RBSE Solutions for Class 12 Biology Chapter 35 Mendel’s Law of Inheritance
RBSE Solutions for Class 12 Biology Chapter 35 Mendel’s Law of Inheritance
Rajasthan Board RBSE Class 12 Biology Chapter 35 Mendel’s Law of Inheritance
RBSE Class 12 Biology Chapter 35 Multiple Choice Questions
Question 1.
The main reason for the success of Mendel was that he:
(a) Selected pea plant
(b) Studied one character at a time in hybrid
(c) Kept record of pedigree
(d) All of the above
Answer:
(d) All of the above
Question 2.
Law of Independent assortment is proved by:
(a) All offspring of the F1 generation is long
(b) Long and dwarf plants produce in 3:1 ratio
(c) Expression of plants having smooth & wrinkled seeds in the F2 generation.
(d) Expression of Long & Dwarf plants in the F2 generation.
Answer:
(c) Expression of plants having smooth & wrinkled seeds in the F2 generation.
Question 3.
A monohybrid cross gives an F2 generation is a characteristic phenotypic ratio of:
(a) 9:3:3:1
(b) 3:1
(c) 1:1
(d) 2:1
Answer:
(b) 3:1
Question 4.
Red and White cross give pink progeny. In this R gene represents:
(a) Hybrid
(b) Recessive
(c) Incomplete dominance
(d) Mutation
Answer:
(c) Incomplete dominance
Question 5.
The effect of the genotype of group AB in human being shows:
(a) Dominant-Recessive
(b) Incomplete dominance
(c) Co-dominance
(d) Complimentary
Answer:
(c) Co-dominance
Question 6.
What is the genotype ratio in a lethal gene?
(a) 1:2:1
(b) 3:1
(c) 2:1
(d) 9:3:3:1
Answer:
(c) 2:1
RBSE Class 12 Biology Chapter 35 Very Short Answer Type Questions
Question 1.
Who is called the father of heredity and why?
Answer:
Gregor Johann Mendel (1822 to 1884) was first to explain the mechanism of transmission of characters from generation to generation. He is, therefore, called as “father of genetics”.
Question 2.
What are the monohybrid and dihybrid phenotypic ratio of the F2 generation?
Answer:
Monohybrid ⇒ 3:1 (Dominant: Recessive)
Dihybrid ⇒ 9:3:3:1
Question 3.
What are the multiple gene characters?
Answer:
- Normally, phenotype expression is controlled by a pair of alleles. But when 3 or more alleles are responsible for one character, it is called multiple alleles.
- Example: ABO blood system.
Question 4.
Where and in which organization Mendel read the research paper of his experiments?
Answer:
Brunn Society of Natural History from 8th Feb. to 8th March 1865.
Question 5.
In which language the original research paper of Mendel was published and what was its title?
Answer:
German
Title: Versuche Uber Pflanzenhybriden.
Question 6.
Who rediscovered Mendel’s work?
Answer:
Hugo De Vries (Holland), E.V. Tschermak (Austria) and K. Correns (Germany)
RBSE Class 12 Biology Chapter 35 Short Answer Type Questions
Question 1.
Differentiate the following:
- Homozygous and Heterozygous
- Dominant and Recessive trait
- Genotype and Phenotype
- Monohybrid Cross and Dihybrid Cross
Answer:
1. Homozygous and Heterozygous:
- There are two genes for every character. If the two genes for a particular character are identical, is said to be homozygous (G homos ⇒ same; zygotes ⇒ a pair). For example, tall plants (TT) & dwarf plant (tt) are homozygous.
- If the two genes for a particular character are unlike, it is said to be heterozygous (G. heteros ⇒ other, zygotes ⇒ a pair). For example, Tt is heterozygous tall.
| Homozygous | Heterozygous |
| Formation of only one type of gametes | Formation of 2 types of gametes |
| Breed true to the particular character | Do not breed true to a particular character |
| It possesses two alleles of a gene | It possesses two identical alleles of a contrasting gene |
2. Dominant and Recessive trait:
- Dominant character:
- Amongst two alleles of a character, the character express in a heterozygous condition is called Dominant character. The incidence is known as Dominance and the gene responsible for it is dominant allele.
- For Example, Tt containing pea plant has T for tallness and t for dwarfness and T is the dominant character.
- Recessive character:
- Amongst Alleles of a character in the F1 generation, which is not expressed is called recessive character. It expresses only in the homozygous condition of character.
- For Example, tt condition express dwarfness of a plant.
3. Genotype and Phenotype:
- Genotype:
- Genetic expression of a feature, which obtained from parents.
- For Example Pure round seed producing parent plants has genotype R R.
- Phenotype:
- The visible expression of the hereditary character possessed by an organism.
- For example, around or wrinkled shape of seed is its phenotype.
4. Monohybrid Cross and Dihybrid Cross:
- Monohybrid cross:
- Across in which inheritance of only one pair of contrasting character is studied.
- Dihybrid cross:
- Across in which inheritance of two pairs of contrasting characters is studied simultaneously.
Question 2.
Define:
- Allele
- Codominance
- Polygenes
- Lethal gene
Answer:
1. Allele:
- Alternative forms of a gene for eg. blue colour and black colour of the eye are two alleles of the colour of the eye gene. An allele is a short form of Allelomorph. Similarly, for the seed of the pea plant, the shape of the seed Round (R) and Wrinkled (r) are two alleles.
2. Codominance:
- The dominant and recessive factor of an allele equally expresses their characters in F1 generation is called codominance. The skin colour in castles is red (RR) and white (RR) is found. The cross of these gives a roan colour of skin in the F1 generation.
- Then cross between Fı generation gives red, roan and white skin castles in the F2 generation. The ratio of red, roan and white coloured skin animals are found 1:2:1 (RR: Rr: RR). The results do not follow Mendelian ratio with regard to colours.
3. Multiple gene or Polygenes:
- The phenotype traits which are governed by three or more genes are called polygenic traits. The polygenic traits show a wide range of phenotypes. Each gene of the polygenic trait contributes to the phenotype but to a small degree.
- Presence of more than one dominant gene makes the phenotype more prominent. All the dominant genes add up their effects to produce a full phenotype. Therefore, the polygenic traits are also called as quantitative traits.
- Inheritance of such traits is called polygenic or quantitative inheritance.
- The polygenes may occupy two or more different loci on the same homologous chromosome pair.
- It was first studied by Galton (1883) in man.
- The well-known examples of a polygenic trait are human skin colour & kernel colour in wheat.
- The other examples of polygenic traits in human beings are – eye colour & body height.
Example: 1 – Human skin colour - The human skin colour is a polygenic effect and controlled by three pairs of genes Aa, Bb, Cc.
- These genes are located on different chromosomes and are inherited independently.
- Each gene contributes to a unit of darkness due to incomplete dominance.
- The skin colour varies from very dark to very light and has many shades of intermediate colours. Hence, a total of 64 phenotypic combinations is possible for skin colours.
4. Lethal gene:
- Some genes besides controlling external characteristics also affect the viability of organisms.
- L. Cuenot in 1905 presented results of his experiment on the body colour of rats. These results were different from Mendel’s law of segregation.
- According to his experiments, the yellow colour of the skin is due to gene Y and grey colour is due to gene y in which Y is dominant over y (grey colour).
- When yellow colour rats were crossed then yellow and grey colour rats in 2:1 ratio produced.
- Genotypically, yellow rats were heterozygous (Yy) and grey rats were homozygous (yy).
- The Yy (heterozygous) yellow colour rats were unable to survive. Hence Y gene is responsible for the yellow colour of rats and it also affects survival capacity.
- The yellow homozygous (YY) rats can not survive, hence homozygous rats are not found at all.
- Hence YY genes are called lethal genes and this incidence is known as Lethality.
- Some lethal genes are lethal only in homozygous recessive condition and called as recessive lethal gene, while dominant lethal gene may be lethal even in heterozygous condition.
Question 3.
What are the reasons for Mendel’s success?
Answer:
Reasons for Mendel’s Law:
- Mendel with his experiments discarded all the theories given by earlier scientists, as these theories were hypothetical and were not based on scientific experiments.
- Initially, Mendel’s work was also not recognized by other scientists. But it was accepted after 34 years of the death of
- Mendel, when three scientists Hugo de Vries, Karl corners and Eric Von Tschermak obtained the same results as of Mendel.
- Mendel is known as “Father of Genetics” for his valuable contribution in the area of heredity.
Achievements of Mendel’s work are mentioned below:
- Scientist discovered dominant and recessive characters in living beings on the basis of Mendel’s work.
- Beneficial characters of different genera are brought together in a genus.
- By using Mendel’s law, the discovery of immunity against diseases, continuity in adverse conditions, flowers and fruits of good quality and in quantity are developed.
- Similarly, Breed of Cow, buffalo and hen are improved.
Question 4.
Describe Incomplete dominance with examples?
Answer:
Incomplete Dominance:
- It was discovered by corners (1903).
- It is also called as blended or partial or mosaic or ‘intermediate inheritance’.
- It is an exception to be an outcome of Mendel’s monohybrid cross.
- It is found in both plants & animals.
- In Mendel’s pea experiments, dominance was essentially complete & there was no difference between the homozygous & heterozygous plants in the expression of dominant character.
- However, there are characters or alleles that are neither dominant nor recessive. In such cases, both the alleles of the contrasting conditions of a character express as a blend.
- As a result, in F1 generation the hybrid produced by crossing two pure individuals does not resemble either of them but is midway between them.
- This expression of an intermediate trait of two pure parents in the F1 hybrids is called as incomplete dominance.
- Example 1: Four O’clock plant (Mirabilis jalapa) & snapdragon plant (Antirrhinum majus) Incomplete dominance in Snapdragon Here the phenotypic and genotypic ratios are same. The ratio 1:2:1 is the characteristics of Incomplete Dominance.
Question 5.
Why Mendel selected Pea plant in his experiments? Explain.
Answer:
Selection of Garden Pea by Mendel for his experiments:
- Mendel carried out his work on garden pea Pisum sativum. He selected this plant because of the following reasons:
- It was easy to grow the plants in open ground and in pots.
- The plant had a short life span and it completes in one season so the study of many generations is possible in a few years.
- The pea plant is small, easy to crossbreed artificially.
- The plant flower being bisexual, and exhibits self-fertilization in nature so that purity of characters can be maintained until many generations but can be easily cross-pollinated experimentally.
- A large number of true-breeding varieties of peas are available or we can say many contrasting characters are found in pea plants.
RBSE Class 12 Biology Chapter 35 Essay Type Questions
Question 1.
Mendel studied which hereditary traits of a Pea. in his experiments, explain.
Answer:
Selection of Characters by Mendel:
- Mendel conducted a breeding experiment on the garden pea plant, Pisurnsativum. He studied the inheritance of seven pairs of contrasting characters in the garden pea & he restricted his experiments to one or two pairs of contrasting traits in each experiment.
- The seven pairs of contrasting characters selected by Mendel are as follows:
| Character | Recessive | Dominant |
| Height of Plant | Dwarf | Tall |
| Position of Flower | Terminal | Auxillary |
| The shape of Pod | Constricted | Inflated |
| Colour of Pod | Yellow | Green |
| The shape of a seed | Wrinkled | Round |
| Colour of Seed Coat | White | Grey |
| Colour of Cotyledon | Green | Yellow |
- Some authors follow the colour of flowers – White and Purple character as a 7th character instead of the colour of cotyledons.
- Mendel found all the above 7 traits to be pure genetically in experiments.
Question 2.
Explain Mendel’s laws in detail.
Answer:
Mendel’s Breeding Experiments:
Mendel did the following experiments to cross plants using one or more contrasting characters at one time –
- Monohybrid cross
- Dihybrid cross
1. Monohybrid cross:
- In this cross, he used one contrasting character at a time.
Experiment:
- He made a cross between pure tall and pure dwarf plants –
- Tall plant: Height 6 feet
- Dwarf plant: Height 1 feet
- Mendel used plants of F1 generation for self-pollination and seeds obtained were sowed. The resulting plants of second filial (F2) generation were both tall and dwarf. in 3:1 ratio. This 3:1 ratio is called monohybrid phenotypic ratio.
- Out of these of dwarf plants were self-pollinated and their seeds again gave all dwarf plants. While the tall plants were self-pollinated and the seeds obtained were sowed they gave both tall and dwarf plants in the 3:1 ratio, phenotypically.
- The genotype of these plants was found to be 1:2:1 (1TT; 2Tt: 1tt).
- The result shows that tall plants homozygous TT and heterozygous Tt while dwarf plants were tt homozygous.
Mendel’s Explanation:
- Hence monohybrid cross produces phenotype ratio 3:1 and genotype ratio 1:2:1 in the F2 generation. The logical conclusion arrived at by Mendel from these points was that Fı plants must have carried a ‘hidden factor’ for dwarfness. Thus
- The second conclusion arrived at by Mendel was that if the F1 plants contain two ‘factors’ for height (the one responsible for tallness which showed up and one for dwarfness which remain hidden).
- Hence pure breed of the tall plant contains two factors (TT) for tallness and dwarf plant contain two factors (tt) for dwarfness.
- These factors are presently referred to as a gene. Out of them one factor from father and another factor from mother came into the offspring.
- The factor which expresses its character is termed Dominant and which could not express in the presence of dominant factor is called Recessive factor.
- Example 1: Seed Shape in Pea.
2. Dihybrid cross:
- In this experiment, Mendel selected two pairs of contrasting characters i.e. Mendel studied the inheritance of two characters at a time. He crossed pea plant with round & yellow seeds (RRYY) with the plant having wrinkled & green seeds.
- In the F1 generation, all pea plants produce round & yellow seeds (Rr Yy), which indicates that round is dominant over the wrinkled shape and yellow is dominant over green colour.
- In F2 generation (resulting by self-pollination of F1 plants), all the four characters were assorted out independent to each other which indicates that seed colour is independent of seed shape.
- The F1 hybrids form 4 types of gametes viz. RY, Ry, rY & ry They on random mating produced & types of the generation having a phenotypic ratio of 9:3:3:1.
- Example: Shape and colour of seeds of a pea plant
- Result of F2 generation:
- yellow Round ⇒ 9 (1, 2, 3, 4, 5, 7, 9, 10, 13)
- yellow Wrinkled ⇒ 3 (11, 12, 15)
- Green Round ⇒ 3 (6, 8, 14)
- Wrinkled green ⇒ 1 (16)
Question 3.
Explain the deviations of principles of Mendel’s laws.
Answer:
principles of Mendel’s laws:
Incomplete Dominance:
- It was discovered by corners (1903).
- It is also called as blended or partial or mosaic or ‘intermediate inheritance’.
- It is an exception to be an outcome of Mendel’s monohybrid cross.
- It is found in both plants & animals.
- In Mendel’s pea experiments, dominance was essentially complete & there was no difference between the homozygous & heterozygous plants in the expression of dominant character.
- However, there are characters or alleles that are neither dominant nor recessive. In such cases, both the alleles of the contrasting conditions of a character express as a blend.
- As a result, in F1 generation the hybrid produced by crossing two pure individuals does not resemble either of them but is midway between them.
- This expression of the intermediate trait of two pure parents in the F1 hybrids is called as incomplete dominance.
- Example 1: Four O’clock plant (Mirabilis jalapa) & snapdragon plant (Antirrhinum majus) Incomplete dominance in Snapdragon Here the phenotypic and genotypic ratios are same. The ratio 1:2:1 is the characteristics of Incomplete Dominance.
Codominance:
- A dominant and recessive factor of an allele equally express their characters in F1 generation is called codominance. The skin colour in cattles is red (RR) and white (rr) is found. The cross of these gives a roan colour of skin in the F1 generation.
- Then cross between F1 generation gives red, roan and white skin cattles in the F2 generation. The ratio of red, roan and white coloured skin animals are found 1:2:1 (RR:Rr:rr). The results do not follow Mendelian ratio with regard to colours.
Differences between Co-dominance and Incomplete dominance:
| Co-dominance | Incomplete dominance |
| The heterozygous shows ratio of both alleles in the F1 generation. For example blood group AB has both the antigens A as well as B. | The heterozygous is of the intermediate type and shows none of the original parental forms. Example Pink colour flower of snapdragon. |
| Both alleles are equally dominant. Both expressed fully and equally. | There is partial dominance of one and hence intermediate type id formed. |
| Examples: ABO blood group and skin colour of cattle. | Examples: the colour of snapdragon flowers and flowers of Four O clock plant. |
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