American geneticist Thomas Morgan studied the patterns of inheritance of genes located on one homologous chromosome.
In the experiments, Morgan used the fruit fly Drosophila, which has qualities important for geneticists: unpretentiousness, fertility, a small number of chromosomes (four pairs), and many clearly expressed alternative traits.
In the fruit fly, genes for body color and wing length are located on the same chromosome.
Morgan performed two types of analysis crosses. When a female recessive in both traits (black with short wings) was crossed with a diheterozygous male (gray with normal wings), 50% of black flies with short wings and 50% of flies with a gray body and normal wings appeared in the offspring.
This means that genes located on the same chromosome were inherited together. Such genes form a linkage group.
The phenomenon of joint inheritance of genes located on the same chromosome, Morgan called linked inheritance.
Other results were obtained when a dihybrid female (gray with normal wings) was crossed with a homozygous recessive male (black with short wings). In this case, the offspring had four phenotypes: 41.5% – gray with short wings, 41.5% – black with normal wings, 8.5% – gray with normal wings, 8.5% – black with short wings.
Morgan concluded that the clutch might be incomplete. It is disrupted as a result of crossing over – the exchange of sites between homologous chromosomes.
In prophase I of meiosis, conjugation of homologous chromosomes occurs. Chromosomes approach each other, and then begin to diverge and form intersections (chiasma). In the process of conjugation between some daughter chromatids, an exchange of sites (crossing over) is possible.
Each of the formed chromatids falls into a separate gamete. As a result of crossing over, crossover gametes arise, the chromosomes of which contain new combinations of genes.
Organisms that result from the fusion of crossover gametes are called recombinant.
Since crossing over does not occur after each conjugation, the number of crossover gametes is always less than the number of non-crossover gametes.
Morgan proved that the frequency of crossing over between homologous chromosomes depends on the distance between genes in the chromosome. The greater this distance, the more often crossing over occurs and the more often recombinant organisms appear.
Recombination frequency (crossing over) = number of recombinants: total number of offspring × 100%.
This value shows the relative distance between linked genes in a chromosome. 1 morganida (1% crossing over), or the percentage of the appearance of recombinant individuals, is taken as a unit of distance between genes.