AVIAN GENETICS

AVIAN GENETICS

SEX- LINKED GENES

A sex-linked gene is any gene located on the "X" chromosome. The sex-linked gene can be recessive, codominant, dominant, wild-type or mutant to other genes within the allelic group. A gene is a gene whether it exists on an autosomal chromosome or a sex chromosome: the structure and function are the same. Just like autosomal genes, sex-linked genes are made up of DNA. This DNA determines the protein to be produced for a biochemical process within the bird's body. The avian X chromosome is believed to contain about 10% of the genome or haploid genetic makeup. Sex-linked genes do not have an allelic counterpart on the "Y" chromosome; therefore, the female will carry only one allele for any sex-linked traits since the female only carries one X chromosome. This one-allelic condition is termed hemizygous in contrast to the homozygous and heterozygous possibilities in the male. The single sex-linked allele that is passed onto the female offspring from the male parent will be expressed by the female offspring. Again, only one sex-linked allele for a trait need be present in the female to be expressed. On the other hand, since the male has two X chromosomes, he will possess two genes for each trait and thus the rules of dominance, recessive and codominance will apply in determining the males phenotype for any sex-linked trait.

Some sex-linked genes for Avian coloration and pattern include:

Phenotype	Genetic Control	Symbol	Other Names given to Phenotype	Remarks
Cinnamon	Recessive	cin	Colored Canary: Brown
Opaline	Recessive	op	Cockatiel: Pearl Bourke's Parrot: Rose Eastern Rosella: Red Peachfaced Loverbird: Rose-headed Black-capped Conure: Yellowsided
Slate	Recessive	sl	Only found in Budgerigars so far
Lutino	Recessive	ino	Galah: Rubino Patagonian Conure: Yellow	Codominant with Lime & Platinum
Lime	Recessive	ino^l	Scarlet-chested Parrot: Isabel Elegant Parrot: Cinnamon Yellow Red-rumped Parrot: Yellow Swift Parrot: Yellow Peachfaced Lovebird: Australian Cinnamon Indian Ringnecked: Pallid	Codominant with Lutino & Platinum
Platinum	Recessive	ino^pl	All use "Platinum" term	Codominant with Lutino & Lime

When the egg is fertilized, the embryo receives 1/2 of its genetic makeup from the female and the other 1/2 from the male. Therefore, when calculating the probable outcome of a cross, only one sex chromosome from the male is combined with one sex chromosome from the female during fertilization. Since each bird has two chromosomes that make up the sex chromosomal pair, there will be four possible outcomes for any cross involving a single sex linked mutation.

To explain sex-linked genes further, I will show two different crosses. The first cross involves a male with the Cinnamon phenotype (that is, a visual Cinnamon plumage color). In order for the male to be a Cinnamon phenotype, the Cinnamon gene must be present on both sex chromosomes (X^cin,X^cin) - the homozygous recessive condition. If you have a Cinnamon male (X^cin,X^cin ) and cross it to a Cinnamon female (X^cin ,Y), all of the offspring will be Cinnamon. See Figure 8 below.

If the Cinnamon gene occurs on just one of the male's sex chromosomes (X^cin,X⁺) he will not display the Cinnamon trait. In this case, the male is heterozygous (that is, contains both the dominant (+) and recessive (cin) alleles). The phenotype for the heterozygous Cinnamon will be the normal wild-type. In Figure 9 below, a Normal Male heterozygous for Cinnamon (X^cin,X⁺) is crossed to a Normal Female (X⁺,Y) hemizygous for the wild-type gene at the Cinnamon locus. You will notice that not only does this produce four different types of offspring, but could also allow early sex determination since only half of the female offspring of this particular cross can display the Cinnamon trait.

<<< Previous

Next >>>

Return to Cabin Software Home Page