epistasis

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EPISTASIS

DEFINITION:

DEFINITION Epistasis come from the Greek – “ epi ” means “upon” “ histani ” means “to place” So it means to place upon or to stand u “A nonreciprocal interaction between two non-alternative forms of genes in which one gene suppresses the expression of another affecting the same part of the organism”.

DEFINITION:

DEFINITION Epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes . The gene whose phenotype is expressed is said to be epistatic , while the phenotype altered or suppressed is said to be hypostatic .

A Cross Involving a Two-Gene Interaction Can Still Produce a 9:3:3:1 ratio:

Inheritance of comb morphology in chicken First example of gene interaction William Bateson and Reginald Punnett in 1906 Four different comb morphologies A Cross Involving a Two-Gene Interaction Can Still Produce a 9:3:3:1 ratio

The crosses of Bateson and Punnett :

The crosses of Bateson and Punnett

CONTINUED.. :

CONTINUED.. F 2 generation consisted of chickens with four types of combs 9 walnut : 3 rose : 3 pea : 1 single Bateson and Punnett reasoned that comb morphology is determined by two different genes R (rose comb) is dominant to r P (pea comb) is dominant to p R and P are codominant (walnut comb) rrpp produces single comb

mechanism of epistasis:

mechanism of epistasis In this mechanism, the pigment melanin is present in both the black and agouti phenotypes. The agouti is a modified black hair in which yellow stripes(the pigment phaeomelanin ) have been added. Thus, with melanin present, agouti is dominant.

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Without melanin, we get an albino regardless of the genotype of the agouti gene because both agouti and black depend on melanin. Albinism is the result of one of several defects in the enzymatic pathway for the synthesis of melanin.

DOMINANT EPISTASIS:

DOMINANT EPISTASIS In dominant epistasis a F2 cross yeilds a typical 12:3:1 ratio. Of the two genes controling the phenotype one is termed the epistatic gene and must be present in homozygose recessive form before the second genes the hypostatic gene, alleles will be expressed in the phenotype.

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Studied in summer squash ( Cucurbita pepo ) Common fruit colors- white,yellow &green White (W) is dominant over colored squash Yellow (Y) is dominant over green squash Pure breeding white fruited variety is crossed with the double recessive green variety,F1 hybrids are all white When the hybrids are selfed-white,yellow &green fruited plants arise in the ratio of 12:3:1

CONTINUED. .:

CONTINUED. . 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

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♂/♀ WY Wy wY wy WY WWYY WWYy WwYY WwYy Wy WWYy WWyy WwYy Wwyy wY WwYY WwYy wwYY wwYy wy WwYy Wwyy wwYy wwyy

RECESSIVE EPISTASIS:

RECESSIVE EPISTASIS Recessive epistasis yields and typical 9:3:4 ration in the second filial cross. The epistatic gene if present in homozygous recessive form will stop the expression of the hypostatic dominant allele.

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Studied in snap dragons nivea has alleles that determine whether any pigment is produced nn genotype - prevents pigment production NN or Nn genotypes - permit pigment color genes to express themselves

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red white NNEE * nnee F1 red * self NnEe ratio is 9:3:4 NE Ne nE ne NE NNEE red NNEe red NnEE red NnEe red Ne NNEe red Nnee pink NnEe red Nnee pink nE NnEE red NnEe red nnEE white nnEe white ne NnEe red Nnee pink nnEe white nnee white

DOMINANT RECESSIVE EPISTASIS:

DOMINANT RECESSIVE EPISTASIS Interaction involves an inhibitory factor which by itself has no phenotypic effect But, when present in the dominant form prevents or inhibits the expression of another dominant gene eg :. Malvidin in primula flowers Malvidin is a O- Methylated anthocyanin responsible for the blue pigments in Primula polyanthus plant

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malvidin )

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Synthesis of malvidin (blue) is controlled by gene K In recessive state(k), malvidin is not synthesized Production is suppressed by gene D, found at completely different locus D allele is dominant to K allele

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KKdd x kkDD P (blue) (white) ↓ KkDd ( selfed ) F1 (white) ↓ F2 ♂/♀ KD Kd kD kd KD KKDD KKDd KkDD KkDd Kd KKDd KKdd KkDd Kkdd kD KkDD KkDd kkDD kkDd kd KkDd Kkdd kkDd kkdd

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Therefore, plants with KkDd genotype will not produce malvidin due to the presence of D allele Thus, white & blue colored flowers producing plants are obtained in the ratio of 13:3 Also known as dominant supression ,since a single allele supresses the expression of someother gene

DUPLICATE DOMINANT EPISTASIS:

DUPLICATE DOMINANT EPISTASIS Genes do not act always in opposition to each other for an interaction to be epistasis Sometimes,2 genes that have identical effect substitute each other Eg :. shepherd’s purse-seed capsule

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Having either A allele or a B allele produces triangular shaped capsule But lack of either will produce oval capsule Thus, if dihybrids are selfed,two phenotypes are produced in a 15:1 ratio

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Thank You!