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9.5. Ligation and recombination

Ligation and recombination

Morgan received the Nobel Prize in 1934 for proving chromosome theory with his laboratory work with the fruit fly Drosophila melanogaster studying the transmission of the "eye color" character.

The Drosophila melanogaster normally has a bright red eye color, but a white-eyed male was discovered. When the male (white eyes) crossed with the normal females (red eyes), all the offspring had red eyes. This was to be expected if it is a dominant inheritance.

When he crossed individuals of F1 (red eyes) he obtained a progeny of 3 (red eyes) by 1 (white eyes), but in which all the white-eyed flies were male.

If it was a reciprocal cross, white - eyed females with male red - eyed, got offspring with all females of the F1 with red eyes, and all males with white eyes.

Morgan related this type of inheritance, in which males inherit the character from the mother and females from the father, with sex. He hypothesized that alleles for eye color (red or white) would be located on the X chromosome, with no information for that trait on the Y chromosome. Females would have two alleles for that gene, but males only one. This would explain the existence of sex-linked characters and that genes are located on chromosomes.

Ligation

Any species has few chromosomes compared to the number of characters, so each chromosome has to contain many genes.

Genes that are on different chromosomes are distributed among the gametes independently of each other, but genes located very closely on the same chromosome tend to be inherited together, which is why they are called linked genes. They can be linked on both autosomes and sex chromosomes.

Logically, if they are linked genes, the phenotypic proportions obtained in their crosses will be different.

For example, when a test cross (AaBb * aabb) is performed in a diheterozygous individual (AaBb) , the result, if they are not linked genes, is an offspring with a 1:1:1:1 ratio (AB:Ab:aB:ab). But if it is a question of linked genes, the phenotypic ratio obtained would be 1:1, since the genes tend to be transmitted together.

If the result obtained is very different from the 1:1:1:1 ratio, it can make us think that there is linkage between the genes. But that genes are linked does not mean that they are always transmitted together, since homologous chromatids, in meiotic prophase I, exchange chromosomal fragments in recombination. Then, the same phenotypes are obtained as the parental ones (those with the highest frequency) and others, less frequent, which are the recombinant ones.

Recombination

As we saw when talking about meiosis, each chromosome  is duplicated and two identical sister chromatids are obtained . Homologous chromosomes pair (synapse) and cross-over occurs between non-sister chromatids (from two different chromosomes). Two of these four chromatids break and join at any point, exchanging genetic material. Thus, there are two chromatids that remain intact, have not crossed over, which are called non-recombinant or parental type, and two other chromatids that are recombinant .

Limits of recombination

When two loci are so far apart within the same chromosome that there is a 100% chance that a chiasm will form between them, they will produce  phenotypes similar to what they would if they were on different chromosomes. That is, 50% of the gametes will be of the parental type (non-recombinant) and 50% of the recombinant type.

The frequency of recombination between two linked genes is constant and characteristic of that pair of genes. It cannot exceed 50%, even when there are multiple crossovers between them. The frequency of recombination increases the further apart these genes are, since the length of the chromosome fragment in which the crossover can occur is greater.

Making a chromosome map

If you know the crossover frequency between two genes, you can know the distance between them, and therefore the position (locus) that each gene occupies within the chromosome.

The recombination unit (or map unit) is called centimorgan, and it is equivalent to the distance between two points on the chromosome, whose recombination frequency will be 1%.

With the data of distances between the genes, chromosome maps can be made, which contain the genetic information of the species.