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Q8:
Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in (F_{1}) generation for a dibybrid cross?
Solution :
Step 1: Establishing Genetic Configuration
Given that the parents are heterozygous for two linked loci (Dihybrid), we define the alleles as $A, a$ and $B, b$. The term "linked" implies that the loci are situated on the same chromosome. The phenotypic distribution depends on the phase of linkage:
- Cis-arrangement (Coupling): Dominant alleles ($AB$) are on one chromosome, and recessive alleles ($ab$) are on the homologous chromosome. Genotype: $\frac{AB}{ab}$.
- Trans-arrangement (Repulsion): Dominant and recessive alleles are cross-linked. Genotype: $\frac{Ab}{aB}$.
Step 2: Analytical Implications of Linkage
Per the Law of Linkage (Morgan, 1910), linked genes do not follow Mendel’s Law of Independent Assortment. Instead, they tend to be inherited together unless separated by crossing over (recombination).
| Inheritance Pattern | Expected Ratio (Independent Assortment) | Result with Complete Linkage |
|---|---|---|
| Phenotypic Ratio | $9:3:3:1$ | $3:1$ (Cis) or $2:1:1$ (Trans) |
Step 3: Calculating Phenotypic Distribution (Assuming Complete Linkage)
If genes are completely linked and in cis-configuration ($\frac{AB}{ab} \times \frac{AB}{ab}$):
- Gametes produced by each parent: $AB$ and $ab$ (in equal proportions).
- Punnett Square setup:
- Phenotypic frequency: 3 (Dominant-Dominant) : 1 (Recessive-Recessive).
Step 4: Incorporating Recombination (Incomplete Linkage)
In biological reality, linkage is rarely 100%. If crossing over occurs, the distance between loci determines the frequency of recombinant phenotypes. If $p$ is the frequency of recombination, the offspring frequency is defined as:
- Parental combinations: $\frac{1-p}{2}$ for each type.
- Recombinant combinations: $\frac{p}{2}$ for each type.
[By the Principle of Recombination Frequency]: As the distance between loci increases, the ratio shifts from $3:1$ toward the Mendelian $9:3:3:1$ ratio.
Final Solution: If linkage is complete, the phenotypic ratio is 3:1. If linkage is incomplete, the phenotypic distribution will deviate from the Mendelian 9:3:3:1 ratio, with parental phenotypes appearing with higher frequency than recombinant phenotypes.
More Questions from Class 12 Biology Principles of Inheritance and Variation EXERCISES
- Q1: Mention the advantages of selecting pea plant for experiment by Mendel.
- Q1: Mention the advantages of selecting pea plant for experiment by Mendel.
- Q2(a): Differentiate between the following – (a) Dominance and Recessive
- Q2(a): Differentiate between the following – (a) Dominance and Recessive
- Q2(b): Differentiate between the following – (b) Homozygous and Heterozygous
- Q2(b): Differentiate between the following – (b) Homozygous and Heterozygous
- Q2(c): Differentiate between the following – (c) Monohybrid and Dihybrid.
- Q2(c): Differentiate between the following – (c) Monohybrid and Dihybrid.
- Q3: A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
- Q3: A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
- Q4: Explain the Law of Dominance using a monohybrid cross.
- Q4: Explain the Law of Dominance using a monohybrid cross.
- Q5: Define and design a test-cross.
- Q5: Define and design a test-cross.
- Q6: Using a Punnett Square, workout 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.
- Q6: Using a Punnett Square, workout 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.
- Q7(a): 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.
- Q7(a): 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.
- Q7(b): 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 (b) dwarf and green.
- Q7(b): 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 (b) dwarf and green.
- Q8: Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in \(F_{1}\) generation for a dibybrid cross?
- Q9: Briefly mention the contribution of T.H. Morgan in genetics.
- Q9: Briefly mention the contribution of T.H. Morgan in genetics.
- Q10: What is pedigree analysis? Suggest how such an analysis, can be useful.
- Q10: What is pedigree analysis? Suggest how such an analysis, can be useful.
- Q11: How is sex determined in human beings?
- Q11: How is sex determined in human beings?
- 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.
- 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.
- Q13(a): Explain the following terms with example (a) Co-dominance
- Q13(a): Explain the following terms with example (a) Co-dominance
- Q13(b): Explain the following terms with example (b) Incomplete dominance
- Q13(b): Explain the following terms with example (b) Incomplete dominance
- Q14: What is point mutation? Give one example.
- Q14: What is point mutation? Give one example.
- Q15: Who had proposed the chromosomal theory of the inheritance?
- Q15: Who had proposed the chromosomal theory of the inheritance?
- Q16: Mention any two autosomal genetic disorders with their symptoms.
- Q16: Mention any two autosomal genetic disorders with their symptoms.
CBSE Solutions for Class 12 Biology Principles of Inheritance and Variation
Chapters in CBSE - Class 12 Biology
- Reproduction in Organisms
- Human Reproduction
- Reproductive Health
- Molecular Basis of Inheritance
- Evolution
- Human Health and Disease
- Strategies for Enhancement in Food Production
- Microbes in Human Welfare
- Biotechnology: Principles and Processes
- Biotechnology and Its Applications
- Organisms and Populations
- Ecosystem
- Biodiversity and Conservation
- Environmental Issues
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