Principles of Genetics and Breeding

7.1.3.1 Incomplete linkage

1. When the linked genes tend to separate on some occasions during the course of inheritance, the phenomenon is known as incomplete linkage.
2. This occurs in the process of crossing over during gametogenesis.
3. The genes that are present on the homologous part of sex chromosomes exhibit the phenomenon of incomplete linkage.
4. The linkage may be autosomal linkage, if the linked genes are present on the autosomes or it may be sex linkage, if the linked genes are present on the sex chromosomes.
5. Cross-overs. In the fruit fly, Drosophila, with which the classical linkage work was done, there was hundreds of known multiallelic loci and only four chromosomes.

In fish, almost the opposite applies: relatively few loci are known and chromosomes are numerous but small.

• In the guppy, however, many dominant colour pattern genes are known and these formed the basic of the first chromosome maps in fish.
• Data on the inheritance of 18 of these patterns was presented by Winge and used to construct a map of the sex chromosomes on which they were located.
• Wing’s principal approach was to estimate the frequency with which genes crossed from the X to the Y chromosome and vice versa.
• Of the 18 genes, nine were never observed on the X chromosome and Winge suggested that they were alleles of the remaining nine, five were sufficiently well represented in the crosses to assess the frequency with which they moved from Y to X or vice versa.
• A female with no colour genes is crossed with a male in which the Y has these three genes, Maculatus (Ma), Tigrinus (Ti) and Luteus (Lu).
• Among the offspring it is possible to distinguish cross- overs as those fish which lack any of the patterns. Winge build up a map of the chromosome.

The possibilities of inter–locus cross – overs for four loci in Guppy Co and Vi on the X and Ma and El as the Y, and the offspring phenotypes can be scored for cross- over as follows:

• Ma El – no cross over
• Ma Co Vi – Cross – over between Ma and Co – position 1
• Ma Vi – Cross- over between Co and Vi – position 1- 4
• Ma – Cross – over between Vi and El – position 5.

When linked characters (or genes) are inherited together through two or more generations,linkage is said to be complete. This phenomenon is rare and is found in the male Drosophila. This happen when chromosomes do not undergo any breakage by accident during gametogenesis.

Linkage groups in fish were first detected by Winge (1923). For example, the Xvi and Xel genesin the guppy are linked on the X chromosome. Although both genes may cross over to the Y chromosome, it is assumed that they are located only on the X chromosome.

Usually, however, chromosomes undergo breakage during gametogenesis. This naturally interfere with the linkage of genes, which separate as a result of the incomplete. The breaking away of chromosomes during gametogenesis is accompanied by rejoining in such a way that corresponding fragments, are exchanged by homologous chromosomes. This phenomenon is known as crossing-over.