In this section, we will try to infer and apply Laws of genetics through problems. Here, we will be discussing inter allelic or intragenic gene interactions.
Complementary Genes
These are two pairs of non-allelic dominant genes, which
interact to produce only one phenotypic trait, but neither of
them (if present alone) produces the trait in the absence of
other. Therefore, for the development of dominant character,
the presence of both the genes is necessary.
Supplementary Genes
These are two independent dominant gene pairs, which
interact in such a way that one dominant gene produces its
effect irrespective of the presence or absence of other. The
second gene when added, changes the expression of first, but
only in the presence of latter.
Inhibiting Genes or Epistasis
It is the interaction between non-allelic genes, in which one
gene masks, inhibits or suppresses the expression of other.
It is of two types:
Dominant Epistasis
Out of the two pairs of genes, the dominant one masks the
expression of other gene pair.
Recessive Epistasis
Out of the two pairs of genes, the recessive gene masks the
activity of dominant gene of the other gene locus. The dihybrid
ratio 9 : 3 : 4 is also considered as recessive epistasis ratio.
Prob lems
If two white flowered strains (CCpp, ccPP) of Lathyrus are
crossed, in F 1 -generation purple flowered plants (CcPp) are
obtained. When these purple flowered plants are allowed to
self-cross, both white and purple flowered plants are
obtained. Give reason, why purple coloured flowers were
obtained in F 2 -generation? Also, predict the phenotypic
ratios of F 2 progeny with the help of Punnett square.Sol.If we consider a cross between plant species with
The genotype of F 2 -generation can be explained as follows:
Parents – CcPp CcPp×
Gametes CP Cp cP cpF 2 -generation
Purple flower : 9White flower : 7CP CCPP
(Purple)CCPp
(Purple)CcPP
(Purple)CcPp
(Purple)Cp CCPp
(Purple)CCpp
(White)CcPp
(Purple)Ccpp
(White)cP CcPP
(Purple)CcPp
(Purple)ccPP
(White)ccPp
(White)cp CcPp
(Purple)Ccpp
(White)ccPp
(White)ccpp
(White)Connectivity with the Laws
Gene C produces an enzyme that catalyses the formation of
colourless chromogen for the formation of anthocyanin pigment.
The gene P on the other hand controls the formation of another
enzyme that catalyses transformation of chromogen into
anthocyanin. Thus, both the genes are complementary. If only one
of them is present the result is colourless or white flowers.
Therefore, due to the presence of complementary genes, in
F- 1 generation plants with purple coloured flowers were obtained.When a deaf-mute man (aaBB) mar ries with a deaf-mute woman
(Aabb), in F 1 -gen er a tion, all nor mal off spring are ob tained. In
F 2 -gen er a tion nor mal and deaf-mute off spring are pro duced in 9 : 7
ra tio. How would you jus tify the pres ence of all nor mal off spring in
F 1 -gen er a tion and re ap pear ance of deaf-mute offspring with
nor mal off spring in F 2 -gen er a tion?Sol.If we consider a cross between a deaf-mute man (aaBB) and a
deaf-mute woman (Aabb).Con nec tivity with the Laws
Deaf-mutism in man is controlled by complementary genes. In man
the power of hearing and speech is an account of interaction of
dominant genes A and B. Whenever, in man anyone of the gene
amongst the two is recessive allelomorph homozygous then this
disease or abnormality develops in man.When a lablab plant with a khaki coloured (KKll) seed coat is
crossed with a lablab plant of buff coloured (kkLL) seed coat, in
F 1 -generation, chocolate coloured plants are obtained. On
self-crossing, in F 2 -generation, all the three colour seed coat, i.e.
khaki, buff and chocolate are obtained. Why plants of
F 1 -generation do not produce seed coat that resemble either of the
parent? Predict the phenotypic ratio of F 2 -generation.Sol.From the facts given in the problem, we can conclude following
results.The F 1 progeny plants when crossed among themselves, produce
offspring (F 2 -generation) represented by the following cross.GENETIC CLASSROOM
INTERACTION OF GENES-I
Purple flowers
(CcPp)White flower
(CCpp)White flower
(ccPP)×F -generation 1aabb
(Deaf-mute)aaBb
(Deaf-mute)Aabb
(Deaf-mute)AaBb
ab (Normal)aaBb
(Deaf-mute)aaBB
(Deaf-mute)AaBb
(Normal)AaBB
aB (Normal)Aabb
(Deaf-mute)AaBb
(Normal)AAbb
(Deaf-mute)AABb
Ab (Normal)AaBb
(Normal)AaBB
(Normal)AABb
(Normal)AABB
AB (Normal)% AB Ab aB ab
&Normal offspring
(AaBb)Deaf-mute man
(aaBB)Deaf-mute woman
(Aabb)×GametesGametesF -generation 2
Normal : 9
Deaf-mute : 7F -generation 1Chocolate
(KkLl)Khaki
(KKll)Buff
(kkLL)×F -generation 1