Gene Interaction | Additional Study Material for NEET PDF Download

GENE INTERACTION

Gene interaction is two types :

(i) Allelic interaction/Intragenic interaction 

(ii) Non allelic interaction/Intergenic interaction 

(i) Allelic interaction/Intragenic interaction: Allelic interaction takes place between allele of same gene which are present at same locus.

Example of Allelic interaction are as follows :–

[1] Incomplete dominance :- According to Mendel's law of dominance, dominant character must be present in F₁ generation. But in some organisms, F₁ generation is different from the both parents.

Both factors such as dominant and recessive are present in incomplete dominance but dominant factors is unable to express its character completely, resulting Intermediate type of generation is formed  which is different from the both parents. Some examples are –

(a) Flower colour in Mirabilis jalapa : Incomplete dominance was first discovered by Correns in Mirabilis jalapa. This plant is called as '4 O' clock plant 'or' Gul-e-Bans'. Three different types of plant are found in Mirabilis on the basis of flower colour, such as red , white and pink.

& When plants with red flowers is crossed with white flower, plants with pink flower obtained in F₁ generation. The reason of this is that the genes of red colour is incompletely  dominant over the genes of white colour. & When, F₁ generation of pink flower is self pollinated then the phenotypic  ratio of F₂ generation  is red, pink, white is 1:2:1 ratio in place of  normal monohybrid cross ratio 3:1. & The ratio of phenotype and genotype of F₂ generation in incomplete dominance is always same.

(b) Flower colour in Antirrhinum majus :- Incomplete dominance is also seen in flower colour of this plant.This plant is also known as 'Snapdragon ' or 'Dog flower'. Incomplete dominance is found in this plant which is the same as Mirabilis.

(c) Feather colour in Andalusian Fowls :- Incomplete dominance is present for their feather colour.

When a black colour fowl is crossed with a white colour fowl, the colour of F₁ generation  is blue.

[2] Co-dominance :- In this phenomenon, both the gene expressed for a particular character in F₁ hybrid progeny. There is no blending of characters, whereas both the characters expressed equally.

Examples :- Co-dominance is seen in animals for coat colour. when a black parent is crossed with white parent, a roan colour F₁ progeny is produced.

When we obtain F₂ generation from the F₁ generation, the ratio of black ; black-white (Roan) ; white animals is  1 : 2 : 1

Note :-  F₂ generation is obtained in animals by sib-mating cross.

It is obvious by above analysis that the ratio of phenotype as well as genotype is 1:2:1 in co-dominance.

Sp. Note :- In incomplete dominance, characters are blended phenotypically, while in co-dominance, both the genes of a pair exhibit both the characters side by side and effect of both the character is independent from each other.

Other Examples of Co-dominance :
(ii) AB blood group inheritance (I^AI^B)
(iii) Carrier of Sickle cell anaemia (Hb^A Hb^S)

[3] Multiple allele :– More than 2 alternative forms of same gene called as multiple allele. Multiple allele is formed due to mutation. Multile allele located on same locus of homologous chromosome.

A diploid individual contains two alleles and gamete contains one allele for a character.

Ex. Blood group - 3 alleles Coat colour in rabbit - 4 alleles

If n is the number of allele of a gene then number of different possible genotype =

Example  of multiple allele : 

1. ABO blood group → ABO blood groups are determined by allele I^A, allele I^B, allele I^O

I^A = dominant

I^B = dominant

I^O = recessive Possible phenotypes - A, B, AB, O

2. Coat colour in rabbit → Four alleles for coat colour in rabbit

Wild type = Full coloured = agouti = C⁺

Himalayan [white with black tip on extremities (like nose, tail and feet)] = c^h

Chinchilla [mixed coloured and white hairs] = c^ch

albino = Colourless = c^a

These alleles show a gradient in dominance  C⁺ > c^ch > c^h > c^a

Possible genotypes –

Coloured = C+C+, C+c^ch, C+c^h C+c^a

Chinchilla = c^chc^ch , c^chc^h , c^chc^a

Himalayan = c^hc^h , c^hc^a

Albino = c^ac^a

Possible genotype  =    = 10 genotypes

Eye colour in Drosophila  and self incompatibility genes in plants are also the example of multiple allelism.

[4] Lethal gene :– Gene which causes death of individual in early stage when it comes in homozygous condition called lethal gene. Lethal gene may be dominant or recessive both, but mostly recessive for lethality. Many of these genes which do not cause definite lethality are called semilethals. In semilethal gene death occurs in late stage.

1. Lethal gene was discovered by L. Cuenot in coat colour of mice.
Yellow body colour(Y) was dominant over normal brown colour(y).
Gene of yellow body colour is lethal.
So homozygous yellow mice are never obtained in population. It dies in embryonal stage.
When yellow mice were crossed among themselves segregation for yellow and brown body colour was obtained in 2 : 1 ratio.

YY - death in embryonal stage modified ratio = 2 : 1

2. In plant lethal gene was first discovered by E. Baur in Snapdragon (Antirrhinum majus)

Homozygous golden leaves are never obtained.

3. Sickle cell anaemia in human. In human, gene of sickle cell anaemia HbS is the example of lethal gene.
When two carrier individuals of sickle cell anaemia are crossed then off-springs are obtained in 2 : 1 ratio.

Sublethal gene but ratio 2 : 1

[5] Pleiotropic gene :– Gene which controls more than one character is called pleiotropic gene. This gene shows multiple phenotypic effect.
For example :

(1) In Pea plant : Single gene influences 

2) In Drosophila recessive gene of vestigial wings also influence the some another characters–

• Structure of reproductive organs
• Longevity (Length of Body)
• Bristles on wings.
• Reduction in egg production.

(3) Examples of  pleiotropic gene in human. 

(a) Sickle cell anaemia - Gene Hb^S_β provide a classical example of pleiotrophy. It not only causes haemolytic anaemia but also results increased resistance to one type of malaria that caused by the parasite Plasmodium falciparum. The sickle cell Hb^S_β allele also has pleiotropic effect on the development of many tissues and organs such as bone, lungs, kidney, spleen, heart.

(b) Cystic fibrosis – Hereditary metabolic disorder that is controlled by a single autosomal recessive gene.
The gene specifies an enzyme that produces a unique glycoprotein.
This glycoprotein results in the production of mucous.
More mucous interfere with the normal functioning of several exocrine glands including those in the skin, lungs, liver and pancreas.

(ii) Non allelic interaction/Intergenic interaction -When interaction takes place between non-allele is called non-allelic gene interaction. It changes or modifies other non-allelic gene.
Examples of non-allelic interaction.

1. Epistasis :- When, a gene prevents the expression of another non-allelic gene, then it is known as epistatic gene and this phenomenon is known as Epistasis.
Gene which inhibit the expression of another non-allelic gene is called epistatic gene and expression of gene which is suppressed  by epistatic gene called hypostatic gene.

Example :- 

Hair Colour in Dog :-

B = Dominant allele for black colour of hairs.
b = Recessive allele for brown colour of hairs.
I = Epistatic gene.
If the genotype bbii for brown colour and BBII for white colour.
Following types of generation will be obtained by following crosses.

It is obviously clear by above analysis, the phenotypic ratio of F_{2 -} generation in epistasis is  - 12:3:1

2. Inhibitory gene - Inhibitory gene itself have no phenotype but inhibits the effect of other non-allelic gene. Non-allelic gene behaves as  recessive.  Inhibitory gene must be in dominant stage & inhibit the effect of only dominant gene.
Ex., Leaf colour in Rice
R – Purple
r – Green
I – Inhibitory gene
R – I – Green – 9
R – ii – Purple – 3
rr – I – Green – 3
rr – ii – Green – 1
13 (Green) : 3(Purple)

3. Complementary Gene :- Two pair of non-allelic genes are essential in dominant form to produce a particular character.
Such genes that act together to produce an effect that neither can produce, it's  effect separately are called complementary genes. Both types of gene must be present in dominant form.

Example :- Colour of flowers in Lathyrus odoratus :-

C – P Purple coloured

C – pp colour less

cc – P colour less

cc – pp colour less

Thus phenotypic ratio of complementary genes = Coloured : Colourless   9  :  7

4. Duplicate Genes :-

Two pairs of non-allelic genes require  are for a character . If any one of them gene is dominant, then this character is expressed such type of gene is called duplicate gene.

Example :- Fruit shape in Capsella. Two pair of non-allelic genes are present in Capsella for triangular shape of fruits.

If any one gene out of them is dominant, the shape of fruit is triangular and no one gene is dominant than fruits will be elongated.

If  TTDD = For triangular shape      

ttdd = For top shape of fruits

Phenotypic ratio of F₂ -> Triangular : Top shaped 15    :   1

5. Additive Gene effect : In additive gene effect if non-allelic gene separately in dominant stage phenotype is same but both gene come dominant stage together phenotype is change due to additive effect. eg.: Fruit shape in cucumber

A – bb→ spherical

aa B→ spherical

A – B – discoid (new phenotype)

aa bb – cylindrical

6. Collaboratory Gene :- Two pairs of non-allelic gene interacting together to produce a new phenotypic character.

Example :- Comb - shape in Chickens -

If, RR = For Rose comb

PP = For Pea comb

Both R & P = For walnut comb

rr pp = for single comb

A new type of phenotype walnut - (Rr Pp) comb is produced by the cross in between Rose comb (RR pp) and Pea comb (rr PP)

Thus, phenotypic ratio of collaboratory gene = Walnut : Rose : Pea : Single   = 9 : 3 : 3 :  1

7. Supplementary gene or Recessive Epistasis :- A pair of gene change the effect of another non-allelic gene, is called supplementary gene.
Example :- Coat colour in Mice.
If alleles,   C = Black coat colour                  

c = Albino (Colourless coat) or (It has no effect)                  
A = Supplementary gene When black coat mice crossed with albino mice, the F₁ generation is Agouti.
It means, here the effect of non allelic gene is changed.

Thus, Recessive epistasis or supplementary gene ratio in

F₂ -   Agouti : Black : Albino

          9    :     3    :    4