Mutations & Development of Twins I. Mutations What is a mutation? Mutation – changes in genetic material (DNA). *Mutations occur when cells make mistakes in copying their own DNA.

2 Major Kinds of Mutations 1. Gene mutation – mutation caused by changes in a single gene. 2. Chromosomal mutation – mutation caused by changes in a whole chromosome. -Sometimes things in the environment can cause mutations to occur. Mutagen – any substance that causes changes in DNA, ex : radiation, UV light, X-rays, smoking, drugs, alcohol, etc.

II. Development of Twins -There are 3 major types of twins : 1. Fraternal twins – occur when 2 eggs are fertilized by 2 different sperm. -They don’t look alike & can be girls, boys, or a girl & a boy.

Identical Twins 2. Identical twins – occur when 1 egg randomly splits into two babies. -They look exactly the same, are the same sex, & have the same genetic make-up.

Conjoined Twins 3. Conjoined (Siamese) twins – occur when1 egg starts to split randomly & then stops before the babies split.