Principles of Genetics and Breeding

13.1.9.3.Combined selection

Within family selection and between families selection can be combined in a single program where best individuals of the best families can be bred together. The advantage of this method is that the additive genetic variation between and within the families can be exploited to have a better selection response. This form of selection is mostly used when the main differences are within rather than between families.

• An example would be different families maintained at different water temperatures, salinities or other environmental variables. The differences between families would be environmental but genetic within a family.
• Using this technique inbreeding will also be reduced as all the families will contribute offspring to the next generation rather than just the best ones.
• To increase the efficiency of family selection and to offset some of the extra costs in following this regime a combination of individual and family selection is frequently used.
• This is the technique which underlines the Norwegian and Atlantic salmon and rainbow trout and Nile tilapia, Oreochromis niloticus (Genetic Improvement of Farmed Tilapia, GIFT) programmes .

Table 3 . Comparison of advantages and disadvantages of individual selection, within-family selection, and between-family selection.

Type of selection	Strategy	Advantages	Disadvantages
Individual	Choose best individuals; family relationships not important.	Best when h2≥0.25; inexpensive;can be done in few ponds; relatively easy to use for 2 or 3 phenotypes; all select fish are the largest; easy to retain large breeding population; least amount of data needed; least amount of record keeping.	Ineffective when h2≤0.15 as VE makes it difficult to choose best fish; asynchronous spawning can cause problems.
Within-family	Choose best individuals with in each family.	Best when h2≤:0.15 and VE affects family more than individuals; can be used with asynchronous spawning; moderately easy to maintain large breeding population; less expensive than between-family.	Moderately expensive; requires many ponds; hard to incorporate 2 or 3phenotypes; small fish can become select brood fish. Requires lots of data and lots of record keeping.
Between-family	Choose best families based on family means; individual values not considered.	Best when h2≤0.15 and VE affects individuals more than families; can because when fish must be killed.	Very expensive; requires many ponds; hard to incorporate 2 or 3phenotypes; small fish can become select brood fish; can lead to high levels of inbreeding. Requires lots of data and lots of record keeping.