From: tittle@ics.uci.edu (Cindy Tittle Moore)
Subject: Re: Speaking of breeding...
Newsgroups: rec.pets.dogs
Reply-To: tittle@ics.uci.edu (Cindy Tittle Moore)
Organization: ICS Dept., UC Irvine
Date: 7 Jan 93 23:49:01 GMT

In <1993Jan7.112657.591@news.wesleyan.edu> RGINZBERG@eagle.wesleyan.edu (Ruth Ginzberg) writes:

>I have an acquaintance who breeds chocolate labs.  That is, she has
>three bitches, who are all chocolate labs, and she breeds them only
>to other chocolate labs.  But the puppies she gets are not all
>chocolates.  In fact, she fairly often gets a couple of yellows in a
>litter.  Could someone explain this in terms that make sense?  If
>'yellowness' is a recessive gene (? is it?)  couldn't it be bred out
>of a line of chocolates?  I'm confused.

You asked for it!  I'll try...

Let's just take yellow and black for a moment.  Yellow is a simple
recessive, black a dominant.  Each lab will have a pair of color
genes, the possible combinations are BB (black), BY or YB (black,
carries yellow), and YY (yellow).  You can get only black puppies if
you breed a BB lab to any other lab; you can get only yellow puppies
if YY and YY are bred to each other.  Other combinations yield both
possiblities.  I assume you understand this part.

But throw in chocolate and now you are talking about a polygenic
trait.  You now have two sets of genes, one set of which influences
the behavior of the other.  Call the first set A and it controls
whether a lab is black or not.  A is black, and a is chocolate and
recessive to A.

As for the second set, call it D, this controls color expression.  D
is dominant and results in color shown, d is not and means no color.

If D is present, then an AA dog is black, an Aa dog is a black dog
carrying chocolate, and an aa dog is chocolate.  If no D is present,
then the dog is yellow, regardless of the A/a combination.  The
combinations are:

	AADD	black only               (BB)
	AADd	black carrying yellow	 (BY)

	AaDD	black carrying chocolate
	AaDd	black carrying chocolate and yellow

        aaDD    chocolate only
	aaDd	chocolate carrying yellow

        AAdd	yellow carrying black
        Aadd	yellow carrying black and chocolate
        aadd	yellow carrying chocolate

      The *only* hard and fast rule of thumb about color combinations
is that when both parents are yellow, they can *only* produce yellows.
This is because a yellow dog has a double recessive at the second
locus, dd.  This means it does *not* have the ability to produce the
dark pigment, neither black or chocolate, regardless of what it is
carrying.  YOu have to have a D for the possibility of a black or
chocolate.

If your dogs have only AADD, AADd, AAdd, then that's the classic
yellow/black I first described.  If your chocolates were all aaDD, you
wouldn't get anything else.  But it's hard to determine which
chocolates are this ;-).  Plus, the sentiment among lab breeders is
that you get the best chocolate colors when you use both chocolates
and blacks in your breeding programs.  You especially want to avoid
the aadd combination because that gives you the dudley nose.

--Cindy

--
Cindy Tittle Moore

Terrell's Chocolate Deduction--Hershe (Labrador Retriever)

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