The VIDYANKER team has thoughtfully prepared the NCERT Solutions for Class 12 Biology Chapter 5, "Principles of Inheritance and Variation." These solutions are designed to help you tackle the NCERT textbook questions with ease. We recommend going through the chapter's theory before diving into the solutions for a deeper understanding. Feel free to share these NCERT Solutions for Class 12 Biology with others—learning is always better when shared!
01. Mention the advantages of selecting pea plant for experiment by Mendel.
Answer
Advantages of choosing pea plant for experimentation by Mendel:
- Peas have many visible contrasting characters like smooth or wrinkled seeds, yellow or green seeds, smooth or inflated pods, green or yellow pods, and tall or dwarf plants.
- It easily undergoes self-pollination that results in offspring with the same traits generation after generation.
- There's also cross-pollination done manually by emasculation. This is where the stamen of the flower is removed without affecting the pistil.
- Pea plants have a short life span and produce many seeds in one generation.
2. Differentiate between the following –
(a) Dominance and Recessive
Dominance | Recessive |
| Dominance allele is able to express itself even in the presence of its recessive allele. | Recessive allele or factor is unable to express its effect in the presence of dominant allele. |
| It does not require another similar allele to produce its effect on the phenotype, e.g., Tt is tall. | It produces its phenotypic effect only in the presence of a similar allele, e.g., tt is dwarf. |
(b) Homozygous and Hetrozygous
Homozygous | Heterozygous |
| It is pure for a trait and breeds true, i.e., gives rise to similar homozygous individuals. | It is rarely pure and produces offspring with different genotypes on selling, e.g., TT. Tt and tt on selfing of Tt individuals. |
| Both the alleles of a character are similar. Example: TT, tt | It carries dissimilar alleles, Example: Tt |
| It produces only one type of gamete. | It produces two different kinds of gametes. |
(c) Monohybrid and Dihybrid
Monohybrid | Dihybrid |
| It is a cross between two pure organisms in order to study the inheritance of a single pair of alleles. | It is a cross between two pure organisms of a species in order to study the inheritance of two pairs of alleles. |
| It produces genotypic ratio of 1: 2: 1 in F2 generation. | It produces a phenotypic dihybrid ratio of 9: 3: 3: 1 in F2 generation. |
3. A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
Answer
By applying the formula 2n (where n = number of loci)
4 loci can produce = 24 = 16 types of gametes.
4. Explain the Law of Dominance using a monohybrid cross.
Answer
This law is known as the law of dominance. It simply states that whenever two genes exist in an organism, the effect of the dominant gene appears while the recessive gene effect goes undetected and thus masked.
When the cross pollination experiment is conducted between two organisms taking a single contrasting character at a time then it is called monohybrid cross.
When two pea plants, one is pure tall (TT) and other is pure dwarf (tt) are crossed. All pants in F1 generation were found to be Tal l(TT). When these tall plants (Tt) were self fertilized, both the tall and dwarf seeds appeared in F2 generation in 3: 1 ratio. Hence, in F1 generation, the dominant character (Tall) appeared, while the recessive character (dwarf) got suppressed, and reappeared in F2 generation. Thus, this monohybrid cross explains the Law of Dominance.
5. Define and design a test - cross?
Answer
A test cross is one in which an individual with an unknown dominant phenotype is crossed with a recessive individual for that trait. It is used to determine whether the individual is homozygous or heterozygous for a trait.
If the unknown is homozygous tall (TT), then crossing with dwarf recessive (tt) gives all tall offspring (Tt).
If the unknown is heterozygous tall (Tt), then crossing with dwarf results in 50% tall (Tt) and 50% dwarf (tt) progeny.
6. 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
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
A cross between tall plant with yellow seeds and tall plant with green seeds will produce
(a) three tall and green plants
(b) one dwarf and green plant
8. 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
If two loci are linked, they lie close on the same locus of a chromosome, they would separate. However, the chromosomes do segregate and end up in different gametes.
9. Briefly mention the contribution of T.H. Morgan in genetics.
Answer
TH Morgan's contribution in genetics are
• Morgan contributed to the understanding of sex-linked traits.
• He carried out several experiments on fruit flies and his dihybrid crosses revealed that genes for some traits did not segregate independently and the F2 ratio was a lot different from 9:3:3:1.
• He inferred that when two genes were situated on the same chromosome, the proportions of parental gene combinations were significantly higher than the non-parental type.
• He established the principle of linkage crossing over, sex-linked inheritance and discovered the relation.
• He established the technique of chromosome mapping.
• He observed and worked on mutation.
10. 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.
11. How is sex determined in human beings?
Answer
Human beings have 23 pairs of chromosomes among which the 22 pairs of autosomes and one pair of sex chromosomes. Females have the composition of pair of XX-chromosomes while the males have XY (heterogametic) composition. The type of male gamete that fuses with the female gamete determines the sex of the baby. It is also quite clear how the sex of the baby should be determined because if the fertilizing sperm will carry the X chromosome, then the baby produced will be a girl. If the fertilizing sperm carries a Y chromosome, then the baby produced will be a boy.
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 child with blood group ‘O’ will have homozygous recessive alleles. Therefore, both the parents should be heterozygous, i.e., the genotype of the father will be IiA and of mother will be IiB.
Blood group: AB
The possible blood groups of other offsprings will be AB, A, B and O
13. Explain the following terms with example
(a) Co-dominance
(b) Incomplete dominance
Answer
Co-dominance: The phenomenon in which F1 generation resembles both the parents and both the parental characters are expressed simultaneously is known as co-dominance.
Example: A plant with yellow coloured flower crossing with a plant of red coloured flower and in F 1 generation, all progeny show orange flowers due to the co-dominance of both alleles for red and yellow colours.
Incomplete Dominance : Incomplete dominance is a phenomenon in which the genes of allelomorphic pairs are not expressed as dominant and recessive but express themselves partially when they are present together in the hybrid.
Example 4' O clock plant exhibit two types of flowers, red and white and hybrids are pink cloured flowers.
14. What is point mutation? Give one example.
Answer
Point mutation is a change in single base pair of DNA molecule. Example: 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.
15. Who had proposed the chromosomal theory of the inheritance?
Answer
Sutton and Boveri proposed the chromosomal theory of inheritance in 1902.
16. Mention any two autosomal genetic disorders with their symptoms.
Answer
Sickle cell anaemia: It is an autosomal linked recessive character that may be passed on from the parents to offspring when both partners are a carrier of the gene or heterozygous.
Some of the major symptoms associated with sickle cell anaemia disease are rapid heart rate, breathlessness, delayed growth and puberty, jaundice, weakness, fever, excessive thirst, chest pain, and low fertility.
Down's syndrome: It is an autosomal disorder-the consequence of a trisomy of chromosome 21.
Symptoms: The affected person is short statured, with a round head, open mouth, protruding tongue, short neck, slanting eyes, and broad short hands. The individual also exhibits retarded mental and physical growth.
