Note: This section only refers to eukaryotes. Bacterial genetic exchange is entirely non-Mendelian.
Some forms of inheritance complicate the pattern of Mendelian inheritance and reassortment of traits. Inheritance may be overlaid by expansion of triplet repeats, or by parental imprinting. A few genes are not inherited by Mendelian principles at all, because they are contained on an extranuclear chromosome of the mitochondria or chloroplast.
A well-known example is Fragile
X Syndrome. This very common cause of mental retardation
results from a region of repetitive DNA sequence that grows longer than
normal with each generation. The length of the sequence--a
triplet repeat, called a microsatellite--encourages
the region of the chromosome to shut down expression of an essential
gene nearby. It also makes the chromosome "fragile;" that region
of the chromosome fails to condense during cell culture for
Fragile X shows incomplete dominance; females who carry the trait will
show partial (but not total) compensation by the wild-type allele on
other X chromosome. The effectiveness of the compensation is partly
related to patterns of X-inactivation.
So far, all the inheritance mechanisms we have considered depend on DNA sequence of bases: A, T, C, G. However, certain sequences in addition contain chemical modification such as methylation. These modifications are added after DNA replication. They add information about how a gene will be expressed.
methylation occurs in the gonads, as sperm or eggs develop.
For example Huntington's disease is autosomal dominant. The onset of disease may occur in childhood, if you inherit the trait through your father; but only much later in life, if you inherit through your mother.
Another disease involving methylation is Fragile X Syndrome. (See your handout.) In Fragile X, a region of repetitive CGG sequence is too long. The length of the repetitive sequence stimulates enzymes to "turn off" expression, including a nearby essential gene, FMR-1. This sequence is "turned off" through methylation in the female, and fails to get "turned back on" in her children. Children of males do not show the syndrome; but the grandchildren have high risk.
Outside the nucleus are the mitochondria, which evolved out of endosymbiotic bacteria. Plants in addition have chloroplasts which arose the same way. Mitochondria have small circular chromosomes which are inherited through the female, because the sperm contribute no mitochondria to the fertilized egg.
mother passes on mitochondrial traits to all of her children. Some Mitochondrial
Diseases have been identified,
including muscle degeneration and some forms of migraine headache.