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|Excerpt from Chapter 2
The Golden Rules of Color Genetics Inheritance
This chapter will cover the modes of inheritance in cockatiels and will explain some of the ground rules
to keep in mind. You may want to periodically re-read this chapter or refer back to the following "golden
rules" whenever you make a connection between them and the chapter your are currently reading.
1) All living organisms, including birds, are made up of cells. Chromosomes are bodies of thread-like
structures, which form in the nucleus of a cell when it begins to divide. The chromosomes carry the
genes which determine a bird's hereditary characteristics or traits.
2) All chromosomes travel in pairs, and in most cases are identical, with the exception of the sex
chromosomes. The sex chromosomes, which determine the sex of the individual, are identical in the
cock but differ in the hen. That is, male cockatiels always have two "X" chromosome (XX) and female
cockatiels always have an "X" and a "Y" chromosome (XY).
3) Each offspring inherits one sex chromosome from each parent. The sire will always donate an "X'
from his "XX" sex chromosomes. The dam will donate either an "X" or a "Y" from her "XY" sex
chromosomes (with a 50/50 possibility of either). If the sire's "X" unites with the dam's "X" to form "XX"
they will produce a cock. If the sire's "X" instead unites with the dam's "Y" to form "XY" they will produce a
hen. Therefore, in birds, it is the female who actually determines the sex of the offspring by whichever
chromosome she donates to the union.
4) Sex-linked colors are the only mutations to be found on the sex chromosomes. Furthermore, the
sex-linked mutations may only be found on the "X" sex chromosomes. The "Y" sex chromosome, being
too small, is unable to carry sex-linked color mutations.
5) In males, sex-linked mutations may be carried in either single or double quantities. If a sex-linked
mutation is carried on both "X" chromosomes (double quantity), the bird will be visual for that sex-linked
mutation. That is, the sex-linked mutation must be carried on both "X" chromosomes in the male for him
to visually show the mutation.
|(Complete and Unabridged)
GOLDEN RULES FOR SEX-LINKED MUTATIONS:
|6) If the sex-linked mutation is carried on only one "X" chromosome, (i.e. single quanity), the male is
merely SPLIT and carries the mutation in hidden form.
|Explanation: in order to be visual, the mutation must be in double quantity or found on BOTH male "X" chromosomes. If only one male "X" chromosome of a pair (XX) is
affected by a single quantity, the remaining "X" chromosome serves to hide or mask the "X" which carries the mutation. The male with this single quantity is called
heterozygous, or as aviculturists prefer to call it, "split."
7) Females (XY) need only carry the mutation of their one "X" chromosome for the mutation to be visual.
Explanation: A gene on the "X" chromosome cannot be covered up and MUST show itself, since the "Y" chromosome is UNABLE to hide or mask it. This is why females
(XY) show sex-linked recessive colors more frequently. As the female (XY) only has ONE "X" sex chromosome, there is simply no other "X" chromosome to mask the "X"
carrying the mutation. Therefore:
A HEN CAN NEVER BE SPLIT TO SEX-LINKED MUTATION! The motto for hens when dealing with SEX-LINKED mutations is: "What you see is what you get!"
Consider the following: A breeder offers to sell you a Normal Grey hen split to Albino. Should you pay the exorbitant price? Absolutely not! Why? Because we know that the
only Albino mutation in cockatiels, at the moment, is a cross mutation called Whiteface Lutino. While a hen can be split to autosomal recessive colors (which we will cover
shortly), we now know that a HEN CAN NEVER BE SPLIT TO SEX-LINKED MUTATIONS, AND LUTINO IS A SEX-LINKED MUTATION! While the hen in question may be split
Whiteface, she certainly is not carrying the Lutino gene in hidden form. Why? Because she either shows a sex-linked mutation, or she does not carry it. (Remember, her
"Y" chromosome of her "XY" pairing cannot carry sex-linked colors OR mask her "X." Therefore, "what you see, is what you get")!
8) Sex-linked mutations are carried on the "X" sex chromosomes and the color or pattern mutation of the young produced from pairings of Normal Grey x mutation, or from
one mutation x another mutation, will be dependent upon which parent is male and which is female. Two easy rules to remember are listed in the statements below.
9) In sex-linked mutations, the sire will always throw DAUGHTERS of the same mutation(s) as himself, and all SONS will be split for his mutation(s).
10) In sex-linked mutations, the dam will always throw SONS split for her mutations. However, her daughters will remain unaffected.
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