In the 1920s. Arthur Boycott discovered the first maternal effect phenotype in t
ID: 79591 • Letter: I
Question
In the 1920s. Arthur Boycott discovered the first maternal effect phenotype in the snail Limnaea peregra. Limnaea has a diploid genome. Like most gastropods, the shell and internal organs normally coil to the right, a phenotype described as dextral. Boycott found a mutant whose body was mirror-image symmetrical to wildtype. The mutant coiled to the left a phenotype called sinistral. Chirality has been shown to be controlled by an autosomal maternal effect gene called s. The dominant s^+ allele causes dextrality and the recessive, loss-of-function s^- causes sinistrality. (a) Limnaea are either hermaphrodites or males; the hermaphrodites can self-fertilize or mate with males. If a pure-breeding line of sinistral Limnaea hermaphrodites are crossed with males from a pure-breeding dextral line. What will be the phenotype of the progeny? (b) Answer part (a) for a cross performed in opposite direction: dextral hermaphrodites crossed to sinistral males. (c) Describe the progeny phenotypes if the F_1 from part (a) self-fertilize. Do the same for the F_1 from part (b). (d) If you now self the F_2 from parts (a) and (b). What results would you expect in the F_3 in each case?Explanation / Answer
1. a) Parents Sinistral female (SS) x Dextral males (S+S+)
gametes S S+
F1 Sinistral (S+S)
F2 1 (S+S+) : 2 (S+S) : 1 (SS) All of them are Destral
F3 Dextral (S+S+) ; 1 (S+S+) : 2 (S+S) : 1 (SS) All Dextral ; Sinistal (SS)
b) Parents Dextral female (S+S+) x Sinistral male (SS)
gametes S+ S
F1 Sinistral (S+S)
F2 1 (S+S+) : 2 (S+S) : 1 (SS) All of them are Destral
F3 Dextral (S+S+) ; 1 (S+S+) : 2 (S+S) : 1 (SS) All Dextral ; Sinistal (SS)
2. a) Limnaea is unisexual organism. Sexex are separate. If pure breed of sinistral female is crossed with pure breed of dextral males, the F1 progeny would be sinistral. (as shown in 1a flow chart)
b) In the reciprocal cross (dextral female is crossed with sinistral females, the F1 progeny would be destral (as shown in 1b flow chart)
c) If sinistral F1 is self fertilised, all the F2 generation would be dextral. Similar dextral F1 is self fertilized all the F2 generation would be dextral.
d) If F2 generation of reciprocal cross is self fertilised all the F3 generation would dextral, all dextral and sinistral in both the reciprocal cross.
3) a) The direction of shell is controlled by a single nuclear gene S+/S.
b) The segregation of nuclear gene S+S is normal and occurs in the F2 generation itself.
c) But phenotypic effet of segretation are visible only one generation after the actual segretation of alleles.
d) This is becuase the direction of coiling is determined by the plane or the direction of the first mitotic division of the zygote,
e) Direction of coiling is determined by some substances already present in the egg as it is produced by female parent.
f) So they would produce the phenotype appropriate for the maternal phenotype.
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