not so easy but understandable INTRA INTER ALLELIC INTERACTION (NON MENDELIAN GENETICS)

1. -Contents:
1. Introduction to Gene Interaction
2. Allelic Gene Interactions
3. Non Allelic Gene Interactions
4. Multiple Factors and Polygenic Gene Interactions
5. Other Kinds of Gene Interactions
1. Introduction to Gene Interaction:
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 genetic interactions 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 inde-
pendent 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 (Table 7.1).
2. Allelic Gene Interactions:
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.

2. In snapdragon and Mirabilis jalapa, the cross between pure bred 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 (Fig. 7.1).
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 F2into (3:1) x (1 :2:1) = 3:6:3:1 :2:1.
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.
Lethal Factor (2:1):
The genes which cause the death of the individual carrying it, is called lethal factor. Recessive lethals 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 recessive lethal, e.g., gene for yellow coat colour in mice.
But many genes are recessive both in their phenotypic as well as lethal effects, e.g., gene producing albino seedlings in
barley (Fig. 7.2).

3. Dominant lethals 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 lethals require a specific condition for their lethal action, e.g.,
temperature sensitive mutant of barley (lethal effect at low temperature).
.
Multiple Alleles:
A gene for particular character may have more than two allelomorphs or alleles occupying same locus of the
chromosome (only two of them present in a diploid organism). These allelomorphs 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
– lA
, IB
, i. 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. Therefore, the possible genotypes of the four blood
groups are shown in Fig. 7.3.

4. 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 colour.
3. Non Allelic Gene Interactions:
Simple Interaction (9:3:3:1):
In this case, two non-alleiic 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
(Fig. 7.5). Similar pattern of inheritance is found in Streptocarpus flower colour (Fig. 7.6).

5. Complementary Factor (9:7):
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 colour. But absence of any one cannot produce anthocyanin causing white flower. So C and P are complementary
to each other for anthocyanin formation (Fig. 7.7).9

6. Involvement of more than two complementary genes is possible, e.g., three complementary genes governing aleurone
colour in maize.
Epistasis:
When a gene or gene pair masks or prevents the expression of other non-allelic gene, called epistasis. The gene which
produces the effect called epistatic gene and the gene whose expression is suppressed called hypostatic gene.
(a) Recessive Epistasis or Supplementary Factor (9:3:4):
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 colour of mice is controlled by two pairs of genes.
Dominant gene C produces black colour, absence of it causes albino. Gene A produces 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 (Fig. 7.8).

7. The grain colour in maize is governed by two genes — R (red) and Pr (purple). The recessive allele rr is epistatic to
gene Pr (Fig. 7.9).

8. (b) Dominant Epistasis (12:3:1):
Sometimes a dominant gene does not allow the expression of other non-allelic gene called dominant epistasis. In summer
squash, the fruit colour is governed by two genes. The dominant gene W for white colour, suppresses the expression of
the gene Y which controls yellow colour. So yellow colour appears only in absence of W. Thus W is epistatic to Y. In
absence of both W and Y, green colour develops (Fig. 7.10).
Inhibitory Factor:
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 colour is due to gene P, and p causing green colour. Another non-allelic dominant gene I inhibit
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 colour expression of P (Fig. 7.11).

9. Polymorphic Gene (9:6:1):
Here two non-allelilc genes controlling a character produce identical phenotype when they are alone, but when both the
genes are present together their phenotypes effect is enhanced due to cumulative effect. In barley, two genes A and B
affect the length of awns.
Gene A or B alone gives rise to awns of medium length (the effect of A is same as B); but when both present, long awn
is produced; absence of both results aweless (Fig. 7.13).

10. ------------------OR------------------
Polymorphic Gene
When two genes govern any character separately, their effect is equal but when both the genes are present together, there
phenotypic effect is increased or raised as if the effects of the two genes were additive or cummulative. It is notable in this
case that both the genes show complete dominance.
“Additive or cummulative effect of genes present at different loci is called polymerism.”
In wheat three types of pericarp colour is found:
Colourless. When a cross is made between plants having deep red (AABB) and colourless (aabb) pericarp, the F1 (AaBb)
has deep red pericarp due to the additive effect of both the genes A and B.
In F2 generation, on an average, 9/16 plants will have dominant alleles of both the genes A and B; as a result these plants
will produce deep red per-carp. 6/16 plants will have light red pericarp since they have either A or B, but not both. The rest
1/16 plant will be homozygous recessive for both the genes and will be therefore, colourless.
Genotypes and phenotypes of the F2 generation:
Duplicate Gene (15:1):
Sometimes a character is controlled by two non-allelic genes whose dominant alleles produce the same phenotype
whether they are alone or together. In Shepherd’s purse (Capsella bursa-pastoris), the presence of either gene A or gene
B or both results in triangular capsules; when both these genes are in recessive forms, the oval capsules produced (Fig.
7.14).

11. Duplicate Gene with Dominance Modification (11:5):
A character controlled by two gene pairs showing dominance only if two dominant alleles are present. Dominant
phenotype will thus be produced only when two non-allelic dominant alleles or two allelic dominant alleles are present.
Such a case is found in pigment glands of cotton (Fig. 7.15).

12. ------------------OR------------------
Duplicate Factor (15:1):
The two pairs of factors which have identical effect are known as duplicate factors.
Or
Characters showing duplicate factor or gene action are governed by two dominant factors. These dominant factors
produce the same phenotype whether they are alone or in pairs.
(Awned and awnless characters in rice plants)
Genotypes and phenotypes of F2 generation:

13. Thus in duplicate gene action, the presence of a single dominant allele of any one of two genes governing the dominant
phenotype e.g., awned type in rice, while the recessive phenotype e.g., awnless is produced only when both the genes are in
the homozygous recessive condition.
A similar case of duplicate factor interaction governs the fruit shape in Bursa, endosperm colour in maize, nodulation in
groundnut and certain characters in many other plants. First of all, the duplicate gene interaction was observed and
explained by G.H. Shull in plant called shepherd’s purse (Capsella bursapastoris.)
5. Other Kinds of Gene Interactions:
Modifiers:
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 responsible for dilution of body colour.
Suppressors:
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).
Pleiotropy:
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.
Atavism:
The appearance of offspring’s which resemble their remote ancestors called atavism.
Penetrance:
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.
Expressivity:
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.
What is the genotype of an individual?
a. Physical appearance
b. Genetic makeup
c. Nature of the individual
d. Blended characteristics of the individual