Carp genetics

[Snape]

I originally posted this on the Redmire forum but thought I would stick up here as well (sorry if it's a bit heavy going).

The question is why are Redmire carp mostly commons, given that the original stocking was 50% commons and 50% mirrors?

Carp genetics is complicated stuff, but I rather like Dick Walker's, now known to be flawed and simplistic, theory of scaling in 1952:
He said there are 3 types of king carp: mirror, leather and common.
Crosses give rise to 3 other types giving 6 in total.
1. Leather - scaleless
2. Mirror - 2 lines of scales along flanks
3. Common - all scaled in regular pattern
4. Leather x Common - odd groups of scales
5. Leather x Mirror - 1 line of scales along flanks
6. Common x Mirror - covered with irregular scales

If this were the case then by now the Redmire carp should be a pretty even mixture of all 6 varieties.

In fact scaling is controlled by 2 genes called S (for scale) and N (for number). There are 2 versions (called alleles) of each gene: S/s and N/n. Each individual will have 2 of each gene and its scale pattern (phenotype) is determined by the combination (genotype).
This gives rise to the 6 possibilities:
S is 'normal' scaling and s is 'mirror' scaling. N reduces the extent of scaling, n has no effect. NN is fatal.
Thus:
SSnn = common carp no. 3 above
Ssnn = fully scaled mirror no. 6 above
ssnn = mirror carp no. 4 above
SSNn = linear carp (1 row of scales) no. 5 above
SsNn = linear mirror carp (1 row of scales and some other mirror type scales) no. 2 above
ssNn = leather carp no. 1 above.

SSNN, SsNN, ssNN - all die on hatching

Walker was wrong saying there are 3 types of King carp because there are only 2 types which will breed true (homozygous): commons and mirrors (Walker's no. 4) If you had a lake full of just commons (SSnn), you would only ever get commons (apart from chance mutations) and if you had a lake full of only type 4 mirrors (ssnn) you would only get mirrors. But as soon as you interbreed commons and mirrors or have any linears, fully scaled or leather's it all mixes up.

With all these combinations why are there still so many commons in Redmire? Shouldn't there be a substantial number of linears, mirrors, fully scaled and leathers with commons being no more abundant?
No - commons will always be the majority because the original stocking was 50% commons and 50% mirrors and the mirrors would probably been a mixture of Walker's type 2,4 & 5. If type 5 was considered a mirror at the time then may be they formed a substantial proportion. But type 5 linears are actually not mirrors as they have no s allele, only S; type 2 linears have one S and one s allele; Type 4 are the only true mirror (ssnn) and they would presumably have only formed a maximum of one third of the mirrors put in (1/6 of the total). This means there are far more S alleles present in the gene pool rather than s. At least 75% (and possibly quite a lot more if the majority of the mirrors were type 5) of the scale alleles would have been S even though Leney put in 50% mirrors, and so commons are more likely as they are SSnn. But what about the N? The linears put in would have all had N alleles but because none of them could have had NN (fatal combination) and the commons (50% of the total) and true mirrors had to be nn this means only 1/6 of the N/n alleles would be N so there would not be so many after all (may be 17%).
This means at least 75% of the S/s alleles in the pool are S and 83% of the N/n alleles are n. So the combination SSnn (common carp) will occur frequently.
Also the presence of the N allele affects viability and development (many fish carrying the N allele do not survive their first winter) and so linears and leathers which have to have it, are disadvantaged (commercial fisheries, especially if growing carp for food as in Eastern Europe, avoid strains carrying the N allele).
So why are linears the second most abundant type over fully-scaled, mirrors and leathers? Because there are 2 ways of getting a linear: SSNn and SsNn but only one way of getting each of the others. So there should be more of them.

And after all that I still think I prefer Walker's version.

Having done a 'back of the envelope' calculation it seems that if we take it that about 75% of the S/s alleles are S and 83% of the N/n alleles are n then we get about:

44% of the fish are true commons with another 22% being fully scaled mirrors - so 2/3 of Redmire fish should be fully scaled.
This leaves 25% as linears (both varieties) but since the N allele is a disadvantage quite a few will die before their first winter.
Only 3% are true mirrors but as they do not have the N allele they are not at a disadvantage.
3% are leathers and they also have the N allele so quite a few die when very young.
3% die on hatching due to having NN.

This seems to fit the observation of the majority of Redmire carp being commons or common-like fully scaled (many fully-scaled mirrors look very similar to commons), linears come next and fare pretty well, but not too many mirrors and very few leathers.

You may well be thinking "hang on, 22% of Redmire fish aren't fully-scaled mirrors". But fully-scaled mirrors can look the same as commons.
See here:
Photo of Hungarian fully-scaled mirror Ssnn
http://www.fao.org/docrep/005/Y2406E/y2406e0b.jpg
Info about it
http://www.fao.org/docrep/005/Y2406E/y2 ... htm#bm3.12

Photo of similar strain but common variety SSnn
http://www.fao.org/docrep/005/Y2406E/y2406e0c.jpg
Info about it
http://www.fao.org/docrep/005/Y2406E/y2 ... htm#bm3.13

Alternatively, as we see relatively few distinctive fully-scaled mirrors come out, it might suggest that the original stock of mirrors had a high proportion of the Walker type 5 fish which are SSNn so there are even fewer s alleles in the gene pool and so fewer fully scaled mirrors would be produced but still the same number of linears (although a higher proportion of SSNn over SsNn)


Two common carp cannot produce a mirror as they would have to be SSnn and by crossing SSnn with SSnn you the only combinations are SSnn so common carp breed true. However, if some of the 90% common carp are actually fully scaled mirrors but they look exactly like the common (see the earlier photos) then you can get mixtures.
Two Ssnn fish will both be fully scaled but they carry the mirror gene.
When crossed they produce:
1/4 commons SSnn, 1/2 fully scaled mirrors Ssnn and 1/4 mirrors ssnn.

Leathers can only occur if one parent is a linear SSNn or SsNn.
two double linears (SsNn) will produce a real mixture though:
1/4 die on hatching, 1/8 single linear, 1/4 double linear, 1/16 mirror, 1/8 leather, 1/16 common, 1/8 fully scaled mirror.

and so on.

A lake with nearly all mirrors but some of the others is possible if there were enough mirrors and leathers at the original stocking. To be a mirror the fish has to be ssnn. If a S or N is present it throws it out. This is a bit like blue eyes. To be blue eyed you have to have both blue eye alleles, if you have just one other colour you can't be blue eyed. This would normally make being blue eyed rare (as in Asia) but in Northern Europe there are so many blue eyed genes it becomes the norm.

There you go.

Test next week!

[Mark]

Thanks for that Snape. :thumb:

[The Sweetcorn Kid]

Excellent Snape.....



....Test!!!??? :think:

[Snape]

....Test!!!??? :think:

Yes SK. You WERE paying attention at the back there WEREN'T you?!

[Mark]

What test Snape, did he pass?

[The Sweetcorn Kid]

....Test!!!??? :think:

Yes SK. You WERE paying attention at the back there WEREN'T you?!

Loud and clear sir!!! :thumb:

[St.John]

My head hurts. Cheers teach.

[StalkingLuke]

Thanks Snape I think I understand but will look again after a nights sleep!

[MadScientist]

don't you just love mendelian genetics?

[PondPikey]

Possibly the most informative forum post ever posted in the history of the internet!

Very good snape! Encore!