12th Botany / Bio Botany Chapter 2 Important 2, 3, 5 Mark Questions 2026
Classical Genetics
Most Repeated Questions
Number next to question tells how many times the question has been repeated in the past few years
TWO MARKS
1.What is back cross?(4)
- Back cross is a cross of F1 hybrid with any one of the parental genotypes.
- Types :
- dominant back cross
- recessive back cross.
- It involves the cross between the F1 offspring with either of the two parents.
2.What are multiple alleles?(5)
Three or more allelic forms of a gene occupy the same locus in a given pair of homologous chromosomes, they are said to be called multiple alleles.
3.Give the names of scientists who rediscovered Mendelism.(2)
Mendel’s experiments were rediscovered by three biologists,
- Hugo de Vries of Holland,
- Carl Correns of Germany and
- Erich von Tschermak of Austria.
4.Define Genetics.(2)
“Genetics” is the branch of biological science which deals with the mechanism of transmission of characters from parents to offsprings.
Genetics – The Science of heredity (Inheritance)
5.What is a Lethal gene? Give ExampleLn2(2)
An allele which has the potential to cause the death of an organism is called a “Lethal Allele”
THREE MARKS
1.What are the reasons for Mendel’s success in his breeding experiment?(5)
- He applied mathematics and statistical methods to biology and laws of probability to his breeding experiments.
- Followed scientific method with accurate & detailed records
- Conducted well-planned experiments with large sample size
- Studied contrasting characters controlled by genes on separate chromosomes
- Selected pure breeding parents, confirmed by self-crossing for many generations
2.What is Atavism? Give an example.(4)
Atavism is a modification of a biological structure whereby an ancestral trait reappears after having been lost through reemergence of sexual reproduction in the flowering plant.
Example : Hieracium pilosellafor
3.Differentiate continuous and discontinuous variation.(2)
| Continuous variation | Discontinuous variation |
| Shows gradual variation with intermediates | Shows distinct categories with no intermediates |
| Controlled by many genes (polygenes) | Controlled by one or few major genes |
| Affected by both genes and environment | Not affected by environment |
| Phenotype : Overlapping and continuous range | No overlapping between phenotypes |
| Quantitative inheritance | Qualitative inheritance |
| Ex : Human height, skin colour | Ex: Style length in Primula, pea plant height |
Differentiate Incomplete Dominance and Co-dominance.(2)
| Feature | Incomplete Dominance | Codominance |
| Definition | One allele is not completely dominant, resulting in an intermediate phenotype | Both alleles are expressed equally in the heterozygote |
| F1 Phenotype | Shows blending (intermediate) (e.g., pink) | Shows both traits together (no blending) |
| Expression of alleles | Alleles show partial expression | Alleles show full and simultaneous expression |
| F2 Ratio | 1 : 2 : 1 (phenotype = genotype) | 1 : 2 : 1 (phenotype = genotype) |
| Example | 4’o clock Plant (Mirabilis jalapa (red × white → pink) | ABO blood group, Camellia flowers |
| Approach | Quantitative for gene expression | Qualitative of both incomplete dominant alleles |
FIVE MARKS
1.Describe dominant epistasis with example?(7)
Definition
- Dominant epistasis is a gene interaction where a dominant allele at one locus masks the expression of alleles at another locus.
- The masking gene is called epistatic gene.
- The masked gene is called hypostatic gene.
Key Concept
- When both genes are present:
- Epistatic gene determines the phenotype
- Hypostatic gene does not express
Example: Summer Squash (Fruit Colour)
- Gene 1 (Epistatic gene):
- W → White (dominant, masks all colours)
- w → Allows colour expression
- Gene 2 (Hypostatic gene):
- G → Yellow (dominant)
- g → Green (recessive)
Cross
- Parents:
- White (WWgg) × Yellow (wwGG)
- F1 Generation:
- All WwGg (White)
- F2 Phenotypic Ratio:
- 12 White : 3 Yellow : 1 Green
Explanation of Ratio
- Since W is epistatic to the alleles ‘G’ and ‘g’, the white which is dominant, masks the effect of yellow or green.
- Homozygous recessive ww genotypes only can give the coloured fruits (4/16).
- Double recessive ‘wwgg’ will give green fruit (1/16).
- The Plants having only ‘G’ in its genotype (wwGg or wwGG) will give the yellow fruit(3/16).
Important Points
- Dominant allele W masks G and g
- Colour appears only in ww condition
- Shows modified dihybrid ratio (12 : 3 : 1)
2.Bring out the inheritance of chloroplast gene with an example.(7)
Definition
- Chloroplast inheritance is a type of cytoplasmic inheritance where traits are controlled by chloroplast genes and passed through the female parent.
Example: Mirabilis jalapa (4 O’ Clock plant)
- Two types of leaves:
- Dark green
- Pale green (variegated)
Key Observations
- In reciprocal crosses, F1 offspring are not identical
- F1 plants show the character of the female parent
- This does not follow Mendelian inheritance
Reason
- Chloroplasts are present in the cytoplasm of ovum (female gamete)
- Male gamete contributes only nucleus, not cytoplasm
- Hence, inheritance is maternal (cytoplasmic inheritance)
Cytoplasmic Male Sterility (CMS)
- Controlled by mitochondrial genes
- Two types of cytoplasm:
- N (Normal) → fertile
- S (Sterile) → male sterile
- Restorer genes (Rf):
- Nuclear genes that restore fertility in S cytoplasm
Combinations
- N cytoplasm + rfrf → Fertile
- S cytoplasm + RfRf → Fertile (restored)
- S cytoplasm + rfrf → Male sterile
3.Explain polygenic inheritance with an example?(3)
Definition
- Polygenic inheritance – Several genes combine to affect a single trait.
- It explains continuous variation and follows Mendel’s laws.
Key Features
- Controlled by two or more genes
- Each gene has additive effect on phenotype
- Produces continuous variation (no clear-cut categories)
- Results in bell-shaped (normal) distribution curve
Example: Wheat Kernel Colour (H. Nilsson–Ehle, 1909)
- Traits controlled by two gene pairs (R₁, R₂)
- Red (R) is dominant, white (r) is recessive
Cross:
- Dark red (R₁R₁R₂R₂) × White (r₁r₁r₂r₂)
- F1 → Medium red (R₁r₁R₂r₂)
F2 Variation (based on number of dominant alleles):
- 4 R genes → Dark red
- 3 R genes → Medium dark red
- 2 R genes → Medium red
- 1 R gene → Light red
- 0 R gene → White
Important Results
- Phenotypic ratio shows continuous gradation
- Produces bell-shaped curve
- Parental types reappear → no blending of genes
- Genes show independent assortment and additive effect
4.Explain incomplete dominance?(2)
Definition
- Incomplete dominance is a condition where one allele is not completely dominant over the other, resulting in an intermediate phenotype.
Example: Mirabilis jalapa (4 O’ Clock plant)
- Studied by Carl Correns (1905)
- Red (R₁R₁) × White (R₂R₂)
F1 Generation
- Genotype: R₁R₂
- Phenotype: Pink (intermediate)
- Shows no complete dominance
F2 Generation (Selfing of F1)
- Phenotypic ratio: 1 Red : 2 Pink : 1 White
- Genotypic ratio: 1 R₁R₁ : 2 R₁R₂ : 1 R₂R₂
- Phenotypic ratio = Genotypic ratio (1 : 2 : 1)
Explanation
- R₁ allele → produces enzyme for red pigment
- R₂ allele → produces defective enzyme
- R₁R₂ → produces less pigment → pink colour
Important Points
- No blending of genes (genes remain separate)
- Parental types reappear in F2
- Follows Mendel’s Law of Segregation
- Shows intermediate phenotype in F1
