Table: Summary of Epistatic Ratios
Genotypes A–B A–bb aaB–
Classical ratio 9 3 3 1
Dominant epistasis 12 3 1
recessive epistasis 9 3 4
Duplicate genes with cumulative effect 9 6 1
Duplicate dominant genes 15 1
Duplicate recessive genes 9 7
Dominant and recessive interaction 13 3Pleiotropy
according to Mendel, a specific gene produces only one specific phenotypic character. but cases have been studied in which one
gene may produce several side effects, i.e., a gene produces a major phenotypic trait but in addition to that influences some other
phenotypic traits also. This phenomenon of a single major gene influencing more than one character (multiple expression) is known
as pleiotropism and such genes are known as pleiotropic genes. Pleiotropy may be due to true pleiotropic genes or the closely
linked genes or group of genes.
It is not essential that all the traits are equally influenced. Sometimes the effect of a pleiotropic gene is more evident in case of one
trait (major effect) and less evident in case of other (secondary effect).
Example: In man, gene producing the disease phenylketonuria also produces a number of abnormal phenotypic traits, which are
collectively known as syndrome. This gene results in short stature, mental retardation, widely spaced incisors, pigmented patches
on the skin and excessive sweating.
In Drosophila, the recessive gene for vestigial wings also affects structure of reproductive organs, reduction in the egg production, shape of
spermathecae, eye colour, reduction in longeivity and the bristles on the wings.
Linkage
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.
Difference between linked and unlinked genes
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.