When a female transmits a single defective gene from one of her two X chromosomes, an X-linked disease may occur. The disease will affect half of her sons, and half her daughters will be carriers. Well-known examples include Duchenne Muscular Dystrophy and the bleeding disorder, Hemophilia A. Typical characteristics of an X-linked type disorder include:
- The disorder is never transmitted from father to son
- But the gene is passed from an affected man to all his daughters and then through them to, on average, half his grandsons.
Disorders of the Mitochondria
Floating in the fluid that surrounds the nucleus of each cell are several hundred small circular bits of DNA called mitochondria. A defective gene in these mitochondria can be passed from a mother to all of her children of both sexes. However, not all offspring will show signs of the disorder. And, affected and carrier males will not transmit the mutation to any of their children!
One example of maternal mitochondrial inheritance is one form of deafness. In this case, a specific mitochondrial gene mutation predisposes a person to develop deafness after taking an antibiotic such as Gentamycin. This antibiotic is commonly used in tiny, premature infants and could cause deafness in genetically susceptible infants.
Y Chromosome Disorders
Genes that pass from father to son do so on the male (Y) chromosome. Mutations or missing genes on the Y chromosome may lead to infertility due to absent sperm or a low sperm count. These disorders will be passed (if a father is able to sire children) to all his sons, but to none of his daughters.
Over 8,500 genetic disorders and traits are transmitted through single genes. As emphasized in my column on the Family Tree, knowledge of the family history is critical in recognizing a pattern of gene transmission through the family. And once you know, you can deal with the condition accordingly with the help of a doctor.