Main provisions of the chromosomal theory
The theory developed by Thomas Morgan and his colleagues is based on the parallelism between the behavior of genes and chromosomes (specifically during meiosis). In its classical form, the theory includes the following fundamental principles:
- Heredity is directly determined by genes, which are physically localized on chromosomes.
- Each gene occupies a strictly defined position on a chromosome — its locus.
- The arrangement of genes on chromosomes is exclusively linear in nature.
- A single chromosome represents a single linkage group.
- The number of such linkage groups in an organism equals the haploid set of its chromosomes.
- An exchange of segments containing allelic genes is possible between homologous chromosomes.
- The distance between genes is directly proportional to the percentage of crossing over between them.
These principles made it possible to explain phenomena that did not fit into classical Mendelian laws, primarily the joint inheritance of traits.
Linked inheritance of genes
Linkage refers to the joint inheritance of genes that are localized on the same chromosome. T. Morgan studied this phenomenon in the fruit fly Drosophila melanogaster, analyzing traits such as body color (gray — dominant gene B, black — recessive b) and wing shape (long — V, vestigial — v).
Complete linkage A type of linkage in which genes are always transmitted together because crossing over does not occur between them. An important biological feature of Drosophila is that complete linkage is observed only in males. In Morgan's experiment, a dihybrid male from the first generation ($BbVv$) was crossed with a recessive female ($bbvv$). According to Mendel's third law (the law of independent assortment), a phenotypic split in a 1:1:1:1 ratio was expected. However, in reality, only two phenotypic classes of progeny were obtained:
- 50% — gray with normal wings ($BbVv$).
- 50% — black with vestigial wings ($bbvv$).
The progeny completely replicated the parental forms. This proved that the genes for body color and wing shape reside on the same chromosome and are inherited inseparably.
Incomplete linkage In Drosophila females, complete linkage is absent; it is periodically disrupted. When crossing a dihybrid female ($BbVv$) with a recessive male, four types of offspring appeared in the progeny:
- Two parental types (gray/long wings and black/vestigial wings) made up the vast majority — a total of 83% (41.5% each).
- Two new types (gray/vestigial wings and black/long wings) made up a total of 17% (8.5% each).
The deviation from an equal ratio (25% each) confirmed the fact of linkage, and the appearance of new combinations indicated that linkage can be disrupted.
Crossing over and genetic distance
T. Morgan explained the mechanism leading to the appearance of new phenotypes in incomplete linkage as the chiasma of homologous chromosomes — crossing over. This is the process of physical exchange of identical segments between chromosomes, occurring at points between genes.
Individuals formed from gametes that underwent crossing over are called recombinant (crossover) individuals. In the Drosophila female experiment described above, their proportion was 17%, which corresponds to the frequency of crossing over between the body color and wing shape genes.
Morgan's law states: genes located on the same chromosome are inherited in a linked manner, with the strength of linkage being inversely proportional to the distance between them. In other words, the farther apart genes are from each other, the weaker their linkage and the higher the probability that crossing over will occur between them.
The unit of genetic distance is 1 centimorgan (1 cM), which equals 1% crossing over. Using the hybridological method and analyzing recombination frequencies, scientists construct genetic maps of chromosomes — diagrams of the relative positions of genes within a single linkage group. Modern science also utilizes physical maps, where distance is determined with high precision in base pairs (bp). One centimorgan roughly corresponds to one million base pairs (1 Megabase).
Briefly: answers to topical questions
How is human chromosome mapping performed using somatic cell hybridization?
Gene mapping via somatic cell hybridization is based on the fusion of different cell types, such as human and mouse cells.
The process includes:
- Cell hybridization — the fusion of different cell types.
- Gradual chromosome loss in hybrid cells.
- Establishment of linkage groups based on traits that disappear simultaneously.
The somatic cell genetics method is used for gene mapping and studying regulatory mechanisms.
Class and exam questions
Why is the number of linkage groups equal to the haploid set of chromosomes?
A linkage group consists of genes located on the same chromosome. Since homologous chromosomes carry identical sets of loci (allelic genes), they represent the same linkage group. Therefore, their total number equals the number of pairs of homologous chromosomes, i.e., the haploid set.
What is the difference between genetic and physical chromosome maps?
Genetic maps reflect the order of gene arrangement and the distance between them in centimorgans (recombination percentages), calculated based on crosses. Physical maps show the actual physical distance between genes in base pairs (bp).
Why does crossing over not occur in Drosophila males?
This is an evolutionarily established biological feature of this species (Drosophila melanogaster). In Drosophila males during meiosis, chiasma formation and chromosome segment exchange do not occur, so genes on the same chromosome are always inherited as a complete block (absolute linkage is observed).
What else to study on this topic
- Mechanisms and stages of crossing over in prophase I of meiosis
- Features of test crosses in hybridological analysis
- Principles and algorithms for constructing genetic maps based on recombination frequency
- Cytological foundations of Mendel's third law and reasons for deviations from it
- Specifics of gametogenesis and reduction division in various model organisms
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