Genetics with the Basics of Breeding - M.P. Myhun - 2008

CHAPTER III. Inheritance of Chromosomal and Non-Chromosomal Genes

3.4. Patterns of Inheritance under Complete and Incomplete Gene Linkage. Crossing Over

In 1906, W. Bateson and R. Punnett published a paper reporting that in certain crosses of sweet pea, the segregation of F₂ offspring did not conform to Mendel's Laws; however, they were unable to explain this phenomenon.

This was later achieved by T. Morgan and his colleagues. In experiments on Drosophila, it was discovered that all genes in these insects tend to be inherited in groups. T. Morgan concluded that each group of genes is associated with Chromosomes.

If genes are located on different chromosomes, they assort into Gametes independently and randomly, in full accordance with chromosome segregation during Meiosis. However, there are far more genes than chromosomes. Humans have about 30,000 genes and 23 pairs of chromosomes; thus, a single chromosome can carry hundreds of genes. This explains why many traits can be inherited together.

A chromosome represents a distinct physical and functional unit. All genes located on the same chromosome are linked to one another by the substrate (body) of the chromosome. Genes belonging to the same chromosome constitute a single linkage group. The number of linkage groups in a diploid Organism equals the number of chromosome pairs, which has been confirmed in experiments with all organisms studied in this regard. T. Morgan formulated the following principle: genes localized on chromosomes form linkage groups, the number of which is limited by the number of homologous chromosome pairs characteristic of the respective species.

The Inheritance of Linked genes has its own characteristics that distinguish it from independent (Mendelian) assortment. The extreme manifestation of linked inheritance is complete linkage, in which the exchange of genes between chromosomes is entirely ruled out.

Linked Gene inheritance has its own notation system. If two genes are completely linked, the dihybrid AaBb is schematically represented as

The continuous horizontal line represents a pair of homologous chromosomes and indicates that the non-allelic genes A and B belong to one of them.

Under complete linkage, genes are inherited as a single group, i.e., as a single gene. This can be demonstrated by crossing organisms that differ in two pairs of traits (a dihybrid cross), whose non-allelic genes are located on the same chromosome, i.e., linked: in F₁, a hybrid with the genotype is formed. Crossing F₁ hybrids with each other yields a genotypic segregation and a phenotypic ratio of 3:1 (as in a monohybrid cross), rather than 9:3:3:1, which would be expected for a dihybrid cross under independent assortment of genes.

A testcross yields a segregation of, i.e., 1:1, rather than 1:1:1:1. Instead of the four phenotypic classes that would appear in a dihybrid cross under independent assortment, only two appear, as in a monohybrid cross.

This can be observed in one of T. Morgan's experiments. In a cross between Drosophila flies with grey bodies and long wings (dominant traits) and flies with black bodies and vestigial wings, all F₁ flies were grey with long wings (dominant traits).

When these hybrids were crossed with each other, the F₂ generation did not show the independent assortment of traits typical of a 9:3:3:1 ratio in Mendelian inheritance. Among the F₂ hybrids, the majority of flies inherited the parental combination of traits (grey long-winged and black short-winged), and only a small fraction exhibited recombinant traits (grey short-winged and black long-winged). This example demonstrates that the genes determining grey body and long wings, and black body and short wings, are inherited together.

It is important to emphasize that complete gene linkage is very rare in nature. In our Drosophila example, it was also incomplete, as a small number of flies exhibited new (recombinant) traits. The vast majority of genes exhibit incomplete linkage. This is because allelic genes of a homologous chromosome pair exchange places; that is, genes from the paternal chromosome can move to the maternal one, and vice versa.

If non-allelic genes are located on the same chromosome, i.e., linked, the only cause of their recombination can be the synapsis of homologous chromosomes during prophase I of meiosis. During synapsis, homologous chromosomes pair up to form bivalents, and at this time, an exchange of homologous segments occurs between chromatids (pachytene stage). The reciprocal EXCHANGE OF GENETIC material between two homologous chromosomes is called genetic recombination.

The process of exchanging genes or homologous segments of homologous chromosomes is called Crossing Over or chromosomal crossover.

Gamete types whose genotypes have recombined in new ways As a result of crossing over, as well as organisms resulting from the fusion of such gametes, are called crossover types. Crossing over that occurs in prophase I of meiosis is called meiotic, but recombination can also occur in somatic Cells—such crossing over is called somatic. Two, three, or four chromatids can be involved in crossing over. The same chromatid can participate in single and multiple exchanges, and chromatids can exchange equal or unequal fragments. The exchange between non-sister chromatids contributes to the increase in recombinant variation in organisms, whereas crossing over between sister chromatids does not alter the genetic Structure and is not expressed in the phenotype.

As emphasized above, complete linkage is a very rare phenomenon in nature; linkage is mostly incomplete. Under incomplete linkage, crossing diheterozygotes yields four phenotypic classes instead of the two observed under complete linkage. However, in contrast to independent (Mendelian) inheritance, these classes never exhibit a 9:3:3:1 ratio.

A testcross under incomplete linkage leads to the appearance of four classes of genotypes and phenotypes, rather than two as in complete linkage.

The number of classes and their phenotypic characteristics resemble the segregation that occurs in a dihybrid testcross under independent assortment. However, the quantitative ratio of individuals in different classes under incomplete linkage does not conform to a 1:1:1:1 ratio. This is because fewer recombinant gametes, Ab and aB, are formed compared to the non-recombinant gametes, AB and ab, which always make up more than 50% (Table 3.4).

Table 3.4

Segregation of traits in dihybrid offspring under independent (Mendelian) and linked inheritance

Crossing over ensures The Emergence of new gene combinations, generating recombinant variation in all organisms without exception, which is crucial for evolution.

T. H. Morgan and his colleagues, in their experiments with Drosophila, found numerous Examples of linked inheritance and demonstrated that gene linkage is, as a rule, incomplete.

One such example of genetic evidence for crossing over (which we discussed above) is the Inheritance of the black (b) gene, which causes a black body in Drosophila, and vestigial (vg), which causes underdeveloped wings. The dominant alleles of these genes determine: b* - grey body, vg* - normal wing length.

When crossing recessive females for these traits with dominant males bb vg vg х b⁺b⁺vg⁺vg⁺, we obtain a dihybrid (b⁺bvg⁺vg) in F₁. In a testcross of dihybrid females with tester males (recessive dihomozygote) b⁺b vg⁺vg х bbvg vg, we obtain four phenotypic classes of flies in the following ratios:

1) black with vestigial wings (genotype bb vgvg) - 41.5%;

2) grey with normal wings (b⁺b vg⁺vg) - 41.5%;

3) grey with vestigial wings (b⁺b vgvg) - 8.5%;

4) black with normal wings (bb vg⁺vg) - 8.5%.

The testcross yielded four groups of offspring in a ratio of 41.5%: 41.5%: 8.5%: 8.5% or 4.9: 4.9: 1: 1 instead of 1: 1: 1: 1.

Thus, the vast majority of the offspring, 83% (41.5%+41.5%), inherit the parental phenotypes, and only 17% (8.5%+8.5%) exhibit recombinant traits.

T. H. Morgan explained the results of this cross by the fact that the b and vg genes are located on the same chromosome, meaning they are linked. However, this linkage is incomplete because, during meiosis in F₁ dihybrid females, crossing over (an exchange of homologous segments of homologous chromosomes) occurred between the loci (chromosome regions) where the b and vg genes are located, resulting in The formation of two 1.1.11 recombinant gametes with a probability of 8.5% each.

Analyzing the results of this and other experiments, and considering the observations of F. Janssens (1909), who discovered chiasmata during meiosis and suggested that they were associated with the exchange of segments between homologous chromosomes, T. H. Morgan provided a correct explanation for the phenomena of complete and incomplete gene linkage. At the same time, cytological Evidence for the existence of crossing over was presented, directly confirming that genes are indeed localized on chromosomes.

It was later discovered that the frequency of crossing over between any two closely linked genes varies within an average range common to all individuals of a species. However, it is not the same for different non-allelic genes.

Thus, the rate or frequency of crossing over varies depending on the ..... of the genes under study, the research subjects, and other factors.

A. Sturtevant, a colleague of T. H. Morgan, hypothesized that the frequency of crossing over between genes located on the same chromosome is proportional to the distance between them, and that genes are arranged linearly on chromosomes.

The genetic distance at which crossing over occurs with a frequency of 1% was named the centimorgan (CM) in honor of T. H. Morgan, serving as the unit of measurement for crossing over.

The rate of crossing over is calculated as the percentage of recombinant individuals relative to their total number in a given cross.

Example. In a testcross, 1000 maize seeds were obtained, 36 of which were recombinant.

The rate of crossing over is

Thus, the rate of crossing over can be shown to depend on the distance between genes. The greater the distance between genes, the higher the probability that they will be separated by Crossing over and end up in different gametes; conversely, the closer the genes are located, the less likely they are to be separated.

This Conclusion allowed T. H. Morgan to derive the following relationship:

The strength of gene linkage is inversely proportional to the distance between these genes.

Therefore, the rate or frequency of crossing over can be used to determine the distance between genes and their relative positions on chromosomes, i.e., to construct genetic maps. Such a map plots the relative positions of genes belonging to the same linkage group.

A genetic map is a diagram showing the relative positions of genes located within the same linkage group.

It is constructed by determining the frequency of crossing over between two, or preferably three, marker genes in dihybrids. In the vast majority of cases, the rate of crossing over is determined using the testcross method.

Example. In a cross, it was established that genes A, B, and C are inherited in a linked manner. There is 5.1% crossing over between genes A and B, and 2.9% between genes B and C. It is necessary to determine THE POSITION OF gene C relative to gene B, as it could be located on either side of it. To do this, we need to know the frequency of crossing over between genes A and C. A testcross showed that this rate is 2.2%. If the crossing over rate between genes A and C were greater than 5.1%, gene C would be located beyond gene B; however, it is smaller, meaning the genes are arranged in the following order: A, C, B.

By introducing new genes into crosses, one can determine their crossover frequency and construct genetic chromosome maps, which have been mapped in the greatest detail for Drosophila, maize, and tomatoes.

To determine the correspondence between the relative arrangement of genes in linkage groups, as determined by crossover data, and their actual physical localization in chromosomes, cytological maps are constructed and then compared.

Comparative studies of these maps have confirmed THE PRINCIPLE OF linear

gene arrangement and the correspondence of gene locations on genetic and cytological maps.

Thus:

Genes are located in chromosomes, arranged linearly, and form linkage groups, the number of which corresponds to the number of chromosome pairs.

Genes localized on the same chromosome are inherited together (linked). The strength of linkage depends on the distance between them.

Crossing over can occur between homologous chromosomes.

Crossing over is a regular process that normally occurs during meiosis, involving the exchange of homologous segments between homologous chromosomes.

Crossing over underlies gene recombination, which provides recombinational variation in successive generations, actively driving the rate of species evolution.

Crossing over occurs in humans, animals, plants, and microorganisms.

Gene linkage and recombination resulting from crossing over is a fundamental biological phenomenon that reflects The Unity of heredity and variation in organisms.