Allopolyploidy refers to polyploidy occurring in an interspecies hybrid. Wheat is a classic example. It is a hexaploid with genomes from three different species. While the species hybrid itself is likely to be sterile, by doubling the chromosomes the homologs can pair with a homolog from their own species. Studies can demonstrate that allopolyploids originate by hybridization. Genetic markers can be traced to the progenitor species. Chromosome painting which involves hybridization of DNA from suspected parent species to the chromosomes of the allopolyploid can be used to confirm suspected origins.
Genetic studies with monoploids can have advantages. In plants, monoploid tissues can be cultured and treated with chemicals to cause mutations. The cells can then be selected for particular traits, the chromosomes doubled with colchicine treatment, and a diploid plant produced with hormone treatments. This can allow distinctive types of plants to be selected which otherwise would be very difficult to isolate.
In fish, clonal lines can be produced by fertilizing irradiated eggs with normal sperm. The fish are made into homozygous diploids by heat-treating the eggs shortly after fertilization. These individuals are reared to maturity and used to produce clonal lines by using sperm to fertilize irradiated eggs, or by allowing eggs to be activated to develop using irradiated sperm. In both cases, heat treatments are used to produce diploids.
Aneuploidy refers to changes from balanced sets (euploid condition) of chromosomes. Trisomic refers to 2n +1 (one extra chromosome), monosomic refers to 2n – 1. Changes are normally the result of nondisjunction.
The ability to G-band (Giemsa band) all of the human chromosomes opened the way to the detailed study of human chromosomes. Trisomy 21 is the most common human chromosome abnormality at birth. It is also known as Down syndrome. The incidence of trisomy 21 rises dramatically in older mothers. Amniocentesis can be used to detect trisomy 21 in fetuses before birth. Trisomy for the other human chromosomes is not generally compatible with survival past birth. Exceptions include trisomy 13 and trisomy 18. 15% of pregnancies end in miscarriage/ spontaneous abortion. Half of those have chromosome abnormalities (most common are XO and triploidy). However, less than 1% of live births have chromosome abnormalities. The most common of those are trisomy 21 and translocations (see text Table 5.1).
Aneuploidy for the sex chromosomes is generally more easily tolerated than aneuploidy for the autosomes. Females include XXX, trisomy X indivdiuals, who are relatively normal and fertile but show some increase in the frequency of mental retardation. The other common female sex chromosome aneuploidy is XO, Turner syndrome. These individuals are short, not fertile and have normal intelligence.
Males include XYY, double Y, resulting in taller than average males and some increase in frequency of mental retardation.. The other common type are XXY, Klinefelter syndrome. These individuals are tall, not fertile and often mentally retarded.
Dosage compensation refers to mechanisms used to even out expression on the X chromosomes in males and females. In Drosophila, each X in females produces half as much transcript (mRNA) as the single X in males. In humans and other mammals all but one X is genetically inactivated early in development. This explains why sex chromosome aneuploids have high viability. The calico female cat is a living example of X chromosome inactivation. Some genes on the short arm of the X which pair with the Y are not subject to X chromosome inactivation. This may account for why XO, XXX and XXY individuals are marginally less healthy than are normal males and females.
(That is as far as the test on Tuesday, February 18 will cover. This corresponds to page 174 in the text).