The Genetics of Snail Shell Coiling

or: Why Are Snail Kings So Rare?

Looking at the frequence, in which right handed (dextral) and left handed (sinistral) Helix pomatia shells occur in nature, it points into the direction of a simple dominant-recessive pattern of heredity. The sinistral form clearly is the recessive character or allele, suppressed in a heterozygote genotype by the dominant wild-type character, according to Gregor Mendel (1822 - 1884). Some crossing experiments, though, give rather unexpected results, which in no way explain the astonishingly small numbers of left handed Helix shells in nature, which amount to only one in several thousand right handed shells, giving them the colloquial name of a snail king.

Following a normal dominant-recessive pattern of heredity, only snails, that carry the recessive allele s for a left handed (sinistral) shell in a homozygote way, thus in the combination ss, s from both parents. Pairing two of those, the generation of their offspring (the so-called F1-Generation) had to be exclusively left-handed, as there is no way of a D (dextral) allele to appear. In the contrary, though, their offspring will be most probably, and almost exclusively, right handed:

		´

How can that be explained, basing on a dominant-recessive pattern of heredity?

The explanation has been found by the American geneticist Alfred Henry Sturtevant in 1923, using the pond snail species Radix labiata: It indeed is a dominant-recessive pattern, but a matrocline one. That means, the way a snail shell coils is influenced not by the respective snail's genotype (the combination of its alleles), but by that of its mother. Thos astonishing fact is caused by snails belonging to the animal group of Spiralia. Their fertilized cells in a spiral. This spiral's direction and thus finally the shell's coiling direction are determined by genetic information in the respective cell's cytoplasm. As the cell is an egg cell, it is the mother's genetic information, not the respective snail's own.

So here is the same crossing pattern, only this time also with each snail's genotype. (D: dominant, right handed (dextral); s: recessive left handed (sinistral)).

		´
	Ds		Ds


DD	Ds	Ds	ss	Ds ?

The fifth juvenile snail is outside of Mendel's counting, the relation between genotypes DD : Ds : ss usually being 1:2:1.

Thinking this experiment further, only one of those juveniles' own offspring, should it survive until maturity will be sinistral: It is the one framed red with the Genotype ss. By all probability, the character "left handed shell" would disappear again in the next generation, suppressed by the "father's" dominant genes. Besides only 5% of juvenile snails survive in nature, which provided another most prominent reason for the rareness of left-handed so-called snail-kings.

Here is a possible crossing pattern for one of numerous possible cases:

		´
	ss		Ds


Ds	Ds	ss	ss	?

I leave it to the reader to think about the further path the offspring of the blue framed snails would take, also for the possible genotype of the fifth snail in the last crossing pattern.

Snail kings - Left handed Helix snails.