Study Notes on Principles of Inheritance and Variations

Biology Principles of Inheritance and Variations

Gene Interaction

• Gene interaction refers to the ways in which different genes influence the expression of various traits.

• Types of gene interaction include:   - One gene / Intragenic / Allelic interaction.   

Examples of Gene Interaction:

1. Incomplete Dominance

2. Co-dominance

3. Multiple Alleles

4. Pleiotropic Gene

Classification of Characters:

• Qualitative Character:
    - Not dependent on the number of dominant alleles.

• Quantitative Character:
    - Dependent on the number of dominant alleles.

A. Incomplete Dominance

• Defined as an exception to Mendel's law of dominance.

• In incomplete dominance, the dominant allele is not fully dominant over the recessive allele, resulting in the heterozygous condition producing an intermediate phenotype.

• As a result, the F1 hybrid does not resemble either parent.

• Discovery: Incomplete dominance was first discovered by Carl Correns in the plant Mirabilis jalapa, commonly known as '4 o'clock plant' or 'Gul-e-Bans'.

(i) Flower Color in Mirabilis jalapa:

• When a plant with red flowers (RR) is crossed with a plant with white flowers (rr):   - The F1 generation displays all pink flowers (Rr).

• Gametic Contribution:   - P generation: Red (RR) x White (rr)
         - F1 generation: All pink (Rr)
    - F2 generation ratios:     - Phenotypic ratio: 1 red : 2 pink : 1 white     - Genotypic ratio: 1 RR : 2 Rr : 1 rr

• Diagrams (visual aids required): Illustrate P generation, F1 generation, and F2 ratios.

(ii) Flower Color in Antirrhinum majus:

• Also known as 'Snapdragon' or 'Dog flower'.

• Displays incomplete dominance similar to Mirabilis jalapa.

(iii) Size of Starch Grains in Pea Plant:

• Detailed genetic mechanism of inheritance for starch grain size dependent on allele combinations is required.

B. Co-Dominance

• Another exception to Mendel's law of dominance.

• In this interaction, both alleles are equally dominant in the heterozygous condition and express independently, hence no intermediate phenotype occurs.

• The F1 hybrid resembles both parents:   

Examples of Co-Dominance:

(i) Coat Color in Cattles:

• When crossing a black parent (B₁ B₁) with a white parent (B₂ B₂):
    - The F1 progeny produces roan color (B₁ B₂).

• Generation Breakdown:   - Parental Generation: Pure Black (B₁ B₁) x Pure White (B₂ B₂)   - F1 Generation: Roan Color (B₁ B₂)   - Sib-mating: Used to produce F2 generation from F1 progeny.   - F2 Genotype Ratios:     - Black (B₁ B₁)     - Roan (B₁ B₂)     - White (B₂ B₂)

• Phenotypic Ratio: 1 black : 2 roan : 1 white

• Genotypic Ratio: 1 B₁ B₁ : 2 B₁ B₂ : 1 B₂ B₂

Other Examples of Co-Dominance:

• (i) AB blood group inheritance (IA IB)

• (ii) Carrier of Sickle cell anemia (HbA HbS)

Summary of Dominance Types:

• In dominance, F1 resembles either of the parents.

• In incomplete dominance, F1 is intermediate between both parents.

• In co-dominance, F1 resembles both parents.

C. Multiple Alleles

• Defined as more than two alternative forms of the same gene.

• Multiple alleles arise from genetic mutations.

• Located at the same locus on homologous chromosomes.

Genetic Basics:

• A diploid individual contains two alleles, while gametes contain one allele for a gene.

• Multiple alleles can be studied in a population but not in a single individual.

Genotype Combinations:

• If n is the number of alleles for a gene, then the types of different possible genotypes can be calculated using the formula:
    Types of Genotypes = n(n +1)

Example of Multiple Alleles: ABO Blood Group

• Responsible gene: I

• Alleles of gene I:
    - IA = dominant
    - IB = dominant
    - i = recessive

• Phenotypes Produced: A, B, AB, O

Genetic Crosses for ABO System:

• Possible number of genotypes = 6 , here n= 3
    n(n+1)/2 , where n= no of allele

Conclusion

• Understanding gene interactions is vital in fields like genetics, breeding, and medical research due to implications in inheritance patterns and phenotypic variations.