MadSci Network: Zoology

Re: How common are 2 albinos in a clutch of 3 blue and gold macaws?

Date: Mon Aug 14 10:27:57 2000
Posted By: Kurt Wollenberg, Post Doc Genetics, North Carolina State University
Area of science: Zoology
ID: 966140711.Zo

Albinism in psittacines (parrots) appears to be reasonably well studied due 
to the presence albinism in budgerigars. I found the following web site with 
lots of information about this:

Generally, albinism is a recessive genetic trait causing a defect in the 
biochemical pathway that produces melanin. The most common form of albinism 
results from a defect in the enzyme tyrosinase, which is used to process the 
amino acid tyrosine into melanin. The common form of melanism in birds can 
be either sex-linked (the gene is on one of the sex chromosomes, which in 
birds are designated as Z and W) or autosomal (the gene is on one of the 
other chromosomes). Because female birds are hemizygous (ZW) if your birds 
were carrying a sex-linked gene for albinism one would expect the female to 
be albino more frequently than the male. As you had two albino males and a 
normal female your parent birds are probably carrying an autosomal allele 
for albinism. 

Is having two out of three offspring albino unexpected? Under normal 
Mendelian inheritance an autosomal recessive trait is expected to be 
phenotypically expressed in one quarter of the offspring. At first glance it 
would appear that 0.67 is much greater than 0.25. If one were trying to make 
a judgement from this single clutch our statistical power to tell if 0.25 is 
significantly different from 0.67 would be reduced by the small sample size 
(3). However, as you mention that these parents have had about 20 normal 
chicks then it would appear that having two albinos out of twenty is not too 
unexpected (0.25 expected, .10 seen). 

Are these males twins? I haven't found any data on twinning in birds, though 
I expect that it would be a difficult phenomenon to judge. One molecular 
genetic technique that could be used to test this is microsatellite 
analysis. This technique has been used to assess paternity and maternity 
(egg dumping) in wild birds. If you could find a genetic lab at a university 
around you this sounds like it would be an interesting one semester project 
for a beginning graduate student. 

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