On the way home from COG last night Lia and I (our homes are nearby and we usually share transport) talked about this bird and Lia asked about why postulate
about there being more than one bird involved.
So I thought about it and will explain. It comes down to mathematics. Although I don’t know the story and I have no knowledge as to whether more than one bird
has actually been observed, given the history, I suggest that the chance of there only being one bird affected, is likely to be small.
Firstly as a recessive character, to show it, the two genes must have come from both parents. Thus, the mutation must have been in at least two birds (its parents)
at the same time. So it is clearly being maintained in that population, before and likely after the birth of that bird. The alternate scenario i.e. the chance of both copies of the gene mutating in the one individual, could be suggested is something of the
order of thousands or more likely many millions to one, so that does not work. Surely most of the mutant genes are in individuals that do not show it. Given that the recessive gene is in the population, if over a period of at least 19 years, with so many birds
breeding, the chances that the combinations as described below, not occurring at all, actually become quite low. Of course it depends how wide the gene pool is, as the extent to which the mutant gene is swamped by the more common normal gene. The mathematics
is rather simple if we know the answers to questions on various assumptions about gene frequency, any selective pressures, any assortative matings........
The variant might be called “blue” although that is not much of a name either, the extent of blue looks normal. It is an absence of red. By comparison blue
budgerigars come from an absence of the gene to make the yellow pigment, thus making the normally yellow parts white and the green parts blue.
An on-line simple explanation of the genetics principles is on many websites, such as this one, devoted to birds
http://feistyhome.phpwebhosting.com/genes.htm...... As pointed out there, although genetics is basically the same for all life, there are variations not impacting on this issue, in that birds are different
to mammals, because the sex chromosomes are opposite (and birds typically have many more chromosomes than mammals).
Philip
From: Philip Veerman [
Sent: Monday, 6 November, 2017 3:10 PM
To: 'Ryu Callaway'; 'canberrabirds chatline'
Subject: RE: [canberrabirds] leucistic Crimson Rosella
Ryu,
Yes. You have listed the normal features of recessive trait inheritance. So assuming the website is
correct and the birds are displaying that particular feature, all that holds. I would not think that leucistic is the right term. However I suggest slight rewording to the following:
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A bird may carry the mutated gene but not show it.
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In this case the “mutant” gene exists at low levels in the population. So most birds do not show or carry the “mutant” gene.
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The 'blue' Rosella must have received the mutant gene from both parents.
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2 normal parents both carrying the recessive gene: on average, all other things being equal, 25% of their offspring will show the 'blue' character, other 75% normal.
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1 blue rosella and the other normal parent carrying the recessive gene: on average, all other things being equal, 50% of their offspring will show the 'blue' character, other half normal.
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1 blue rosella and the other normal parent that is not carrying the recessive gene will only produce normal looking offspring. Although all them will carry the mutant and the normal gene.
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2 blue rosella parents: all of any offspring will show the 'blue' character.
To explain why: for example “25% chance of 'blue' offspring” of the heterozygous couple, only applies in regard to just each one offspring..... For example,
if they end up having 20 + offspring, there is a vastly higher chance than 25% of them of having at least one 'blue' offspring – on average 5 out of 20.......
There also can be all sorts of other factors that can influence these pure genetic ratios up or down.
As for Sue’s other question, I’d guess 19 years is quite feasible for a wild rosella, but it is more likely not to be the same bird but could well be related.
“First saw one” suggests to me there is more than one, a little ongoing population there that carry the gene and rarely is shown.
Philip
From: Ryu Callaway [
Sent: Sunday, 5 November, 2017 10:45 PM To: Sue Lashko; canberrabirds chatline
Subject: Re: [canberrabirds] leucistic Crimson Rosella
And a similar bird reported to Canberra Nature Map in September from Banks
My simplistic interpretation of that I think is:
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A bird may carry the mutated gene but not show it
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For a Crimson to be 'blue' both its parents must carry the mutant gene
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2 normal parents both carrying the recessive gene: 25% chance of 'blue' offspring
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1 blue rosella and the other normal parent carrying the recessive gene: 50% chance of 'blue' offspring
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2 blue rosellas: 100% chance of 'blue' offspring
