Final answer:
In humans, the most common disorders involving abnormalities in the number of autosomes are trisomy's of chromosomes 13, 18, and 21, with chromosome 21 trisomy leading to Down syndrome. These trismic individuals have genetic imbalances due to an excess in gene dose. The incidence of these trisomy's increases with parental age, particularly maternal age.
Step-by-step explanation:
The most common disorders involving abnormalities in the number of autosomes are trisomies of chromosomes 13, 18, and 21. In humans, a normal euploid individual has 22 pairs of autosomes and one pair of sex chromosomes. A person with a trisomy has three copies of an autosome instead of two. The trisomy of chromosome 21 leads to Down syndrome, which is the only autosomal trisomy where an affected individual may survive to adulthood, although it is associated with mental and physical impairments and specific facial appearance. The other common trisomy's that may result in live births, but with varying degrees of survival, are trisomy 18 and 13, which generally lead to more severe developmental and health problems compared to Down syndrome.
Autosomal trismic individuals suffer from genetic imbalances due to an excess in gene dose, which can disrupt normal cell functions and development. The risk of nondisjunction, which is the failure of chromosomes to separate properly during meiosis and leads to trisomy, increases with the age of the parents, especially the maternal age. Furthermore, most other autosomal trisomy's are not compatible with life and usually result in miscarriage.