In the 1920s, Arthur Boycott discovered the first maternal effect phenotype in t

Question

In the 1920s, Arthur Boycott discovered the first maternal effect phenotype in the snail Linnaea peregra. Linnaea 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) Limmaea 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

a) Parent generation= pure breeding sinistral female x pure breeding dextral male
dd DD
F1= Sinistral (Dd)
(Since the mother is sinistral and the trait is maternal)

b) Parent generation= pure breeding sinistral male x pure breeding dextral female
dd DD
F1= All dextral Dd
     (Since the mother is dextral and the trait is maternal)

c) F2 from a

Sinistral male (Dd) x Sinistral female (Dd)

All dextral
Since the genotype of female parent carries the dominant (D) allele; So all progeny is dextral only
F2 from b
dextral female (Dd) x Dextral male (Dd)

All dextral (Since the mother has one dominant allele)
d) F3 from a
3 dextral: 1 sinistral (since one of the mother is "dd")

F3 from b
3 dextral: 1 sinistral (Since one of the parent has "dd")

D d D DD
Dextral Dd
Dextral d Dd
Dextral dd
Dextral

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