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The below mentioned article article provides notes on mendelian inheritance.
In Mendel’s dihybrid experiments the F1 double heterozygote always showed independent assortment of the two pairs of genes at the time of gamete formation. In fact the law was justified only because the two genes were not linked to each other. This was true because each gene was located on a different chromosome.
Mendel studied seven characters and the pea plant has only seven pairs of chromosomes. It followed therefore that in Mendelian inheritance one character must be located on one chromosome.
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Sutton in 1903, working on the Chromosome Theory of Heredity pointed out that the number of characters which obeyed Mendel’s laws when studied singly in monohybrid crosses, was much more than the number of chromosome pairs to which they could be assigned. This means that there must be many genes located on the same chromosome.
If we focus attention on any two genes located on the same chromosome and perform a dihybrid cross, we cannot expect the two genes to assort independently as in Mendelism. On the contrary the two genes must be linked and show a tendency to be inherited together. If this is true, then the two phenotypes controlled by these genes must also appear together in an individual more often than if there was independent assortment.
Stating it in another way, genes on the same chromosome are linked and tend to be transmitted together in a single unit. Obviously then the 9:3:3:1 ratio typical for a dihybrid Mendelian cross would be expected to become modified for two genes that are not assorting independently due to linkage.
Indeed Bateson, Saunders and Punnett (1905, 1906) found results of a dihybrid cross in sweat peas different from those expected in independent assortment. The experiment involved two characters, flower colour (purple vs. red) and pollen shape (long vs. round). When two varieties of sweet peas, one purple long the other red round were crossed, the F1 progeny did not appear in the expected 9:3:3:1 ratio.
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Instead, plants with purple long and those with red round combinations were more frequent than expected. It will be noticed that these are the same combinations that were present in the parents.
The new combinations purple round and red long which were not present in the parents were observed in F2 with lesser frequency (Fig. 8.1). The fact emerges that when genes are linked, parental combinations occur more frequently than re-combinations.
The same cross was repeated by Bateson and Punnett in a different way. This time the parents had a different combination of characters namely Purple Round and Red Long. The F1 plants were all Purple Long. The F2 again showed the parental combinations (Purple Round and Red Long) in a higher frequency and the re-combinations (Purple Long and Red Round) in a lower frequency than expected.
Bateson and Punnett applied the testcross method to the above mentioned crosses in sweet peas. They crossed the F1 double heterozygote (Purple Long PpLl) with the double recessive parent (red round ppll).
In Mendelian inheritance such a cross indicates that F1 plants are producing four types of gametes in equal frequency, which combine with the single type of gamete from the recessive parent to produce F2 progeny of four types in the ratio of 1:1:1:1. Instead, the actual ratio observed by Bateson and Punnett was 7:1:1:7.
The above experiment clearly indicates that the two genes for flower colour and pollen shape are located on the same chromosome. The genes are said to be linked. During meiosis linked genes tend to pass en bloc to the same gamete and are responsible for the appearance of parental combinations in the resulting progeny.
The small number of re-combinations in the progeny are due to the fact that some amount of recombination does take place between the linked genes at the pachytene stage of meiosis. Incidentally, none of the dihybrid crosses performed by Mendel had linked genes.
Genes for all seven characters studied by him showed random assortment. It was later demonstrated cytologically that sweet pea has seven pairs of chromosomes. It was also shown genetically that the gene for each of the seven characters studied by Mendel was located on a separate chromosome. Had Mendel studied some more characters that were linked, he would not have been able to interpret his results and his Principle of Independent Assortment may not have been formulated.
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Bateson coined the term coupling for referring to the situation where two dominant alleles of a gene are both present in one parent and the two recessive alleles in the other. Thus both dominant genes pass together into one gamete in one parent and both recessives together in the gamete of the other parent as in the cross purple long (PPLL) and red round (ppll).
The term repulsion was applied when the parents were heterozygous such as in the cross PpLl and PpLl. In this case the two dominant genes come from the different parents so that they are said to be in repulsion.
In other words in the case of two linked loci in a double heterozygote if the two dominant alleles are on one chromosome and the two recessives on the other (PL/pl) the linkage relationship is said to be in the coupling phase.
But when the dominant allele of one locus and the recessive allele of the other occupy the same chromosome (pl/pL) linkage is said to be in repulsion phase. The terms coupling and repulsion are now of historical interest only.
