Model Answer
0 min readIntroduction
Genetic linkage refers to the phenomenon where genes or DNA sequences located close together on the same chromosome tend to be inherited together during meiosis, deviating from Mendel's law of independent assortment. This occurs because the physical proximity of loci reduces the likelihood of separation through crossing over. The concept of coupling and repulsion phases, initially observed by Bateson and Punnett in 1905 in sweet pea plants, describes the specific arrangements of alleles of linked genes on homologous chromosomes. Thomas Hunt Morgan later explained these as two aspects of the same phenomenon: linkage. Understanding these phases is crucial for predicting inheritance patterns and constructing genetic maps.
Coupling and Repulsion Phases of Linkage
Genetic linkage describes the tendency of genes located close to each other on the same chromosome to be inherited together. The arrangement of alleles of two linked genes on homologous chromosomes can occur in two distinct configurations, known as coupling and repulsion phases, which significantly influence the outcome of genetic crosses.
1. Coupling Phase (Cis Configuration)
In the coupling phase, also known as the cis arrangement, the dominant alleles of two linked genes are present on one homologous chromosome, while their corresponding recessive alleles are located on the other homologous chromosome. This means that both dominant alleles (e.g., A and B) are on one chromosome, and both recessive alleles (e.g., a and b) are on the other homologous chromosome. The parental gametes typically carry these combinations (e.g., AB and ab).
- Allelic Arrangement: AB/ab
- Formation: Often arises from a cross between two homozygous parents (e.g., AABB x aabb). The F1 generation will be heterozygous (AaBb) with alleles in the coupling phase.
- Inheritance Pattern: In a test cross (e.g., AaBb x aabb), the coupling phase leads to a higher frequency of offspring with the parental combinations (AB and ab phenotypes) compared to recombinant phenotypes (Ab and aB).
2. Repulsion Phase (Trans Configuration)
Conversely, in the repulsion phase, also known as the trans arrangement, a dominant allele of one gene is located on the same homologous chromosome as a recessive allele of the other linked gene. This implies that one chromosome carries a dominant allele for one gene and a recessive allele for the other (e.g., A and b), while the homologous chromosome carries the recessive allele for the first gene and the dominant allele for the second (e.g., a and B). The parental gametes in this configuration would be Ab and aB.
- Allelic Arrangement: Ab/aB
- Formation: Can arise from a cross between parents where dominant and recessive alleles are segregated differently (e.g., AAbb x aaBB). The F1 generation will be heterozygous (AaBb) but with alleles in the repulsion phase.
- Inheritance Pattern: In a test cross, the repulsion phase also results in a higher frequency of parental types (Ab and aB phenotypes) than recombinant types (AB and ab phenotypes), but the specific parental combinations are different from the coupling phase.
The distinction between coupling and repulsion phases is crucial for calculating recombination frequencies and constructing accurate genetic maps. The frequency of parental versus recombinant offspring in a test cross reveals the strength of linkage and the phase in which the alleles are arranged.
| Feature | Coupling Phase (Cis) | Repulsion Phase (Trans) |
|---|---|---|
| Allelic Arrangement on one chromosome | Both dominant alleles (e.g., AB) or both recessive alleles (e.g., ab) | One dominant and one recessive allele (e.g., Ab or aB) |
| Example Genotype | AB/ab | Ab/aB |
| Parental Gametes (typically) | AB, ab | Ab, aB |
| Expected Test Cross Progeny | Higher frequency of AB and ab | Higher frequency of Ab and aB |
Conclusion
The coupling and repulsion phases are fundamental concepts in understanding genetic linkage, representing the two primary arrangements of alleles of linked genes on homologous chromosomes. While Bateson and Punnett initially described these phases, it was Morgan who elucidated their connection to genes residing on the same chromosome. These arrangements dictate the relative frequencies of parental and recombinant genotypes observed in offspring, offering invaluable insights into gene mapping and the genetic architecture of organisms. Recognizing these phases is essential for geneticists and breeders to accurately predict inheritance patterns and manipulate desirable traits.
Answer Length
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