Principles of Genetics and Breeding

11.1.2 Qualitative traits

• Qualitative traits can be defined by simple discrete categories (e.g. colour variants) and are often under the control of just one or two loci, while quantitative trait require measurement or enumeration and are usually controlled by many genes (e.g. weight at age, numbers of fin rays).
• In selecting for simply-inherited traits, the breeder’s or hatchery manager's task is to identify genotypes of individuals for loci of interest and select those individuals with the most favourable genotypes.
• In other words, the breeder’s job in selecting for polygenic traits is to identify breeding values of individuals for traits of importance and to select those individuals with the best breeding values.
• Qualitative phenotypes are the easiest to observe simply because an individual falls either into one discrete, descriptive, non-overlapping category or another. There is no gradation.
• Examples of such phenotypes are albino vs normally pigmented channel catfish, blue vs normally pigmented common carp, veil tail vs round tail guppies, saddleback vs normal Tilapia aurea, spotted vs unspotted platyfish, line vs leather vs mirror vs scaled common carp and black vs red tilapia.
• Because individuals fall into discrete categories, the distribution of individuals in these categories forms certain ratios, e.g., 3:1 ; 1:0 ; 1:1 ; 9:7 ; 9:3:3:1 ; etc.
• The genetics of qualitative phenotypes is simple. It is often called “Mendelian genetics ” in honour of Gregor Mendel who discovered it. These phenotypes are usually controlled by one or two genes. The alternate forms of a phenotype (for example, blue vs yellow) are produced by the alternate forms of a gene (alleles). Often, the normal phenotype is called the “common” or “wild-type” phenotype, while the others are referred to as “mutant” phenotypes.
• Qualitative phenotypes can also greatly increase the value of farmed fish. For example, in the United States, body colour of fathead minnow has a major effect on its market price. Similarly, the Red tilapia are sold for higher price than black or grey coloured.
• Fish farmers need only to look at the ornamental fish farming industry to realize the importance of qualitative phenotypes. The value of an ornamental fish is determined by its colour, colour pattern, fin shape, eye shape, etc.
• Qualitative phenotypes can be divided into two major categories : autosomal and sex-linked.
• Autosomal phenotypes are those that are controlled by genes located on an autosome (a chromosome other than a sex chromosome). Autosomal genes are inherited and expressed identically in males and females (unless a sex hormone is needed for phenotypic expression). To date, all qualitative phenotypes that have been deciphered in fish are autosomal.
• Sex-linked phenotypes are controlled by genes located on the pair of chromosomes that determine sex. Sex-linked genes are inherited and expressed differently in males and females. Sex-linked genes are known only in ornamental fish and most information about this type of inheritance comes from guppy and platyfish.