👉 In genetics, a trisomy is a condition where an individual has two copies of each chromosome. This means that there are two extra chromosomes in the female and two extra chromosomes in the male form. For example, if you have 23 chromosomes (one for each sex), but one of them is missing, then you would have 22 chromosomes instead. This is called a trisomy 21. Trisomies can be caused by genetic disorders or environmental factors such