How semi-identical twins are made

1 Two sperm enter the same egg

Sperm 1

Egg

Sperm 2

2 The egg now has three sets of chromosomes

Father’s

first set

Mother’s

set

Father’s

second set

3 Three cells develop

Mother-father

(X-Y)

Mother-

father

(X-X)

Father-father

(X-Y)

4 The cells divide and multiply

Father-father

X-Y cells die

5 The ball of cells splits with different ratios of X-X and X-Y cells

Twin 1

Twin 2

6 Semi-identical twins develop in the womb

Twin 1

Twin 2

1 Two sperm enter the same egg

2 The egg now has three sets of chromosomes

Father’s first set

Sperm 1

Mother’s set

Sperm 2

Egg

Father’s

second set

4 The cells divide and multiply

3 Three cells develop

Mother-father (X-Y)

Mother-

father (X-X)

Father-father

X-Y cells die

Father-father (X-Y)

5 The ball of cells splits with different ratios of X-X and X-Y cells

6 Semi-identical twins develop in the womb

Twin 1

Twin 2

Twin 1

Twin 2