Model Answer
0 min readIntroduction
Genetic linkage, a fundamental concept in heredity, describes the tendency of genes located close together on the same chromosome to be inherited together during meiosis. This deviates from Mendel’s law of independent assortment. The coupling and repulsion hypotheses explain the varying phenotypic ratios observed in linked genes. Understanding these hypotheses and the subsequent technique of three-point test cross is vital for constructing chromosome maps, which visually represent the relative positions of genes on chromosomes and are crucial for predicting inheritance patterns and understanding genome organization.
Coupling and Repulsion Hypothesis
The coupling and repulsion hypotheses, proposed by Bateson and Punnett, explain the deviations from Mendelian ratios observed when dealing with linked genes. These hypotheses are based on the arrangement of alleles on homologous chromosomes during gamete formation.
Coupling (Cis Configuration)
Coupling refers to the arrangement of dominant alleles on one chromosome and recessive alleles on the other homologous chromosome. For example, if a plant is heterozygous for two linked genes, say ‘A’ and ‘B’, the coupling configuration would be AB/ab. During gamete formation, the parental combinations (AB and ab) are more frequent than the recombinant combinations (Ab and aB) because the linked genes tend to be inherited together.
- Parental Gametes: AB and ab
- Recombinant Gametes: Ab and aB
- Phenotypic Ratio: A higher proportion of parental phenotypes is observed in the offspring.
Repulsion (Trans Configuration)
Repulsion refers to the arrangement where a dominant allele on one chromosome is linked with a recessive allele on the homologous chromosome. Using the same example, the repulsion configuration would be Ab/aB. Here, the parental combinations are Ab and aB, while the recombinant combinations are AB and ab. Similar to coupling, parental gametes are more frequent.
- Parental Gametes: Ab and aB
- Recombinant Gametes: AB and ab
- Phenotypic Ratio: Again, a higher proportion of parental phenotypes is observed.
The difference in phenotypic ratios observed in coupling and repulsion configurations is due to the different arrangements of alleles and the resulting frequency of recombinant gametes.
Three-Point Test Cross for Chromosome Mapping
A three-point test cross involves analyzing the inheritance of three linked genes simultaneously. This method allows for determining the gene order and calculating the distances between the genes based on recombination frequencies. The procedure involves the following steps:
Step 1: Selecting the Parentals
Choose a homozygous individual with three linked genes (e.g., AABBc/aabbc) and cross it with a homozygous recessive individual (aabbc/aabbc). The F1 generation will be heterozygous for all three genes (AaBbCc/aabbc).
Step 2: Performing the Test Cross
Cross the F1 heterozygous individual with a homozygous recessive individual (aabbc/aabbc). This test cross allows for observing the phenotypic ratios of the offspring, revealing the linked genes and their recombination frequencies.
Step 3: Analyzing the Offspring
Analyze the offspring to determine the frequency of each phenotypic class. The most frequent phenotypic classes represent the parental combinations, while the least frequent classes represent the double crossover events.
Step 4: Determining Gene Order
The gene order is determined by identifying the double crossover events. The gene that lies between the other two will have the lowest recombination frequency with each of them. For example, if the double crossover frequency is lowest for the arrangement ABC, then the gene order is A-C-B.
Step 5: Calculating Recombination Frequencies and Map Distances
Recombination frequency (RF) is calculated as the number of recombinant offspring divided by the total number of offspring. The RF is expressed as a percentage and represents the distance between two genes in map units (mu) or centimorgans (cM). 1% RF = 1 cM or 1 mu.
Example:
Suppose in a three-point test cross involving genes A, B, and C, the following recombination frequencies are observed:
- RF (A-B) = 10%
- RF (B-C) = 5%
- RF (A-C) = 15%
This indicates that the gene order is A-B-C, with the distance between A and B being 10 cM, between B and C being 5 cM, and between A and C being 15 cM (10 + 5).
| Gene Pair | Recombination Frequency (%) | Map Distance (cM) |
|---|---|---|
| A-B | 10 | 10 |
| B-C | 5 | 5 |
| A-C | 15 | 15 |
Conclusion
In conclusion, the coupling and repulsion hypotheses provide a framework for understanding the inheritance patterns of linked genes, while the three-point test cross is a powerful technique for constructing chromosome maps. These concepts are fundamental to understanding genetic linkage, recombination, and the organization of genomes. Accurate chromosome mapping is crucial for identifying genes responsible for inherited traits and diseases, and for advancing our understanding of evolutionary relationships. Further advancements in genomic technologies continue to refine and enhance our ability to map genomes with increasing precision.
Answer Length
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