Epistasis

Epistasis is a Greek word that means standing over .Bateson used it to describe the masking effect in 1909.
An interaction between a pair of loci in which the phenotype effect of one locus depends on the genotype at the second locus.
Genes whose phenotypes are ;
Expressed,epistatic.
Altered or suppressed hypostatic.

1. EPISTASIs Question Discuss 1. Epistasis. 2. The types of epistasis and their statistical inferences Amos Kenyi Dickson Bugema university

2.   Introduction Definition Kinds of Epistasis (і) Dominant Epistasis. (ii) Recessive epistasis (iii) Duplicate Recessive Genes (iv) Duplicate Dominant Genes (v) Duplicate Genes with Cumulative Effect (vi) Dominant Recessive Interaction References CONTENTS

3. introduction Epistasis is a Greek word that means standing over .Bateson used it to describe the masking effect in 1909. An interaction between a pair of loci in which the phenotype effect of one locus depends on the genotype at the second locus. Genes whose phenotypes are ; Expressed epistatic. Altered or suppressed hypostatic.

4. Definition The term epistasis describes a certain relationship between genes, where an allele of one gene (e.g., ‘spread’) hides or masks the visible output, or phenotype, of another gene (e.g., pattern). Epistasis is entirely different from dominant and recessive, which are terms that apply to different alleles of the same gene (e.g., ‘bar’ is dominant to ‘ barless ’ and recessive to ‘check’).

5. Epistasis is the phenomenon of the effect of one gene ( locus ) being dependent on the presence of one or more 'modifier genes', the genetic background. Epistasis occurrs when one allele of a gene masks the expression of alleles of another gene.

6. The masking of the phenotypic effect of alleles at one gene by alleles of another gene. A gene is said to be epistatic when its presence suppresses the effect of a gene at another locus. Epistatic genes are sometimes called inhibiting genes because of their effect on (suppressed) other genes which are described as hypostatic.

7. Difference between dominance and epistasis Dominance Epistasis Involves intra-allelic gene interaction. Involves inter-allelic gene interaction. One allele hides the effect of other allele at the same gene pair. One gene hides the effect of other gene at different gene loci.

8. Kinds of Epistatic Interactions   In epistasis less than four phenotypes appear in F2. (і) Dominant Epistasis. (12:3:1) (ii) Recessive epistasis.(9:3:4)(Supplementary interaction) (iii) Duplicate Recessive Genes (9:7) (Complementary Genes) (iv) Duplicate Dominant Genes. (15:1) (v) Duplicate Genes with Cumulative Effect (9:6:1) (vi) Dominant Recessive Interaction (13:3)

9. 1. Dominant Epistasis. (12:3:1) When a dominant allele at one locus can mask the expression of both alleles (dominant and recessive) at another locus, it is known as dominant epistasis. In other words, the expression of one dominant or recessive allele is masked by another dominant gene. This is also referred to as simple epistasis. This type of dominant epistasis modifies the classical ratio of 9:3:3:1 into 12:3:1

10. Example: Studied in summer squash ( Cucurbita pepo ) An example of dominant epistasis is found for fruit colour in summer squash. There are three types of fruit colours in this cucumber, viz., white, yellow and green. White colour is controlled by dominant gene W and yellow colour by dominant gene G. White is dominant over both yellow and green.

11. The green fruits are produced in recessive condition ( wwgg ). A cross between plants having white and yellow fruits produced F 1 with white fruits. Inter-mating of F 1 plants produced plants with white, yellow and green coloured fruits in F 2 in 12 : 3 : 1 ratio

12. The effect of dominant gene ’Y’ is masked by the dominant gene ’W’ (epistatic gene) P WWYY X wwyy (white) ↓ (green) F1 WwYy (white) (selfed) F2 White:Yellow:Green 12 : 3 : 1 ♂/♀ WY Wy wY wy WY WWYY WWYy WwYY WwYy Wy WWYy WWyy WwYy Wwyy wY WwYY WwYy wwYY wwYy wy WwYy Wwyy wwYy wwyy

13. 2. Recessive epistasis. (9:3:4) (Supplementary interaction) When recessive alleles at one locus mask the expression of both (dominant and recessive) alleles at another locus, it is known as recessive epistasis. This type of gene interaction is also known as supplementary epistasis.

14. In horses, brown coat color ( B ) is dominant over tan ( b ). However, how that gene is expressed in the phenotype is dependent on a second gene that controls the deposition of pigment in hair. The dominant gene ( C ) codes for the presence of pigment in hair, whereas the recessive gene ( c ) codes for the absence of pigment. Example

15.   3. Duplicate Recessive Genes (9:7) (Complementary Genes) When recessive alleles at either of the two loci can mask the expression of dominant alleles at the two loci. If both gene loci have homozygous recessive alleles and both of them produce identical phenotype the F 2 ratio 9:3:3:1 would be 9:7  

16. Example Bateson and Punnett observed that when two white flowered varieties of sweet pea, Lathyrus odoratus were crossed, F 1 progeny had purple flowers. When F 1 was selfed , the F 2 ratio showed the presence of both purple and white flowered varieties in the ratio 9:7. The purple colour of flower in sweet pea is governed by two dominant genes say A and B. When these genes are in separate individuals ( AAbb or aaBB ) or recessive ( aabb ) they produce white flower. Other examples are; Maize colour Human mutism Etc. Example

17. Example

18. In this case dominant alleles on both locus are required hence wherever A and B both are present they result into purple effect masking the white. This is because A and B alleles modified the colorless precursor by showing their effects

19. 4. Duplicate Dominant Genes. (15:1) When a dominant allele at either of two loci can mask the expression of recessive alleles at the two loci, it is also called duplicate gene action. If a dominant allele of both gene loci produces the same phenotype without cumulative effect, i.e., independently the ratio will be 15:1. The duplicate genes are also called pseudoalleles

20. Example A good example of duplicate dominant epistasis is awn character in rice. Development of awn in rice is controlled by two dominant duplicate genes (A and B). Presence of any of these two alleles can produce awn. The awnless condition develops only when both these genes are in homozygous recessive state ( aabb ). A cross between awned and awnless strains produced awned plants in F 1 . Inter-mating of F 1 plants produced awned and awnless plants in 15 : 1 ratio in F 2 generation

21. The allele A is epistatic to B/b alleles and all plants having allele A will develop awn. Another dominant allele B is epistatic to alleles A/a. Individuals with this allele also will develop awn character. Hence in F 2 , plants with A-B-(9/16), A-bb-(3/16) and aaB -(3/16) genotypes will develop awn.

22. The awnless condition will develop only in double recessive ( aabb ) genotype (1/16). In this way only two classes of plants are developed and the normal dihybrid segregation ratio 9 : 3 : 3 : 1 is modified to 15 : 1 ratio in F 2 . Similar gene action is found for nodulation in peanut and non-floating character in rice.

23.   5. Duplicate Genes with Cumulative Effect. (9:6:1) Both the dominant non allelic alleles, when present together, give a new phenotype, but when allowed to express independently, they give their own phenotypic expression separately. In the absence of any dominant allele, the recessive allele is expressed. Such gene interaction is known as polymeric gene interaction. The joint effect of two alleles appears to be additive or cumulative, but each of the two genes show complete dominance, hence they cannot be considered as additive genes.

24. Example A well-known example of polymeric gene interaction is fruit shape in summer squash. There are three types of fruit shape in this plant, viz., disc, spherical and long. The disc shape is controlled by two dominant genes (A and B), the spherical shape is produced by either dominant allele (A or B) and long shaped fruits develop in double recessive ( aabb ) plants.

25. A cross between disc shape (AABB) and long shape ( aabb ) strains produced disc shape fruits in F 1 . Inter-mating of F 1 plants produced plants with disc, spherical and long shape fruits in 9 : 6 : 1 ratio in F 2

26. Here plants with A—B—(9/16) genotypes produce disc shape fruits, those with A-bb-(3/16) and aaB -(3/16) genotypes produce spherical fruits, and plants with aabb (1/16) genotype produce long fruits. Thus in F 2 , normal dihybrid segregation ratio 9:3:3: 1 is modified to 9 : 6 : 1 ratio. Similar gene action is also found in barley for awn length.

27. 6. Dominant Recessive Interaction (13:3) In this type of epistasis, a dominant allele at one locus can mask the expression of both (dominant and recessive) alleles at second locus. This is also known as inhibitory gene interaction. An example of this type of gene interaction is found for anthocyanin pigmentation in rice.

28. Example The green colour of plants is governed by the gene I which is dominant over purple colour . The purple colour is controlled by a dominant gene P. When a cross was made between green ( IIpp ) and purple ( iiPP ) colour plants, the F 1 was green. Inter-mating of F 1 plants produced green and purple plants in 13 : 3 ratio in F 2 . This can be explained as follows.

29. Here the allele I isepistatic to alleles P and p. Hence in F 2 , plants with I-P-(9/16), I-pp (3/16) and iipp (1/16) genotypes will be green because I will mask the effect of P or p. Plants with iiP -(3/16) will be purple, because I is absent.

30. In this way the normal dihybrid segregation ratio 9 : 3 : 3 : 1 is modified to 13 : 3 ratio. Similar gene interaction is found for grain colour in maize, plumage colour in poultry and certain characters in other crop species.

31. References: Hartl,D.L ., & Jones,W.E ., (1998) “Genetics Principles and Analysis” ed : 4 th Jones and Bartlett Publishers International London,UK, pp: 19,20,61-63 Miko , I., (2008) Epistasis: Gene interaction and phenotype effects.  Nature Education  1(1) Richards,J.E . & Hawley, R. S., (2010) “ The human genome” ed : 3 rd Academic Press, pp: 31 Verma,P.S ., & Agarwal,V.K ., (2004) “Cell biology, Genetics, Molecular Biology, Evolution and Ecology” ed : 24 th S.Chand and Company Ltd,Ram Nagar, New Delhi. Pp: 45-56 http://www.biologydiscussion.com/genetics/gene-interactions/top-6-types-of-epistasis-gene-interaction/37818 http://www.yourarticlelibrary.com/biology/6-most-important-kinds-of-epistasis-biology/6436/