XYY syndrome, also known as Jacob's syndrome, is a condition which affects only males and is where they have an extra Y chromosome. The cause of XYY syndrome is due to a mistake in cell division during the production of the male sex cell, the sperm cell, leading to 47 chromosomes rather than the normal amount of 46. Jacob's syndrome is a rare disease, effecting 1 in 1000 new-born male babies. Although there's a chance that sufferers of the condition are slightly taller than average, they show no physical abnormalities. Whilst there usually isn't any physical defects, many of the sufferers have learning difficulties, specifically in speech and motor skills.