AIPMT PMT Post-Mendelian Genetics
• Although Gregor Mendel developed his ground : breaking principles of inheritance in the mid-1800s, the importance of his work went largely unnoticed by the scientific community until the early 1900s.
• At that time, numerous researchers began to conduct experiments that upheld many of Mendel’s ideas; however, they also discovered several situations that represented apparent deviations from these principles.
• Linkage was first suggested by Sutton and Boveri (1902-1903) when they propounded the famous “chromosomal theory of inheritance”.
• Bateson and Punnett (1906) while working on sweet pea (Lathyrus odoratus) found that the factors for certain characters do not show independent
assortment. They suggested that the alleles coming from the same parent tend to enter the same gamete and to be inherited together (coupling). Similarly, the same genes coming from two different parents, tend to enter different gametes and to be inherited separately and independently (repulsion).
Morgan (1910) while working on Drosophila stated that coupling and repulsion are two aspects of the same phenomenon, which he described as ‘linkage’. He defined linkage as ‘the tendency of the genes, as present in the same chromosome, to remain in their original combination and to enter together in the same gamete’.
In 1911, Morgan and Castle proposed ‘the chromosome theory of linkage’. It states that:
– Linked genes occur in the same chromosome.
– They lie in a linear sequence in the chromosome.
– There is a tendency to maintain the parental combination of genes except for occasional crossovers.
– Strength of the linkage between two genes is inversely proportional to the distance between the two, i.e., two linked genes show higher frequency of crossing over if the distance between them is higher and low frequency if the distance is small.
• Linked genes are those genes which occur on the same chromosome while unlinked genes are the ones found on different chromosomes.
• Unlinked genes show independent assortment, a dihybrid ratio of 9:3 :3 :1 and the dihybrid or double test cross ratio of 1 : 1 : 1 : 1 with two parental and two recombinant types.
• The linked genes do not show independent assortment, remain together and are inherited en block producing only parental type of progeny.
• They give a dihybrid ratio of 3 : 1 and a test cross ratio of 1 : 1.
Why didn’t Mendel observe linkage?
It is quite often said that Mendel was very fortunate not to run into the complication of linkage during his experiments. He used seven genes and the pea has only seven chromosomes. Some have said that had he taken just one more, he would have had problems. This however, is a gross over simplification. The actual situation, most probably, is shown in the following table given below. This shows that Mendel worked with three genes in chromosome 4, two genes in chromoome 1, and one gene in each of chromosome 5 and 7. It seems at first glance that, out of the 21 dihybrid combinations Mendel theoretically could have studied, no less than four (that is, a-i, v-fa, v-le, fa-le) ought to have resulted in linkages. As found, however, in hundreds of crosses and shown by the genetic map of the pea, a and i in chromosome 1 are so distantly located on the chromosome that no linkage is normally detected. The same is true for v or le on the one hand, and fa on the other, in chromosome 4. This leaves v-le, which ought to have shown linkage.
Mendel, however, seems not to have completed the cross of this particular combination and thus, presumably, never made the appropriate cross to obtain both genes segregating simultaneously. It is therefore not so astonishing that Mendel did not run into the complication of linkage, although he did not avoid it by choosing one gene from each chromosome.
|Character pair used by Mendel||Alleles in modern terminology||Located
|Seed colour, yellow-green||l-i||1|
|Seed coat and flowers, coloured-white||A-a||1|
|Mature pods, smooth expanded-wrinkled idented||V-v||4|
|Inflorescences, from leaf axis-umbellate in top of plant||Fa-fa||4|
|Plant height >0.5-1 m||Le-le||4|
|Unripe pods, green-yellow||Gp-gp||5|
|Mature seeds, smooth-wrinkled||R-r||7|
Arrangement of linked genes
• In an individual, which is heterozygous for two pairs of linked genes, the linkage can be either of the following two types:
(i) The dominant genes of both pairs are located in one member of the chromosome pair and their recessive alleles are located in the other chromosome of the pair. This arrangement is known as cis-arrangement and the heterozygotes with such arrangement (AB/ab) are known as cis-trans and cis arrangement heterozygtes.
(ii) The dominant gene of one pair and the recessive gene of other pair are located on one chromosome of the pair and the recessive gene of the first pair and dominant gene of the second pair are located in the second chromosome pair (Ab/aB). This arrangement of a dominant and recessive gene in the same chromosome of the chromosome pair is known as trans-arrangement.