Model Answer
0 min readIntroduction
The chromosome theory of inheritance, proposed by Sutton and Boveri in the early 20th century, established that genes reside on chromosomes and are the units of heredity. A crucial extension of this theory is the chromosome theory of linkage, which explains how genes located close together on the same chromosome are inherited together more frequently than genes located far apart. This phenomenon, first observed by Bateson and Punnett in 1905 while studying sweet peas, challenged Mendel’s law of independent assortment. Understanding linkage is fundamental to comprehending genetic mapping and predicting inheritance patterns, and forms the basis for many modern genetic analyses.
Chromosome Theory of Linkage: A Detailed Explanation
The chromosome theory of linkage states that genes located on the same chromosome tend to be inherited together during meiosis, unless separated by crossing over. This means that the alleles of linked genes do not assort independently as predicted by Mendel’s law of independent assortment. The strength of linkage is directly proportional to the physical distance between the genes on the chromosome – the closer the genes, the stronger the linkage and the lower the frequency of recombination.
Methods for Determining Linkage
1. Two-Point Test Crosses
The most common method for determining linkage is the two-point test cross. This involves crossing an individual heterozygous for two genes under investigation with a homozygous recessive individual. The progeny are then analyzed to determine the frequency of parental and recombinant phenotypes.
- Parental Types: These are the phenotypes that resemble the original parental combinations.
- Recombinant Types: These are the phenotypes that result from crossing over between the two genes.
The recombination frequency (RF) is calculated as:
RF = (Number of recombinant progeny / Total number of progeny) x 100
If the RF is low (less than 50%), the genes are considered linked. A higher RF (close to 50%) suggests that the genes are either unlinked or located on different chromosomes.
Example: Drosophila Experiment
Morgan’s experiments with Drosophila melanogaster provided strong evidence for linkage. He crossed flies heterozygous for gray body color (G) and normal wings (W) with black body color (g) and vestigial wings (w). The results showed that the parental combinations (gray body, normal wings and black body, vestigial wings) were much more frequent than the recombinant combinations (gray body, vestigial wings and black body, normal wings). This indicated that the genes for body color and wing shape are linked on the same chromosome.
Let's assume the following results from a test cross:
| Phenotype | Number of Progeny |
|---|---|
| Gray body, Normal wings (Parental) | 420 |
| Black body, Vestigial wings (Parental) | 410 |
| Gray body, Vestigial wings (Recombinant) | 80 |
| Black body, Normal wings (Recombinant) | 90 |
| Total | 1000 |
RF = ((80 + 90) / 1000) x 100 = 17%
Since the RF is 17%, the genes for body color and wing shape are linked.
2. Calculating Genetic Distance and Map Units
The recombination frequency is often used to estimate the genetic distance between genes. One map unit (mu) or centimorgan (cM) is defined as 1% recombination frequency. Therefore, in the Drosophila example above, the genes are approximately 17 map units apart.
Limitations of Two-Point Crosses
Two-point crosses can underestimate the actual distance between genes, especially when dealing with multiple crossovers. They also don't provide information about the gene order.
Three-Point Test Crosses
Three-point test crosses overcome the limitations of two-point crosses by analyzing three linked genes simultaneously. This allows for the determination of gene order and more accurate estimation of genetic distances. The principle relies on identifying the double crossover events, which help in establishing the correct gene sequence.
Conclusion
The chromosome theory of linkage revolutionized our understanding of inheritance, demonstrating that genes are not always inherited independently. Methods like two-point and three-point test crosses, based on analyzing recombination frequencies, allow us to determine linkage, calculate genetic distances, and construct genetic maps. These maps are crucial tools in understanding genome organization and predicting inheritance patterns, with applications in areas like plant and animal breeding, and human genetic disease diagnosis. Further advancements in molecular genetics have refined these techniques, providing even more precise insights into the complexities of gene linkage and recombination.
Answer Length
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