II. Sex-Linked Genes: Dark - Blond - White
A sex-linked gene is any gene located on the sex chromosome or the "Z" chromosome. The male ringneck has two Z chromosomes. Females have only one Z plus a "W" (W means no chromosome). There are three genes (alleles) located on the sex chromosome. An allele is an alternative form of a gene pair. The three alleles (alternative genes) located on the sex chromosome are dark, blond, or white. The wild type or normal is the color standard of reference in the Ringneck Dove (Streptopelia risoria). This color is a dark gray with a violet head and breast. Blond can be thought of as a dilute and white as an extreme dilute of the normal color.
Dark is dominant to both blond and white. Blond is dominant to white. Any two of the three alleles can be carried on the two Z chromosomes of the male. The dark male may carry two dark alleles and is therefore homozygous dark or he may carry a dark and a blond or white allele and is therefore heterozygous dark. The dark female will carry only the dark allele on their single Z chromosome. The single allele condition is referred to as hemizygous in contrast to the homozygous or heterozygous possibilities of the male.
When the egg is fertilized, the embryo receives 1/2 of its genetic makeup from the male and 1/2 from the female. During fertilization, one sex chromosome from the male is combined with one sex chromosome from the female. Because each bird has two chromosomes, which make up the chromosomal pair, there will be four possible outcomes from any mating.
Because dark is the dominant allele, a homozygous dark male mated to a dark or blond or white female will produce only dark offspring. The male offspring from the homozygous dark male mated to the blond or white will be heterozygous dark (they carry either blond or white). A homozygous blond male mated to a blond or white female will produce only blond offspring with the male offspring from the blond male to white female being heterozygous blond (carrying white).
A useful tool (because it is depicted graphically) to determine the results when using a dark heterozygous male mated to a female of any of the three sex-linked colors is the Punnet Square. First, let's establish the gene symbols we will be using.
D+ = Dark or wild type or normal
dB = Blond (recessive to D+)
dw = White (recessive to D+ or dB)
W = W chromosome (which signifies a female and lacks the "d" locus)
The sex-linked locus is called the "d" locus because of the "dilution" effect of the two mutants, blond and white. The upper case letters indicate a degree of dominance. The lower case letters are used for a recessive gene. Superscript letters are used to identify the multiple alleles (D+ for dark, dB for blond, dw for white).
Using these symbols the genotype for a dark, heterozygous for white male is D+//dw
The genotype for a blond female is dB//W. The following example is the Punnett Square for the "d" locus when these two birds are paired.
Here's the Punnet Square for the "d" locus. We will place the male gametes across the top horizontal of the square and the female gamete down the left vertical of the square.
Offspring Genotypes and Phenotypes
1/4 D+//dB = Dark (heterozygous blond) male
1/4 dB//dw = Blond (heterozygous white) male
1/4 D+//W = Dark female
1/4 dw//W = White female
From these results, you see that females show the sex-linked gene they receive from their father. Selective breeding then allows you to determine the sex at hatching. A dark (or blond) female mated to a white male will have white daughters and dark (or blond) sons. Likewise, a dark female mated to a blond male or (white male) will have dark sons and blond (or white) daughters.
For a more detailed and scientific discussion of the sex-linked genes, refer to Dr. Wilmer J. Miller's website (www.ringneckdove.com)
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