Perhaps you’ve seen movies like "Alien 3" that portray men with an extra Y chromosome as homicidal maniacs. In reality, an extra Y chromosome may go undetected and have no noticeable side-effects. The condition, however, is not always entirely benign and can adversely affect a boy’s growth and learning abilities.

Chromosomes are compact packages of DNA and proteins containing the genes that encode the genetic features of an organism. Humans have 23 pairs of chromosome, with one pair member donated by each parent. The first 22 pairs are called “somatic,” but the last pair forms the sex chromosomes. These come in two flavors, X and Y. A female has two X chromosomes; a male is XY. An extra Y chromosome is termed XYY syndrome or Jacob's disorder. Fathers of XYY syndrome children typically have the normal XY complement of sex chromosomes. The chances of bearing an XYY child are about 0.1 percent of male babies. In the United States, mothers deliver about 10 XYY boys a day.

The disorder happens by chance at the time of fertilization and doesn’t predict that brothers will also have the same syndrome. The first signs that a boy is XYY involve learning to speak -- about half such boys have delayed speech acquisition or sub-par reading skills. Early therapy can usually solve the problem. Possible other symptoms include delayed motor skills, weak muscles and involuntary movements. Most XYY boys average normal to slightly lower intelligence, although it is possible to have above-average intelligence as well. Boys with the syndrome might be more temperamental, less able to cope with stress, physically active and easily distracted. XYY boys tend to grow faster and taller than the general population.

Many boys with XYY syndrome mature into men who can hold a job and seek secondary education. According to the United Kingdom’s Genetic Alliance, about 75 percent of XYY adults are employed, in a wide variety of jobs. Men with the syndrome are just as likely to have normal sex lives and normal children as do men in the general population. Offspring are no more likely to have the syndrome than are the children of parents with the regular number of chromosomes.

Boys with XYY syndrome are no more aggressive than are boys without it. No increase in serious mental illness exists. You are no more likely to become a criminal due to the syndrome. Homosexuality rates equal those in the general population. Special hormone therapy is not needed as males with the syndrome usually have normal hormone levels. Genetic testing is available for pregnant women, but the risks of having a second XYY child are no greater than having the first. Some XYY males are mosaic: Only some of their body cells have the extra Y chromosome, and their symptoms vary with the proportion of affected cells.