Hello Students. Are you Searching for NCERT Solutions for Class 12 Biology Chapter 5? If yes then you are most welcome to NCERTian. Here we have provided you with the complete Question and Answers of Chapter 5: Principles Of Inheritance And Variation. These solutions are written by expert teachers and faculties keeping the need of students in mind.
| Chapter | 5. Principles Of Inheritance And Variation |
| Subject | Biology |
| Class | Twelve |
| Category | NCERT Solutions for Class 12 |
Our NCERT Solutions for Class 12 Biology is a great resource for students preparing for boards or competitive exams, such as the NEET. These Biology Class 12 Solutions are made by expert faculties, keeping the latest curriculum in mind. Besides helping students with understanding the concepts of Biology, these solutions are also helpful in writing accurate answers that are vital to score full marks in examinations. On this page, we have given the Class 12 Biology Chapter 5: Principles Of Inheritance And Variation Solutions.
NCERT Solutions for Class 12 Biology Chapter 5
Principles Of Inheritance And Variation Solutions
Q1) Mention the advantages of selecting pea plant for experiment by Mendel.
Answer) Mendel selected pea plants to carry out his study on the inheritance of characters from parents to offspring. He selected a pea plant because of the following features:
- Peas have many visible contrasting characters such as tall/dwarf plants, round/wrinkled seeds, green/yellow pod, purple/white flowers, etc.
- Peas have bisexual flowers and therefore undergo self pollination easily. Thus, pea plants produce offsprings with same traits generation after generation.
- In pea plants, cross pollination can be easily achieved by emasculation in which 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.
Q2) Differentiate between the following
(a) Dominance and Recessive
(b) Homozygous and Heterozygous
(c) Monohybrid and Dihybrid.
Answer a)
| Dominance | Recessive |
|---|---|
| A dominant factor or allele expresses itself in the presence or absence of a recessive trait. | A recessive trait is able to express itself only in the absence of a dominant trait. |
| For example, tall plant, round seed, violet flower, etc. are dominant characters in a pea plant. | For example, dwarf plant, wrinkled seed, white flower, etc. are recessive traits in a pea plant. |
Answer b)
| Homozygous | Heterozygous |
|---|---|
| It contains two similar alleles for a particular trait. | It contains two different alleles for a particular trait. |
| It produces only one type of gamete. | It produces two different kinds of gametes. |
Answer c)
| Monohybrid | Dihybrid |
|---|---|
| It is a cross between parents differing in only one pair of contrasting characters | It is a cross between parents differing in two pairs of contrasting characters |
| Example – a cross between a dwarf and a tall pea plant | Example – a cross between yellow wrinkled seed and a green rounded seed |
Q3) A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
Answer) Locus is a fixed position on a chromosome, which is occupied by a single or more genes. Heterozygous organisms contain different alleles for an allelic pair. Hence, a diploid organism, which is heterozygous at four loci, will have four different contrasting characters at four different loci.
For example, if an organism is heterozygous at four loci with four characters, say Aa, Bb, Cc, Dd, then during meiosis, it will segregate to form 8 separate gametes.
If the genes are not linked, then the diploid organism will produce 16 different gametes. However, if the genes are linked, the gametes will reduce their number as the genes might be linked and the linked genes will be inherited together during the process of meiosis.
Q4) Explain the Law of Dominance using a monohybrid cross.
Answer) Law of Dominance states that when two genes are present in an organism then dominant gene expresses itself and recessive gene does not show its effect and remain hidden.
When a cross pollination experiments is conducted between two organisms taking a single contrasting character at a time 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 F1generation, the dominant character (Tall) appeared and the recessive character (dwarf) got suppressed, which reappeared in F2 generation. Thus, this monohybrid cross explains Law of Dominance.
Q5) 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.
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 plants 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
Q8) Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in F1 generation for a dihybrid cross?
Answer) The co-existence of two or more genes in the same chromosome is termed as linkage. If the genes are located close to each other and on the same chromosome, they are inherited together and are referred to as linked genes. If two heterozygous parents exhibit linkage, then the outcome is as follows:
Parents BbLl x BbLl
Genotype Blue long Blue long
Phenotype in F1 in all the possible may exhibit parental characters as the genes are linked completely. With all the possible genotypes in F1 progeny can display blue long type of phenotype in the above-mentioned example. However, if there is an incomplete linkage, the parental combination will comparatively be more than the newer combinations which are less in number.
Q9) 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.
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:
- Serves as a powerful tool which can be used to trace the inheritance of a particular trait, disease or an abnormality
- It is helpful for genetic counsellors to suggest couples about the possibility of having children with genetic abnormalities such as colour blindness, haemophobia, thalassaemia, sickle cell anaemia, etc
- The analysis is helpful in indicating the origination of a trait and its flow in ancestors
- It is helpful in suggesting that Mendel’s principles can be applied to human genetics with some alteration such as quantitative inheritance, sex-related linkage and characters
- Helpful in reasoning why marriage between close relatives is harmful
- Helpful in extensive research in the field of medical science
Q11) How is sex determined in human beings?
Answer) Human beings have 23 pairs of chromosomes in which 22 pairs of autosomes and one pair of sex chromosomes. The females have a pair of XX-chromosomes (homogametic) and the males XY (heterogametic) composition. The sex of the baby is determined by the type of male gamete that fuses with the female gamete. If the fertilizing sperm contains X chromosome, then the baby produced will be a girl and if the fertilizing sperm contains Y chromosome, then the baby produced will be a boy.
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) 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.
A cross between heterozygous parents will produce progenies with AB blood group (IAIB) and O blood group (ii).
Q13) Explain the following terms with example
(a) Co-dominance
(b) Incomplete dominance
Answer a) 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.
Answer b) Incomplete dominance is the phenomenon of inheritance where none of the contrasting characters in a pair are dominant. In the F1 hybrid, the expression of the trait is a fine mixture of the expression of the two factors. For instance, the Mirabilis jalapa produces pink flowers when a hybrid cross between a red and white flower is carried out.
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 inheritance?
Answer) Sutton and Boveri proposed the chromosomal theory of inheritance in 1902. They linked the inheritance of traits to the chromosomes.
Q16) Mention any two autosomal genetic disorders with their symptoms.
Answer) These disorders are caused as a result of a defect in the gene found on the autosomes. Some autosomal genetic disorders are – Down’s syndrome, sickle cell anemia, Patau syndrome, Phenylketonuria.
Symptoms of Down’s syndrome:
- Flat hands, short neck
- Broad forehead
- Partially open mouth, furrowed tongue
- Mongolian type eyelid fold and stubby fingers
- Stunted psychomotor, physical and mental development
- Heart deformities and deformity of other organs
- Underdeveloped genitalia and gonads
Symptoms of Sickle cell anemia:
- Shape of RBCs change from elongated to sickle-shaped (curved) under the influence of low oxygen tension
- These sickle-shaped RBCs are more rapidly destructed than the normal ones causing anaemia
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