Cytogenetics Questions and Answers – Epistasis

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This set of Cytogenetics Multiple Choice Questions & Answers (MCQs) focuses on “Epistasis”.

1. In case of two gene interaction, the gene which is masking the expression of another is called_____________ and the gene whose expression is masked is called_______________
a) Dominant, recessive
b) Recessive, dominant
c) Epistatic, hypostatic
d) Hypostatic, Epistatic
View Answer

Answer: c
Explanation: In case of single gene two alleles which suppress one another we use dominant/ recessive relations. But in case of interaction between two genes we use epistatic for the one that suppresses the expression of another and hypostatic for the one whose expression is suppressed.
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2. Epistasis is the interaction between_____________ genes.
a) 2
b) 4
c) 8
d) 16
View Answer

Answer: a
Explanation: Epistasis is the interaction between two genes where the genotype of one location affects the expression of genotype at the other location.

3. Epistasis is the interaction between two genes producing a new phenotype.
a) True
b) False
View Answer

Answer: b
Explanation: In case of epistasis there is suppression of expression of one phenotype but there is no production of new phenotype. This is how it differs from gene interaction.
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4. What is the dihybrid phenotypic ratio for recessive epistasis in an F22 generation?
a) 9:3:3:1
b) 9:6:1
c) 9:3:4:1
d) 9:3:4
View Answer

Answer: d
Explanation: In case of recessive epistasis the expression of A/- b/b and a/a b/b are the same where B is the epistatic locus and A is the hypostatic locus. So the dihybrid ratio is modified to 9:3:4.

5. In mouse agouti locus is hypostatic to pigment development locus. If C is for pigment development, and A is for agouti, and recessive alleles in agouti locus gives black colour and recessive in pigment locus gives white colour, what will be the phenotype of A/- c/c and a/a c/c.
a) Agouti, white
b) Black, agouti
c) Black, black
d) White, white
View Answer

Answer: d
Explanation: In this case the C gene is epistatic, so unless colour is developed due to C/- there will be no expression of A locus. In both of these cases, C locus has c/c/ alleles, so both the mice are white.
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6. The cross between a white mouse and a black mouse _______________
a) Can’t give a Black mouse
b) Can’t give a White mouse
c) Can’t give a Agouti mouse
d) Can give all combinations
View Answer

Answer: d
Explanation: Agouti gene is hypostatic to the color locus, in black mouse the combination is a/a C/- and in albino or white it’s -/- c/c. So if there is any dominant agouti gene in white mouse it can be expressed in the F1 where C locus would be C/-, so all combinations are possible.

7. Which one of the following has dominant allele in both gene locus?
a) Black
b) Albino
c) Agouti
d) White
View Answer

Answer: c
Explanation: To give an agouti mouse the gene combination should be A/- C/-, so it must have atleast one dominant gene in both the gene locus.
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8. Coat colour in Labrador retriever is controlled by epistasis. B specifies black pigment, b is for brown pigment (recessive) and E locus determines the expression of B locus. Which one of the following is wrong?
a) bb EE gives yellow
b) Bb EE gives black
c) bb Ee gives brown
d) BB EE gives black
View Answer

Answer: a
Explanation: According to the rules of recessive epistasis, at least one dominant allele at the E locus is needed for expression at the b locus. As in option bb EE gives yellow the E locus is dominant so b locus is expressed to give chocolate or brown colour not yellow.

9. Epistasis can produce a ratio of 12:3:1.
a) True
b) False
View Answer

Answer: a
Explanation: This is the phenotypic ratio observed in case of dominant epistasis. Then the presence of a dominant allele in one locus will produce the same effect irrespective of the allele at the other locus.
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10. In case of dominant epistasis which of the following will have the same expression, when A is the epistatic locus?
a) A/a B/b and a/a B/b
b) A/A b/b and A/A B/b
c) a/a b/b and A/a b/b
d) a/a b/b and a/a B/B
View Answer

Answer: b
Explanation: According to the description of dominant epistasis given, A is the epistatic locus, thus presence of even one set of dominant gene at A locus will give the same effect irrespective of the B locus.

11. Dominant epistasis ratio with A as epistatic gene 12:3:1 corresponds to ________________
a) A/- B/- : A/- b/b : a/a B/-
b) a/a B/- : a/a b/b : a/a b/-
c) A/A B/-: A/- B/-: a/a b/b
d) A/- -/- : a/a B/-: a/a b/b
View Answer

Answer: d
Explanation: In case of dominant epistasis, if there is atleast one dominant allele here A in the epistatic locus, the gene combination at B locus will not affect the phenotype. So the combination at B can be written as -/-.

12. Which one of the following is not the case of epistasis?
a) Fur colour in mouse
b) Fruit colour in summer squash
c) Fruit shape in summer squash
d) Coat colour in labrador
View Answer

Answer: c
Explanation: White Fur colour in mouse and coat colour in Labrador is the case of recessive epistasis, and Fruit color in summer squash being a case of dominant epistasis, fruit shape of summer squash is gene interaction and not epistasis.

13. Which of the following is incorrect about the graying of coat colour of horse?
a) The graying is a case of dominant epistasis
b) The colour of the fur and skin gradually turns grey
c) The G/G or G/g shows greying
d) Coat colour of horse with G/- combination will grey irrespective of what was its colour to start with
View Answer

Answer: b
Explanation: The Greying is seen only in the fur colour of the mature horse and not in the coloring of the skin. The rest of the options are correct and fit for the case f dominant epistasis.

14. In case of summer squash, the W locus shows dominant epistasis over the Y locus. W locus develop white colour while /- give yellow and y/y gives green. If you cross a yellow and a green summer squash you can’t get______________
a) Yellow
b) Green
c) White
d) You can get all
View Answer

Answer: c
Explanation: In both the parents in this case the W locus must have homozygous recessive gene. Thus, the cross can never give a heterozygous case in W locus which is needed for development of white colour.

15. In case of summer squash which of the following is not right.
a) The W allele produces a White substance
b) A white substance is converted to Yellow when dominant Y allele is present
c) The transformation of white substance to yellow is via green intermediate
d) The green intermediate has combination at the Y locus y/y
View Answer

Answer: a
Explanation: In this case there is a white substance initially produced which is converted to yellow via green intermediate. Y allele promotes the green to yellow conversion. White the W allele blocks the white to green conversion so further pathway is also blocked giving white summer squash.

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