Question

Shell orientation in snails is due to a maternal effect gene. A true breeding sinistral (recessive) is crossed to a true breeding Dextral (dominant). The offspring from that cross are self-crossed. What will be the expected ratio of shell types?
A. All sinistral
B. All Dextral
C. Half sinistral, half Dextral
D. 3/4 Dextral, 1/4 sinistral
E. 3/4 sinistral, 1/4 Dextral

Answer 1

Sinistral and dextral are the two types of relative direction.

If the opening is on the left then the shell (Limnaea peregra) is left-handed, or 'sinistral'.

In most snails, shells are dextral .

dextral : on the right side or the right hand.

One of the classical examples of detailed relationship between maternal genotype and egg cytoplasm “phenotype” was studied in snails .

Looking into the opening of the shell it can be seen that in one strain the shell always coils to the left (sinistral) whereas in the other shell always coils to the right (dextral).

In the cross dextral (female) x sinistral (male) all the F, progeny have dextial coils that implies dextral is dominant over sinistral. However, in the F1 x F1 cross (i.e., inbreeding) all the F1 snails are also dextral. The reciprocal cross ( dextral (female)x sinistral (male)) produces F1 progeny that are all left coiler. In this case F1 x F1 cross also yields only dextral coils.

it is clear that coiling of snails is not determined by individuals own genes but by those of mother. The coiling is phenotype in the offspring is controlled by the genotype of the mother.

In simple words ,

when dextral female is crossed to a sinistral male : the result will be all dextral.

when sinistral female is crossed to a dextral male :the result will be all sinistral.