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Gene interaction
- 1. Gene interaction By: Dr. Sunita Sangwan Assistant professor Dept. of Botany Govt. College Bhiwani
- 2. Content • Introduction • Types of gene interaction • Allelic/non epistatic interaction • Complete dominance • Incomplete dominance • Co-dominance • Non-allelic/epistatic interaction • Complementary gene interaction • Epistasis • Inhibitory gene interaction • Duplicate gene interaction
- 3. Introduction • The phenomenon of two or more genes affecting the expression of each other in various ways in the development of a single character of an organism is known as gene interaction • Most of the characters of living organisms are controlled/ influenced/ governed by a collaboration of several different genes. • Mendel and other workers assumed that characters are governed by single genes but later it was discovered that many characters are governed by two or more genes. • Such genes affect the development of concerned characters in various ways; this lead to the modification of the typical dihybrid ratio (9:3:3:1) or trihybrid (27:9:9:9:3:3:3:1). • In gene interaction, expression of one gene depends on expression (presence or absence) of another gene.
- 4. Types of gene interaction Gene interaction is of two types: Allelic interaction Non- allelic interaction
- 5. Allelic/non-epistatic interaction Allelic genes are genes located in identical loci of homologous chromosomes. So interaction between alleles at one locus is called Allelic interaction. This type of interaction gives the classical ratio of 3:1 or 9:3:3:1 Allele interacts with each other in complex ways. • Complete dominance • Co-dominance • Incomplete dominance
- 6. Complete dominance • Mendel’s laws describe a relatively simple pattern of inheritance i.e. each character is determined by one gene, for which there are only two alleles, one is completely dominant to the other. This type of gene interaction is called complete dominance. • In complete dominance both heterozygotes (Aa)and dominant homozygotes (AA) have the same phenotype. • In complete dominance the dominant allele completely masks the recessive one
- 7. Complete dominance • For example purple flowers are dominant over white flower. A homozygous purple flower is crossed with homozygous white flower then the phenotype of the offsprings in F1 generation is purple flowers.
- 8. Complete dominance • F1 (Pp) plant self fertilize and produce gametes ‘P’ and ‘p’. • This results in the two phenotype in the F2 generation. • So phenotypic ratio in monohybrid cross is 3:1 • Genotypic ratio in monohybrid cross is 1:2:1.
- 10. Incomplete dominance • Incomplete dominance (partial dominance) where dominance of an allele over other is not complete . • In incomplete dominance the genes of the allelomorphic pair express themselves partially when present together in hybrid. • As a result heterozygotes (Aa) are phenotypically intermediate between two homozygous types (AA x aa) • The genotypic and phenotypic ratios for the F2 generation are the same ie 1:2:1. • Ex. Mirabilis jalapa or 4 O’ clock plant, Snapdragon plant Snapdragon plant Mirabilis jalapa or 4 O’ clock plant
- 11. Incomplete dominance Mirabilis jalapa Snapdragon
- 12. Co-dominance • Co-dominance is a kind of gene interaction, in which the heterozygotes express both dominant phenotypes. • One allele for a trait is not dominant over the other. • The heterozygotes condition produce a phenotype in which both variations appears as both the alleles are expressed equally. • Co-dominance is most clearly identified when the protein products of both alleles are detectable in heterozygous organisms . • Ex- flower color in Camellia, human blood group
- 13. Co- dominance RR • Red Rr • Red and white rr • white
- 14. Co-dominance • In human, an example is AB type of ABO blood system.
- 15. Non-allelic/epistatic Interaction • Mechanism of Non-allelic interaction • Most cell processes are a set of reactions linked together into a pathway. Each of the reactions is controlled by a different enzyme, and each enzyme is the product of a separate gene. Molecule A Molecule B Molecule CEnzyme 1 Enzyme 2
- 16. Types of Non- allelic /epistatic gene interaction • Complementary gene interaction (9:7) • Dominant epistasis/Masking gene interaction (12:3:1) • Recessive epistasis/Supplementary gene interaction (9:3:4) • Inhibitory gene interaction/Dominant suppressor (13:3) • Duplicate gene interaction (15:1)
- 17. Complementary gene interaction (9:7) • Ex- Flower colour in sweet pea It is a kind of gene interaction where the expression of a character is determined by presence of two dominant genes of different allelomorphic pairs simultaneously (A & B) Precursor Enzyme c Intermediate Enzyme P Molecule C Gene C Gene P Gene c inhibit the effect of gene C Gene p inhibit the effect of gene P
- 18. Complementary gene interaction (9:7) CP Cp cP cp CP 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 Example : sweet pea (Lathyrus odoratus) flower colour 9 C _P_ :3 C_pp: 3 ccP_:1 ccpp Purple White
- 19. Complementary gene interaction (9:7) • C and P products controlling different steps of anthocyanin synthesis pathway . Since anthocyanin production requires the action of the product of C as well as the product of P , both step must be successfully completed for anthocyanin production and deposition in flower petals .
- 20. Epistasis • An allelic pair suppresses the activity of another gene, they are found in same locus of the homologous chromosomes. This phenomenon is known as epistasis. • The gene whose expression is inhibited is called as hypostatic gene. • The gene that inhibit the expression of another gene is called epistatic gene. • Epistasis has two types, they are i. Dominant epistasis ii. Recessive epistasis
- 21. Dominant epistasis (12:3:1) • When the dominant allele at one locus (homozygous dominant or heterozygous) prevent the expression of one or more alleles at another locus (homozygous or heterozygous), 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
- 22. Example- Colour of Summer squash fruit Genotype Fruit colour Gene Actions 9 W_G_ white Dominant white allele negates effect of G allele 3W_gg white Dominant white allele negates effect of g allele 3wwG_ yellow Recessive colour allele allows yellow allele expression 1wwgg Green Recessive colour allele allows green allele expression
- 23. Recessive epistatic/Supplementary gene interaction (9:3:4) • Supplementary genes are two non-allelic genes, in which the first gene can produce its effect whether, the second gene is present or not, but the second (supplementary) gene produces its effect only in the presence of the first gene. • OR • Supplementary genes are genes that both contribute to a single characteristic, where one gene can mask the effect of the other. You may also think of supplementary genes in terms of one gene producing a characteristic and the second as only being able to ‘supplement’ this characteristic. First gene- express & produce colour alone Second gene- does not express so does not produce any colour But second gene in presence of first gene produce a new colour
- 24. • Example: Coat colour in mice The above enzymes are both coded for by different genes found on completely different chromosomes. For simplicity sake we can assume enzyme A is coded for by gene A and that the recessive form of the gene (the a allele) does not code for a functional protein at all. Similarly enzyme C is coded for by the gene C and that the recessive form of the gene (the c allele) also does not code for a functional protein. To produce chemical compound 2 the individual will need at least one dominant allele for gene A To produce chemical compound 3 the individual will need at least one dominant allele for both gene A & C
- 25. Supplementary gene interaction/ recessive epistasis • Therefore the presence of one gene can mask the effect of the other. For instance if an individual has the genotype aaBB none of the third chemical compound is produced. Thus the presence of aa (homologous recessive) masks the effect of the B allele. So it also represent recessive epistasis. • The phenotypic ratio of Supplimentary gene interaction/recessive epistasis is always 9:3:4
- 26. Inhibitory gene interaction/Dominant Suppressor (13:3) • Certain genes have the ability to suppress the expression of a gene at a second locus. • When dominant allele of one gene locus (B) in homozygous (BB) and heterozygous (Bb) condition produce the same phenotype • The F2 ratio becomes 13:3 instead of 9:3:3:1 • While homozygous recessive (bb) condition produces different phenotype. • Homozygous recessive (bb) condition inhibits phenotypic expression of other genes so know as inhibitory gene action
- 27. Inhibitory gene interaction/Dominant Suppressor (13:3) Example: Malvidin production in Primula • Certain genes have the ability to suppress the expression of a gene at a second locus. • The production of the chemical malvidin in the plant Primula is an example. • Both the synthesis of the chemical (controlled by the K gene) and the suppression of synthesis at the K gene (controlled by the D gene) are dominant traits. • The F1 plant with the genotype KkDd will not produce malvidin because of the presence of the dominant D allele. • What will be the distribution of the F2 phenotypes after the F1 was crossed?
- 28. • The ratio from the above table is 13 no malvidin production to 3 malvidin production. Because the action of the dominant D allele masks the genes at the K locus, this interaction is termed dominant suppression epistasis. • Suppressor - a genetic factor that prevents the expression of alleles at a second locus; this is an example of epistatic interaction Genotype Phenotype and genetic explanation 9 K_D_ no malvidin because dominant D allele is present 3 K_dd malvidin productions because dominant K allele present 3 kkD_ no malvidin because recessive k and dominant D alleles present 1 kkdd no malvidin because recessive k allele present
- 29. 9 K_D_ : 3K_dd : 3kkD_: 1kkdd White blue white So the ratio White :blue is 13:3 K Malvidin D Mechanism of malvidin synthesis KD Kd kD kd KD Kd kD kd KKDD KKDd KkDD KkDd KKDd KKdd KkDd Kkdd KkDD KkDd kkDD kkDd KkDd Kkdd kkDd kkdd KKdd x kkDD (blue) (white) KkDd (white) Selfed X
- 30. Duplicate gene interaction (15:1) • When dominant allele of both gene loci produce the same phenotype without cumulative effect • Because either gene can provide the wild phenotype, this interaction is called Duplicate gene interaction • In that case the ratio becomes 15:1 instead of 9:3:3:1 • Example1- flower colour in bean.
- 31. PR Pr pR pr PR Pr pR pr PPRR PPRr PpRR PpRr PPRr PPrr PpRr Pprr PpRR PpRr ppRR ppRr PpRr Pprr ppRr pprr PPRR x pprr (purple) (white) PpRr (purple) Selfed 9 K_D_ : 3K_dd : 3kkD_: 1kkdd Purple White So the ratio is modified from 9:3:3:1 to 15:1
- 32. Ex 2- Shape of Shepherds purse plant • In shepherds purse plant seed capsule occurs in two shapes i.e. triangular and ovoid shapes. • Ovoid shape seed capsule occurs when both genes are present in homozygous recessive condition
- 33. Gene interaction Inheritance pattern A_/B_ A_/bb aa/B_ aabb Ratio Complementary At least one dominant allele from each of two genes needed for phenotype 9 3 3 1 9:7 Recessive epistasis Homologous recessive genotype at one locus masks expression at second locus 9 3 3 1 9:3:4 Dominant epistasis Dominant allele at one locus masks expression at second locus 9 3 3 1 12:3:1 Duplicate genes One dominant allele from either of two genes needed for phenotype 9 3 3 1 15:1 Dominant suppressor Dominant allele at one locus inhibit the dominant allele at second locus 9 3 3 1 13:3
Editor's Notes
- In complete dominance, the dominant allele must produce enough of protein product so that a single copy of the dominant allele (as in heterozygote) give the maximum phenotypic response.
- So incomplete dominance is a kind of inheritance of allelic genes where a cross between organisms with two different phenotypes (AA x aa) produces offspring with a third phenotype that is a blending (Aa) of parental traits. Incomplete dominance is manifested when the interacting enzymes are slightly different in their activity.
- For example, when red Mirabilis plant are crossed with white Mirabilis plants, all the F1 hybrids have pink flowers. This third phenotype results from the heterozygote flowers having less red pigment than the red homozygotes. The self breeding of F1 hybrids produces F2 offsprings with a phenotypic ratio of 1 red: 2 pink: 1 white. The genotypic and phenotypic ratios for the F2 generation are the same ie 1:2:1.
- In human, an example is AB type of ABO blood system. The heterozygote fully expresses both alleles. Blood type AB individuals produce both A and B antigens. Since neither A nor B is dominant over the other and they are both dominant over ‘O’ they said to be co-dominant.
- In the hypothetical pathway above, moleculeA is converted into molecule B by enzyme1 and molecule B is then processed to become molecule C by enzyme 2. if either enzyme 1 and enzyme 2 is defective, molecule C cannot be manufactured, producing a mutant phenotypes. The result of defects in such pathways leads to modified Menedelian phenotypic ratio for crosses..
- If two genes are involved in a specific pathway and functional products from both are required for expression, then one recessive allelic pair at either pair would result in the mutant phenotype. Here any gene in recessive form become non functional and does not synthesize the functional enzyme.
- Complete dominance at both gene pairs; however, when either gene is homozygous recessive, it hides the effect of the other gene.
- When out of two genes, the dominant allele (e.g., A) of one gene masked the activity of allele of another gene (e.g., B) • Then A gene locus is said to be epistatic to the B gene locus • Dominant allele A express itself only in the presence of either B or b so such type of epistatic is know as dominant epistatic • The allele of hypostatic locus express only when the allele of epistatic locus present in homozygous recessive condition
- 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. The green fruits are produced in recessive condition (wwgg). A cross between plants having white and yellow fruits produced F1 with white fruits. Inter-mating of F1 plants produced plants with white, yellow and green coloured fruits in F2 in 12 : 3 : 1 ratio
- The dominant allele for gene A produces an enzyme responsible for the production of a pigment melanin (chemical compound 2). Thus as long the individual has one dominant allele A, they will be brown. However, the dominant allele for gene C produces an enzyme which increases the level of Melanin expression. Thus if an individual has at least one dominant allele for both gene A and gene C, they will be black. In a heterozygous cross (AaCc x AaCc) the following offspring are produced: Note that although three of the albino individuals have the dominant allele C (which increases the expression of melanin), no melanin is produced to begin with and thus no colour can be produced. Therefore the final phenotypic ratio for Supplementary genes is always: 9 : 3 : 4
- Complete dominance at both gene pairs; however, when one gene is homozygous recessive, it hides the phenotype of the other gene
- Complete dominance at both gene pairs; however, when either gene is dominant, it hides the effects of the other gene
- For this pathway a functional enzyme A or B synthesize anthocyanin from a common precursor. The product gives purple colour to the flower of bean. therefore, only one dominant allele at either loci is required to generate the product.
- Dominant allele at either locus is capable of catalyzing the conversion of precursor of anthocyanin and producing dominant phenotypes. Conversely if recessive homozygous allele are present at both loci no functional gene product is produced and synthesis pathway does not completedThus if a pure line bean plant with coloured flower ( genotype PPRR) is crossed with plant with white flower (genotype pprr) and resulting F1 plants are selfed, a modification of the dihybrid 9:3:3:1 will be produced ie 15:1.