Re: genetics questions ...

[Stuart Hall <sturob@swbell.net>]

>If a characteristic such as tail eyespots is sex-linked then the female
>carries a gene on the x-chromosome which determines tail ornamentation. In
>females the presence of two of these genes on the two X chromosomes means
>that the tail is clear of tail spots. In males the same gene on its  X
>chromosome will be unpaired because the Y chromosome is basically an empty
>chromosome ( i.e. they are hemizygous for the tail ornamentation gene ). So
>when this combination is expressed it is read to code for eyespots instead
>of a clear tail. So X+X+ = clear tail ( + symbols meant to be superscripted
>and do not reflect an addition of X to X but rather the tail ornamentation
>gene located on the X chromosome ) and X+Y = spotted tail and because the X-
>gene does not exist it is not possible for females to possess tail eye
>spots.

Not exactly.  The problem with your description is that if your gene 
X+ gives a clear tail, then males should all have clear tails.  The 
X-linked situation you're going for would be as follows:

X+ = clear tail, dominant		X- = spotted tail, recessive

X+X+, X+X-  ==> female fish with clear tail

X+Y	    ==> male fish with clear tail

X-Y         ==> *male fish with spotted tail

X-Y-        ==> female fish with a spotted tail

If it's purely a sex-linked trait, then you'd have to have some 
recessive gene for spots which is only expressed in the male because 
the Y couldn't contain the dominant gene for no spots.  I don't think 
this is the situation we're talking about, or there'd be a lot of 
males with clear tails, and a few females without tails.

>3. If tail eye-spots are a sex-linked gene, does that mean that the choice
>of females used in a breeding program to increase the number of tail spots
>is irrelevant or could the number of tail spots be determined by a gene
>which is not located on an autosome ?

It's more likely to be a gene which needs a male hormone (i.e. 
testosterone) for expression.  Young juvenile fish may not have any 
spots, but as they age, the males develop tail spots.  That would 
imply, if every male has them, that whatever's responsible for spot 
formation would need something present in males which is absent in 
females.

But, the fish situation presents certain problems:

(1)  Sex determination in fish may or may not be solely due to 
genetics: there are many reported cases of sex changing in the family 
Cichlidae.  For example, if you've got a trio of Crenicara 
punctulatum, a male and two, spawning females, and the male dies, one 
of the females will become a male.  This is not a case of a 
subdominant male assuming female dress, but actually being a 
functioning female, then a functioning male.  Hermaphrodism is also 
not uncommon in saltwater fish.

(2)  Things like coloration often have several genes (like eye color 
in humans), so it may be even more complicated.

Don't forget that, in humans, the Y chromosome is far from empty.  It 
contains the gene for the testosterone receptor.  There are 
phenotypic (apparent) females in this world who are actually men (XY) 
without a receptor for testosterone.



Stuart
-- 
Stuart Hall
(sturob@swbell.net)
(gasdocstu@my-deja.com)


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