1. 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. 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.

3. 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.

4. 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.

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