Aussie Finch Forum

Hi all,

Can someone explain what the term 'single factor' actually means as far as genetics in birds are concerned? In laymen's terms please.

It applies to one mutation only - European yellow, also called yellow back. This yellow is not the same as Australian yellow.

The gene that causes the mutation is located on the W chromosome. A cock bird has two Zs, a hen has one W and one Z. The gene is co-dominant which means one yellowback gene is equivalent to one normal (green gene). Lets give the gene Y for yellow G for green. Because the genes are co-dominant a bird that has one of each looks somewhere between normal and yellow, it needs two copies of the gene to lose the dark (green and black) colours.

A cock bird can have (only) one of the following combinations:
GG. This bird is normal coloured. Technically it is 0 factor but we don't say that.
GY (or YG, the same thing just spelt differently). This bird is a single factor bird as it has one yellow gene. This bird's green is lighter and the black head ring and bib go smoky grey / smoky blue.
YY. This bird is a double factor bird ie it has a double dose of the yellow gene. This bird is yellow (perhaps with an olive "wash") instead of green and the black becomes white or a very pale blue.

As a hen only has one W she cannot be double factor and so if she has one Y gene she is yellow much the same looking (but paler) as a double factor cock.

Co dominance inheritance patterns are unusual - most mutations are recessive meaning that normal masks the effect of the gene unless a bird has two copies (one on each chromosome) or dominant where the existence of a single gene masks the normal gene on the other chromosome.

Remember all birds have two sets of chromosomes one from each parent and many mutations are on chromosomes other than the ones which determine sex.

If all the genetics bothers you, just remember the bit in blue and yellow text

A Cock bird has two sex chromosones (ZZ) and a hen one (Z) chromosone and one (W) chromosone (ZW).

A colour like yellow back is called sex linked and acts on the Z chromosone only and not the W chromosone.

So if a hen has the yellow colour on it's Z chromosone then it will be visually Yellow back. (Hens can not be DF)

If a cock only has the yellow colour on one of his Z chromosones then he is SF Single Factor, if he has yellow on both his Z chromosones then he is DF Double Factor.
Purple breasted Cock Gouldians SF Yellow will have a dilute green back and a DF Yellow will have a complety yellow back.
White breasted Cock Gouldians SF Yellow will have a yellow back with a blue ring around the head, a DF will have a more intense yellow back and a white ring around the head.

Simba, the co-dominance is crucial and affects the terminology. A cock with a sex linked recessive gene (eg albinism) is split for that trait, it is not said to be a single factor bird.

Natamambo,

According to what I have read, Males are the homogametic sex (ZZ), while females are heterogametic (ZW), but you have said males are (WW) is this correct ?

I understand your comments about Co-Dominance but the poster asked for it to be explained in Laymen's and so I tried to keep it simple.

Opps, sorry Simba. Fixed. Too late at night .

I know what you mean about trying to keep it simple, just being extra careful about another misconception being spread ascross the web just coz "it said so on AFF" .

Thank you all for your help. Makes much more sense now.

Purple breasted Cock Gouldians SF Yellow will have a dilute green back

Looks like I have a SF Yellow Cock bird.

One very relavant point. SF cocks do not have a black ring round the head. It is pale instead. Just found that the most obvious difference, at first glance.
LML

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