Saturday, May 14, 2011

To Inbreed, line-breed, or outcross?

Before you go on reading this article, please also read ‘Introduction to Mendelian Genetics’ if you are not already familiar with genetics.


As breeders, novice or expert level, our goal in breeding rats is more than just ‘to reproduce’. You might want to eliminate a bad trait, to enhance a desired trait in your rats, or simply to breed for health. Let’s try to understand the three breeding methods, and determine which one would be the best choice for your breeding goal.


Inbreed – to breed closely related pairs (e.g. parent-offspring pair, sibling pair)

Line breed – to breed a pair that are not closely related, but within a family tree or lineage (e.g. nephew and aunt)

Outcross – to breed with an unrelated mate


Look at the graph below. You decided to invest in a pair of rats (preferably with good traits) as the foundation parents. From these pair, you would later on form your own line and open up possibilities to introduce new traits.

Once a family tree (line) has been formed, you can decide to:

  • breed an offspring back to its parent (inbreed) to enhance a nice trait
  • make a test inbreeding, to reveal any defects or recessive traits within the line; 
  • breed rats within the family tree, to introduce minute improvements (line breed); or 
  • breed an unrelated pair to introduce a new trait, which is currently not found within the line (outcross)


Here is a summary of the three breeding methods, what they can do and what to expect.


  • It reveals any good or bad recessive trait that is present in the line. 
  • It helps the breeder to decide whether or not to discontinue the line. 
  • If the inbreeding pair produces defects, that is because inbreeding reveals any recessive bad trait that was already present in the line. Your wisest option is to discontinue breeding the line. 
  • Genes are uniform, no variations are introduced; the offspring are essentially identical copies of their predecessors, even after hundreds of generations. 
  • Never breed parents that have the same flaw! (e.g. both hooded parents have bad saddle/spine) 
  • Do not breed a pair where only one good trait is present but other flaws are present! (e.g. berkshire rat has solid deep black color, but the white marking on its belly extends wildly up to the side) 
  • Remember to breed the best pair when it comes to inbreeding. 

Line breeding 

  • Rats that are not closely related are bred. (e.g. aunt and nephew) 
  • Does not introduce new traits, like in outcrossing. 
  • There is not much genetic variation, but is done to make some minor corrections on flaws
  • Parents with nice traits are picked to produce, more or less, offspring with nice traits, without too much variation. 


  • Introduces new traits into the line (e.g. color dilution) 
  • If the line needs improvement on some traits (e.g. color) 
  • Some problems may be introduced as a ‘trojan’ into the offspring if one of the outcross parents has a recessive bad trait. 
  • If the breeder does not determine any bad traits first through inbreeding, these bad genes may be passed on without being detected, only to re-surface after many generations. 


The breeding method to use will depend on your goal. Is it to enhance a good trait (inbreed), to reveal recessive traits (inbreed), to make improvements on the line (linebreeding), or to introduce new traits (outcrossing)?

The absolute rules that apply to all is that you should breed the best pair as much as possible, and discontinue breeding those that show defects. The end result is a happy rat, with good health and longevity!

Please read further about this topic by clicking here: "To inbreed, line breed, or outcross: Some scenarios"

Check out my photo album for 

Brooks, E. (2005) Outcrossing, linebreeding and inbreeding in Rats (An Introduction). Spoiled Ratten Rattery. Retrieved May 12, 2011 from 

Grant, K. (2006 March 26) Breeding methods. Rat breeding guide. Retrieved May 12, 2011 from

Mcckail, Y. Inbreeding, Linebreeding & Outcrossing. Carawatha Rodents. Retrieved May 12, 2011 from 

Royer, N. & Robbins, K. (2008) Beyond the Basics: Outcrossing, Line Breeding, Inbreeding. AFRMA: Breeding. Retrieved May 12, 2011 from

Schwartz, M. & Mills, L. (2004 February 10) Gene flow after inbreeding leads to higher survival in deer mice. Retrieved May 12, 2011 from
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