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Qualitative Inheritance in Poultry: Sex-Linked Traits and Auto-Sexing
Qualitative inheritance in poultry involves traits controlled by specific genes, including several sex-linked characteristics of economic importance. These traits, such as plumage color, barring, feathering rate, and sex-linked dwarfism, have important applications in poultry breeding, particularly in auto-sexing and the development of specialized breeding lines.
Many characters that fowl possess are inherited equally from the sire and dam. The genes responsible for such characters are carried on the numerous pairs of chromosomes common to both sexes, with half of the autosomes being of paternal origin and half of maternal origin in an individual chicken.
There are certain characters, however, that are transmitted from the dam to the sons only and not to her daughters, although some characters are transmitted from the sire to both sons and daughters alike. These characters that are transmitted from the dam to the sons only are called sex-linked characters because the genes that determine such characters are located on the sex chromosomes.
These phenomena have been successfully employed to identify the sex of day-old chicks through specific mating. Breeds that facilitate such mating are called auto-sexing breeds. This has been widely utilized in commercial poultry breeding.
Before the advent of vent sexing, some sex-linked genes influencing feather pattern or color were successfully employed to identify the sex of day-old chicks.
Sex-linked characters are those characters that are carried through the sex chromosomes or “Z” chromosomes; i.e., the genes responsible for these traits are located on the “Z” chromosomes. Since males are homogametic for the “Z” chromosome, they have “ZZ” chromosomes.
Hence, the genes for these traits will also have diploid numbers. Whereas females are heterogametic and have only one “Z” chromosome. Therefore, the genes for these traits will be present only in single copies (haploid). Due to this peculiarity and the differences between the sexes, the phenotypic expression of these traits varies with sex.
The Z chromosome is the 5th largest chromosome and carries several thousand genes. All these genes are responsible for sex-linked characters. Important sex-linked traits or characters that are of economic importance in commercial poultry breeding are:
- Silver and golden plumage
- Barring and non-barring
- Slow and fast feathering
- Sex-linked dwarfism
Of these four traits, the first three are used for auto-sexing of day-old chicks and hence are also called auto-sexing traits, while the fourth is employed for developing dwarf dam lines in broilers.
Golden and Silver Plumage
Silver plumage in the Light Sussex breed is due to a dominant gene “S”, while golden plumage is due to its recessive allele “s” in Rhode Island Red and New Hampshire. In auto-sexing, the female line should possess the dominant gene (in haploid, hemizygous form), and the male line should possess the recessive gene (in diploid, homozygous form). If the opposite cross is carried out, all progeny will possess the dominant character of the male, and the males will be homozygous dominant. If the male parent is heterozygous dominant, 50% of both male and female offspring will exhibit the dominant character, while the remaining 50% will exhibit the recessive character.
Barred and Non-Barred
Barring (B) is dominant over non-barring. This gene is present on the sex chromosomes. There is also an additive effect in barred males (BB), which have wider bars than (BW) females.
Slow Feathering and Fast Feathering
The slow-feathering gene (late feathering) (K) is dominant over the fast-feathering gene (early feathering) (k), which is found in White Leghorn and White Plymouth Rock (WLH and WPR).
- Fast or early feathering: The primaries are longer and the coverts are shorter.
- Slow or late feathering: The primaries are shorter and the coverts are equal in length.
All the above sex-linked traits are passed from the dam to the male progeny and from the sire to the female progeny in the F1 generation in a criss-cross manner. Hence, this type of inheritance is also called criss-cross inheritance.
Sex-Linked Dwarfism
In broiler breeding, the gene dw for sex-linked dwarfism, present on the “Z” chromosome, is utilized to reduce the cost of maintaining the female parent stock. The female parent stock will be about 40% smaller in body weight, occupy 40% less space in the breeder house, and consume 40% less feed, but without any loss in hatching egg production. Hence, the offspring produced from the dwarf dam line are phenotypically of normal size, and there is no economic loss to the breeder.
Unlike in the case of auto-sexing, in sex-linked dwarfism, the sire must carry the dominant genes and the dams the recessive genes because the recessive effect must not be expressed phenotypically in the F1 generation.
