a gene are equally dominant. ABO blood group in human beings is an example of co-dominance. The blood group character is controlled by three sets of alleles, namely, IA, IB, and i. The alleles, IA and IB, are equally dominant and are said to be co­­-dominant as they are expressed in AB blood group. Both these alleles do not interfere with the expression of each other and produce their respective antigens. Hence, AB blood group is an example of co-dominance.

(b) Incomplete dominance : Incomplete dominance may be defined as the partial /expression of both alleles in a heterozygote so that the phenotype is intermediate between those of two homozygotes. In this none of the allele is completely dominant, e.g., Pink colour flower in dog flower. In Mirabilis jalapa & Snapdragon or dog flowers, there are two types of flower colour in pure state-red & white. When the two types of plant are crossed, the hybrid or plants of Fj generation have pink flowers. The pink colour apparently appears either due to mixing of red & white colours (incomplete dominance) or expression of a single gene for pigmented flower which produces only pink colour.

Q14. What is point mutation? Give one example.

Answer: Point mutation is a change in a single base pair of DNA by substitution, deletion, or insertion of a single nitrogenous base. An example of point mutation is sickle cell anaemia. It involves mutation in a single base pair in the beta-globin chain of haemoglobin pigment of the blood. Glutamic acid in short arm of chromosome II gets replaced with valine at the sixth position.

Q15. Who had proposed the chromosomal theory of the inheritance?

Answer: The chromosomal hypothesis of inheritance was presented by Sutton and Boveri in 1902. This states that chromosomes serve as carriers for gene inheritance.

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

Answer: Sickle cell anaemia: This is an autosome linked recessive trait that can be transmitted from parents to the offspring when both the partners are carrier for the gene or heterozygous.
Symptoms: Rapid heart rate, breathlessness, delayed growth and puberty, jaundice, weakness, fever, excessive thirst, chest pain, and decreased fertility are the major symptoms of sickle cell anaemia disease.

Down’s syndrome: It is an autosomal disorder that is caused by the trisomy of chromosome 21.
Symptoms: The individual is short statured with round head, open mouth, protruding tongue, short neck, slanting eyes, and broad short hands. The individual also shows retarded mental and physical growth.

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