Detailed Information about interaction of genes

1. INTERACTION OF GENES
S. Madhumitha
BSc.Zoology(II)
PSG College of Arts an Science

2. What is Gene Interaction?
When expression of one gene depends on the presence or
absence of another gene in an individual, it is known as
. The interaction of genes at different loci that affect the
same character is called epistasis.
The term epistasis was first used by Bateson in 1909 to
describe two different genes which affect the same character, one
of which masks the expression of other gene. The gene that masks
another gene is called epistatic gene, and the gene whose
expression is masked is termed as hypostatic gene. Epistasis is also
referred to as inter-genic or inter-allelic gene interaction.

3. Characteristic of Gene Interaction
1.Number of genes:
The epistasis gene interaction always involves two or more genes.
This is an essential feature of .
2.Affect same character:
The epistatic genes always affect the expression of one and the
same character of an individual.
3.Expression:
The phenotypic expression of one gene usually depends on the
presence or absence of epistatic gene. The gene which has masking
effect is called epistatic gene and the gene whose effect is masked is
known as hypostatic gene.

4. 4.Modification of Dihybrid Segregation Ratio:
Epistasis leads to the modification of normal dihybrid or tri-hybrid
segregation ratio in F2 generation.
5.Genetic control:
Epistasis is usually governed by dominant gene, but now
cases of recessive epistasis are also known.

5. In sometimes two dominant genes controlling the
same character produce a new phenotype in F1 when they come together
from two different parents. Such case of was observed by
Bateson and Punnett for comb shape in poultry.
There are three types of comb shape in poultry, viz., rose, pea and
single. The comb shape is controlled by two pairs of alleles. The rose comb is
governed by a dominant gene R and pea comb by a dominant gene P. The
single comb is governed by two recessive genes (rrpp).
When a cross was made between rose (RRpp) and pea (rrPP), a new
phenotype called walnut developed in F1. The walnut comb developed as a
consequence of combining two dominant alleles R and P together in F1.
Inter-mating of F1 birds produced four types of combs, viz., walnut, rose pea
and single in 9 : 3 : 3 : 1 ratio in F2 generation.

6. Fig 1.1: Gene Interaction for Comb Shape in Poultry

7. When a cross was made between rose (RRpp) and pea
(rrPP), a new phenotype called walnut developed in F1. The walnut
comb developed as a consequence of combining two dominant
alleles R and P together in F1. Inter-mating of F1 birds produced
four types of combs, viz., walnut, rose pea and single in 9 : 3 : 3 : 1
ratio in F2 generation.
Here individuals with R-P-(9/16) genotypes produce
walnut comb, because two dominant genes together produce
walnut comb. Individuals with R-pp (3/16) will give rise to rose
comb, and those with rrP-(3/16) genotypes will produce pea comb.
The single comb (1/16) will develop from a double recessive,
genotype.

8. Gene interactions: Allelic and Non-Allelic
Mendelian genetics does not explain all kinds of inheritance for which
the phenotypic ratios in some cases are different from Mendelian ratios (3:1 for
monohybrid, 9:3:3:1 for di-hybrid in F2). This is because sometimes a particular
allele may be partially or equally dominant to the other or due to existence of
more than two alleles or due to lethal alleles. These kinds of
between the alleles of a single gene are referred to as allelic or intra- allelic
interactions.
Non-allelic or inter-allelic interactions also occur where the
development of single character is due to two or more genes affecting the
expression of each other in various ways.
Thus, the expression of gene is not independent of each other and
dependent on the presence or absence of other gene or genes; These kinds of
deviations from Mendelian one gene-one trait concept is known as Factor
Hypothesis or Interaction of Genes.

9. Fig 2: Different types of Allelic and Non-
Allelic interaction

10. Allelic or Intra-Allelic Gene Interaction
The alleles of one gene can interact in several different ways
at the functional level, resulting in variations in the type of dominance
and in markedly different phenotypic effects in different allelic
combinations.
The In which two alleles preset on the
same gene locus on the two homologous the chromosome of a gene
interact together for phenotypic expression is called Allelic or Intra
Allelic gene interaction. This allelic gene interaction modifies the
Mendelian monohybrid. Phenotypic F2 ratio i.e. 3:1 to 1:2:1.
The Example of this Interaction is:
*Incomplete Dominance or Blending Inheritance(1:2:1)
*Co-dominance
*Lethal Factor(2:1)
*Multiple Alleles

11. Incomplete Dominance or Blending Inheritance (1:2:1)
A dominant allele may not completely suppress other allele, hence a
heterozygote is phenotypically distinguishable (intermediate phenotype) from
either homozygotes.
In Snapdragon and Mirabilis jalapa, the cross between pure breed red-
flowered and white-flowered plants yields pink-flowered F1 hybrid plants
(deviation from parental phenotypes), i.e., intermediate of the two parents. When
F1 plants are self-fertilized, the F2 progeny shows three classes of plants in the ratio
1 red: 2 pink: 1 white instead of 3:1

12. Therefore, a F1 di-hybrid showing incomplete dominance for both
the characters will segregate in F2 into (1 :2 : 1) X (1 :2 : 1 ) = 1 :2 : 1 : 2 : 4 :
2:1 : 2 : 1. And a F1 di-hybrid showing complete dominance for one trait
and incomplete dominance of another trait will segregate in F2 into (3:1)
x (1 :2:1) = 3:6:3:1 :2:1.

13. Pictures showing the Incomplete Dominance in
snapdragon.

14. Co-dominance
Here both the alleles of a gene express themselves in the
heterozygotes. Phenotypes of both the parents appear in F1 hybrid rather
than the intermediate phenotype. In human, MN blood group is controlled
by a single gene.
Only two alleles exist, M and N. Father with N blood group
(genotype NN) and mother with M blood group (genotype MM) will have
children with MN blood group (genotype MN). Both phenotypes are
identifiable in the hybrid. F2 segregates in the ratio 1M blood group: 2 MN
blood group : 1 N blood group.
Over-dominance
Sometimes the phenotype of F1 heterozygote is more extreme
than that of either parents. The amount of fluorescent eye pigment in
heterozygous white eyes of Drosophila exceeds that found in either
parents.

15. Lethal factor(2:1)
The genes which cause the death of the individual carrying it, is
called lethal factor. are expressed only when they are in
homozygous state and the heterozygotes remain unaffected. There are genes
that have a dominant phenotypic effect but are e.g., gene for
yellow coat color in mice.
But many genes are recessive both in their phenotypic as well as
lethal effects, e.g., gene producing albino seedlings in barley.
are lost from the population because they cause death of the
organism even in a heterozygous state, e.g., epiloia gene in human beings.
Conditional lethal require a specific condition for their lethal action, e.g.,
temperature sensitive mutant of barley (lethal effect at low temperature).

16. Fig 3: Inheritance of Lethal Gene in Mice and Barley. Fig 4: Lethal Gene in Mice.
Balanced lethals are all heterozygous for the lethal genes; both
dominant and recessive homozygotes will die, e.g., balanced lethals
system in Oenothera. Gametic lethal make the gametes incapable of
fertilization, e.g., segregation distorter gene in male Drosophila
Semi-lethal genes do not cause the death of all the individuals,
e.g., xentha mutants in some plants.

17. Multiple alleles
A gene for particular character may have more than two
or alleles occupying same locus of the chromosome (only two of them present
in a diploid organism). These make a series of multiple alleles.
Human ABO blood group system furnishes best example. The gene for
antigen may occur in three possible allelic forms – . The allele for the A
antigen is co-dominant with the allele I8 for the B antigen. Both are completely
dominant to the allele i which fails to specify any detectable antigenic
structure.

18. Fig 5: ABO blood groups and their genotypes In human
Therefore, the possible genotypes of the four blood groups are
shown in Fig.5

19. Fig 6: Compatibility in self-sterile plants in tobacco
Self-sterility in tobacco is determined by the gene with many
different allelic forms. If there are only three alleles (s1, s2, s3), the possible
genotypes of plants are s1s2, s1s3, s2s3 (always heterozygous), homozygous
genotypes (s1s1, s2s2, s3s3) are not possible in a self-incompatible species.
In this case, pollen carrying an allele different from the two alleles
present in the female plant will be able to function resulting in restriction of
fertility (Fig 7.4).Iso-alleles express themselves within the same phenotypic
range, e.g., in Drosophila several alleles (W+s ,W+c,W+g) exhibit red eye color.

20. Non-Allelic or Inter-Allelic Gene Interaction
Simple Interaction (9:3:3:1)
In this case, two Non-allelic gene pairs affect the same character.
The dominant allele of each of the two factors produces separate
phenotypes when they are alone. When both the dominant alleles are
present together, they produce a distinct new phenotype. The absence of
both the dominant alleles gives rise to yet another phenotype.
The inheritance of comb types in fowls is the best example where
R gene gives rise to rose comb and P gene gives rise to pea comb; both
are dominant over single comb; the presence of both the dominant
genes results in walnut comb . Similar pattern of inheritance is found in
Streptocarpus flower color.

21. Fig 7: Inheritance of comb type in fowl Fig 8: Inheritance of flower colour in streptocarpus

22. Complementary factor(9:1)
Certain characters are produced by the interaction between two
or more genes occupying different loci inherited from different parents.
These genes are complementary to one another, i.e., if present alone they
remain unexpressed, only when they are brought together through suitable
crossing will express.
In sweet pea (Lathyrus odoratus), both the genes C and P are
required to synthesize anthocyanin pigment causing purple color. But
absence of any one cannot produce anthocyanin causing white flower. So C
and P are complementary to each other for anthocyanin formation.
Involvement of more than two complementary genes is
possible, e.g., three complementary genes governing aleurone color in
maize.

23. W. Bateson and R.C. Punnett crossed two white flowered varieties
of sweet pea (lathyrus odoratus). In the experiment all the plants of F1,
generation developed red flowers instead of expected white flowers. These
F1, plants, on self-pollination, produced red and white flowered plants in the
ratio 9: 7 in F2 generation.
Fig 8: Punnett square by W.Bateson and R.C.

24. Fig 10: Inheritance of flower color In Lathynus odoratus Fig 11: Different flowers of Lathynus odoratus

25. Red flowered plants with both complementary genes C and R = 9, White
flowered plants having either ‘C’ or ‘R’ = 7.
From the checker board it is clear that the red coloration of the flower is
due to interaction of two complementary factors C and R. The parents developed
white flowers because they lacked either gene ‘C’ or gene ‘R’ in their genotypic
expressions CC cc and RR rr. F1, hybrids received both the complementary genes C
and R, hence all the flowers in that generation were red.
The ratio 9: 7 obtained in F2 generation is actually a modification of
normal dihybrid ratio (9:3:3: 1).
Similar cases are reported in many other plants. In Sorghum (jowar), for
example, there are two white grained races which, when crossed, produced F1,
hybrids with brown grains. When brown grained F1 plants were self-pollinated,
they produced brown grained and white grained plants in the ratio 9:7.
In paddy, the plants of two varieties with colourless stigmas, when
crossed, produced F1, hybrids with purple stigma. By selfing the F1, hybrids, plants
with purple and white stigmas were obtained in the 9: 7 ratio in F2.

26. Epistasis(12:3:1)
According to Mendel’s laws of inheritance, when two contrasting
characters (allelomorphs) are brought together, one of them dominates
over the other in F1. In the phenomenon of epistasis, two independent non-
allelic genes affecting the same trait of an individual interact in such a way
that one over-masks the expression of the other.
Here the gene which prevents the expression of another non-
allelic gene is said to be epistatic and the suppressed gene is said to be
hypostatic.
In cholam (sorghum caudatum), the pearly grain colour is
dominant over chalky. Pearly colour develops when dominant gene ‘Z’ is
present, and chalky colour develops in absence of it. Another dominant
gene ‘W which is responsible for the red colour of seeds, is dominant over
the gene Z.
When ‘ W’ is present, the grain will always be red irrespective of
the presence of other gene Z. In the absence of W, the grain colour will be
pearly or chalky depending upon the presence or absence of gene Z.

27. :
In the squashes (cucurbita pepo), Sinnot has shown that there are three
common fruit colors, white, yellow and green. Sinnot has shown that white
was dominant over both yellow and green and yellow was dominant over
green only.
In a cross between white variety (genotype WW YY) and green variety
(genotype wwyy), the F, progeny appeared white fruited (genotype Ww Yy)
which on selfing gave a 12 white: 3 Yellow: 1 green ratio. Here ‘ W’ acts as an
epistatic factor and ‘ Y’ is active only with ‘w’;

28. Fig 12: Inheritance of curcurbita pepo
From the above experiment, it is clear that each fruit color is
being governed by two non-allelic genes W and Y. If gene W is in
dominant condition, dominant Y gene will not show its effect. Gene Y is
active only when it is combined with recessive ‘w’. V.

29. Supplementary factor(9:3:4)
Sometimes interaction involves two different types of genes in which one
type can express itself independently but other has no expression of its own. When
two such genes interact, they produce a new character In Sorghum (Jowar) the
black purple color (P) is dominant over brown (p). Blackish pigmentation is found
to change to red in certain crosses
The modifying factor Q borne on another chromosome has no expression
of its own in presence of other recessive factor ‘p’. Similarly, the recessive factor
‘q’ is also without any phenotypic effect but when both dominants P and Q are
brought together, the blackish purple colour is changed to red.

30. Fig 13: Example for supplementary factor

31. An example of similar type is met within rats and rodents. In rats, there
are three common skin or coat colors. These are Black, Albino, and Wild type
Agouti (yellow hair with black bases). The black coat color is governed by
dominant gene ‘C’ and albino color by dominant gene ‘A’.
The agouti color is governed by interaction of two dominant genes A and C.
‘A’ does not show any visible effect if it is alone, but the gene C is able to express
itself independently.
When a true breeding black rat is crossed with an albino carrying the
dominant gene A, all the F1, hybrids are of the agouti type. When F1 individuals are
inbred, the F1, generation is represented by agouti, black and albino in 9: 3: 4
ratios.
In this case, homozygous recessive condition of a gene determines the
phenotype irrespective of the alleles of other gene pairs, i.e., recessive allele hides
the effect of the other gene. The coat color of mice is controlled by two pairs of
genes.
Dominant gene C produces black color, absence of it causes albino. Gene A pro-
duces agouti colour in presence of C, but cannot express in absence of it (with cc)
resulting in albino. Thus recessive allele c (cc) is epistatic to dominant allele A.

32. Fig 14: Inheritance of coat color In Mice and different coat colored Mice.

33. Inhibitory factor(13:3)
In this modification or gene interaction, one dominant factor
inhibits or suppresses the phenotypic expression of another dominant
gene found on different chromosome. This may be explained by taking
example of pigmentation of leaf in rice plants. Here purple colour of the
leaf is due to factor Lp and green due to lp. The purple colour is dominant
over the green.
There is another factor T. If dominant factor T is present with the
factor Lp, the purple colouration (due to factor Lp) is inhibited and the
leaf becomes normal green. The factor T has thus no visible effect of its
own but it inhibits the colour expression of gene Lp.

34. Inhibitory factor is such a gene which itself has no phenotypic effect but
inhibits the expression of another non-allelic gene; in rice, purple leaf color is
due to gene P, and p causing green color. Another non-allelic dominant gene I
inhibits the expression of P but is ineffective in recessive form (ii). Thus the
factor I has no visible effect of its own but inhibits the color expression of P.
In a cross between purple pigmented (ii Lp Lp) and a green (I I lp lp) rice
plant, the F1, plants were all green (I i Lplp) and in F2 there appeared green and
purple pigmented plants in 13: 3 ratio.

35. Fig 15: example for inhibitory factor.
White Wyandotte and white leghorn are two breeds of fowls with white
plumes. The white plumage of the white wyandottes is recessive to the coloured
plumage. The white leghorns possess a colour factor which is prevented from
being expressed by an inhibiting gene.
If the inhibiting factor is symbolised by ‘I’ and the colour factor by ‘C, the
genotype of white leghorn can be expressed by CCII and that of white Wyandotte
by ccii.
When white Leghorns (CCII) and white Wyandotte (ccii) are crossed, the F1,
hybrids are all white (Cc Ii). When the F, hybrids are inbred, the offsprings in
F2 appear in the ratio 13 White:3 coloured.

36. Inhibitory factor with partial dominance(7:6:3)
Fig 16 :Inheritance of hair direction in guineapig.
Sometimes an inhibitory gene shows incomplete dominance thus
allowing the expression of other gene partially. In guineapig, hair direction
is controlled by two genes. Rough (R) hair is dominant over smooth (r) hair,
other gene I is inhibitory to R at horinozygous state (II) but in heterozygous
state (II) causes partially rough.

37. Multiple factor and Polygenic Inheritance.
Though some characters (qualitative) show discontinuous variation but a
majority of characters (quantitative, e.g., height, weight, etc.) exhibit
continuous variation. Yule, Nilsson-Ehle, East suggested that quantitative
variation is controlled by large number of individual genes called polygenic
systems and the inheritance could be explained on the basis of multiple factor
hypothesis.
The hypothesis states that for a given quantitative trait there could be
several genes, which are independent in their segregation and had cumulative
effect on the phenotype.
Kernel color in wheat is a quantitative character and controlled by two
different genes. The heterozygote is intermediate in color between the two
homozygotes. Both the dominant genes have small and equal (or almost equal)
effects on seed color. F1 heterozygote for two genes will segregate in F2 in the
ratio 1:4: 6:4:1.

38. The intensity of seed color depends on the number of dominant alleles
present, i.e., their effects are cumulative in nature. It is now known that there
are three genes involved in kernel color in wheat, thus a F, heterozygous for all
three genes will segregate in F1 in the ratio 1 : 6 : 15 : 20 : 15 : 6 : 1.
Fig 17: Inheritance of kernel color In wheat.

39. Skin color in human beings is under polygenic effect, the number of gene
pairs involved may be two or more than two, possibly four or five.
The number of genes involved in polygenic inheritance can be calculated from
the frequency of parental type using the formula 1/4n (n = number of gene
pairs).
If parental type obtained is one out of every 64 offspring’s (1/64), then the
number of genes involved will be three (4n = 64 = 43).
Fig 17: Inheritance of Skin color in human.

40. Other kinds of gene interactions
Genes which modify the phenotypic effect of a major gene called modifying gene.
They reduce or enhance the effect of other gene in quantitative manner, e.g., genes respon-
sible for dilution of body color.
Genes which will not allow mutant allele of another gene to express resulting in wild
phenotype called suppressor gene, e.g., Su-s in Drosophila suppresses the expression of
dominant mutant gene star eye(s).
Gene having more than one effect (multiple effects) are called pleiotropic
genes. They have a major effect in addition to secondary effect. In Drosophila, the
genes for bristle, eye and wing significantly influence the number of facets in bar-
eyed individuals

41. The appearance of offspring’s which resemble their remote
ancestors called atavism.
The ability of a gene to be expressed phenotypically to any degree
is called penetrance. Penetrance may be complete, e.g., in pea,
expression of R allele for red flower in homozygous and heterozygous
conditions. It may be incomplete, e.g., dominant gene P for Polydactyly
in human, sometimes does not show polydactylous condition in
heterozygous state.
A trait though penetrant, may be quite variable in its phenotypic
expression, e.g., in human polydactylous condition may be penetrant in left
hand but not in the right.