Chromosomes

About 63% of known bird species have a diploid chromosome number (2n) of 74-86. A further 24% have 2n = 66-74.

Extremes at the lower end of the spectrum are the trumpeter hornbill Ceratogyman bucinator and stone curlew Burhinus oedicemus (with 2n = 40 and 42 respectively). The highest recorded chromosome number in birds is in the common kingfisher Alcedo athis (2n = 132 or 138) or the Grey (or Southern) Go-away bird Corythaixoides concolor (2n = 136-142). Understandably, accurate chromosome counts become difficult with these numbers. [Source: Christidis, 1990, Animal Cytogenetics 4, Chordata 3 B: Aves]

In general, chromosome number in birds is both higher than in most other vertebrates, and less variable. The typical avian karyotype is composed of macro- and microchromosomes, with a continuous size distribution. Exceptions include the falcons and parrots, where chromosomal fusions and other rearrangements are common, and have reduced chromosome number and size variation in these groups as a whole.

For a recent review of this topic, see: The evolution of the avian genome as revealed by comparative molecular cytogenetics, Cytogenet Genome Res. 2007;117(1-4):64-77

up a homologous pair. They act similarly in their roles, but they are not homologous (the same). The X chromosome in humans is much longer than the Y chromosome and also contains many more genes.

Different kinds of tests are:

Serum screening

This is a blood test performed at 15 to 20 weeks. It measures the levels of pregnancy hormones in the mother's blood. By combining these results with the mother's age, then it is possible to calculate the risk of the baby having Down's syndrome. If the woman is considered low risk after the blood test, then nothing further is usually necessary. If the result suggests she is at high risk of having an affected baby, she will be offered an amniocentesis test to see whether the baby is affected or not. The blood test is not foolproof and, occasionally, a Down's baby will be born to a mother with a low-risk result. A high-risk result does not mean that the baby is affected, it just means that the woman may wish to consider a diagnostic test. Nuchal translucency measurement

Using ultrasound at 11 to 14 weeks it is possible to measure the fluid space at the back of the fetus' neck. Down's babies have a bigger space than unaffected pregnancies. By combining this measurement with the mother's age and sometimes with hormone measurements in the mother's blood, it is possible to calculate the risk of carrying an affected baby. Again, if the risk is high then the woman may consider a diagnostic test. Diagnostic tests

A chromosome abnormality can only be made with certainty by analysing cells from the fetus in the genetics laboratory. A diagnostic test is performed when: * a woman is considered at high risk because of the screening test. * a woman is considered at high risk because she has previously had a pregnancy affected by a chromosomal or genetic disorder. * an ultrasound examination has detected features or abnormalities indicating an increased risk of a chromosome abnormality. * a woman requests it because she is concerned that her baby has a chromosome abnormality. * There are two types of diagnostic test - amniocentesis and chorion villous biopsy (CVS). Which of these tests a woman is offered will vary according to the hospital in which she plans to give birth. It is important that the mother understands the implications and the purpose of these tests.